cvs: phpdoc-ru /chapters security.xml
| From: | Alexey Asemov | Date: | Wed, 25 Dec 2002 16:40:35 +0000 |
| Subject: | cvs: phpdoc-ru /chapters security.xml | ||
| Groups: | php.doc.ru | ||
| Request: | Send a blank email to doc-ru+get-56@lists.php.net to get a copy of this message | ||
alexws Wed Dec 25 11:40:35 2002 EDT
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Index: phpdoc-ru/chapters/security.xml +++ phpdoc-ru/chapters/security.xml <?xml version="1.0" encoding="iso-8859-1"?> <!-- EN-Revision: 1.49 Maintainer: alexws Status: in-progress --> <!-- $Revision: 1.1 $ --> <chapter id="security"> <title>Áåçîïàñíîñòü</title> <simpara> PHP - ýòî ìîùíûé ÿçûê ïðîãðàììèðîâàíèÿ è èíòåðïðåòàòîð. Áóäó÷è èíòåãðèðîâàííûì â âèäå ìîäóëÿ â Web-ñåðâåð, èëè âûïîëíÿÿñü êàê îääåëüíîå <acronym>CGI</acronym>-ïðèëîæåíèå, îí ìîæåò îáðàùàòüñÿ ê ôàéëàì, èñïîëíÿòü êîìàíäû è îòêðûâàòü ñåòåâûå ñîåäèíåíèÿ íà ñåðâåðå. Ýòè îñîáåííîñòè äåëàþò âñå, ÷òî âûïîëíÿåòñÿ íà Web-ñåðâåðå, íåáåçîïàñíûì. PHP ñïåöèàëüíî ðàçðàáàòûâàëñÿ äëÿ òîãî, ÷òîáû áûòü áîëåå áåçîïàñíûì ÿçûêîì äëÿ íàïèñàíèÿ CGI-ñîâìåñòèìûõ ïðîãðàìì, íåæåëè Perl èëè Ñ, è, åñëè ïðàâèëüíî âûñòàâëåíû íàñòðîéêè êîìïèëÿöèè è âûïîëíåíèÿ, à òàêæå ïðè íàëè÷èè äîñòàòî÷íîãî îïûòà ïðîãðàììèðîâàíèÿ, PHP äàñò âàì íåîáõîäèìîå ñîîòíîøåíèå ãèáêîñòè è áåçîïàñíîñòè. </simpara> <simpara> Ïîñêîëüêó ñóùåñòâóåò ìíîæåñòâî ðàçëè÷íûõ âàðèàíòîâ èñïîëüçîâàíèÿ PHP, ñóùåñòâóåò î÷åíü ìíîãî íàñòðîåê, êîíòðîëèðóþùèõ åãî ïîâåäåíèå. Áîëüøîé âûáîð îïöèé îçíà÷àåò, ÷òî âû ñìîæåòå èñïîëüçîâàòü PHP äëÿ ìíîãèõ öåëåé, îäíàêî òàêæå çíà÷èò, ÷òî åñòü òàêèå êîìáèíàöèè ýòèõ îïöèé è íàñòðîåê ñåðâåðà, êîòîðûå ïðèâåäóò ê ïðîáëåìàì áåçîïàñíîñòè. </simpara> <simpara> Ãèáêîñòü íàñòðîéêè PHP êîíêóðèðóåò ñ ãèáêîñòüþ ïðîãðàììèðîâàíèÿ. PHP äëÿ ñîçäàíèÿ ñåðâåðíûõ ïðèëîæåíèé ñî âñåìè âîçìîæíîñòÿìè ðÿäîâîãî ïîëüçîâàòåëÿ ñèñòåìû, èëè äëÿ ïðîñòîãî âêëþ÷åíèÿ ôàéëîâ íà ñòîðîíå ñåðâåðà, êîòîðîå íå ïðåäñòàâëÿåò îïàñíîñòè â õîðîøî êîíòðîëèðóåìîé ñèñòåìå. Òî, êàê ïîñòðîåíà ñèñòåìà, è íàñêîëüêî îíà áåçîïàñíà, ïðàêòè÷åñêè ïîëíîñòüþ çàâèñèò îò ðàçðàáîò÷èêà, èñïîëüçóþùåãî PHP. </simpara> <simpara>  íà÷àëå ãëàâû äàþòñÿ îñíîâíûå ñîâåòû ïî ðåàëèçàöèè áåçîïàñíîñòè, îáúÿñíÿþòñÿ ðàçëè÷íûå êîìáèíàöèè îïöèé íàñòðîéêè è ñèòóàöèè, â êîòîðûõ îíè ìîãóò áûòü áåçîïàñíî èñïîëüçîâàíû. Òàêæå ïðèâîäÿòñÿ ðàçëè÷íûå ñîîáðàæåíèÿ ïî ÷àñòè ïðîãðàììèðîâàíèÿ äëÿ ðàçíûõ óðîâíåé áåçîïàñíîñòè. </simpara> <sect1 id="security.general"> <title>Îñíîâíûå ïîëîæåíèÿ</title> <simpara> Ïîëíîñòüþ áåçîïàñíàÿ ñèñòåìà ïðàêòè÷åñêè íå ìîæåò ñóùåñòâîâàòü, ïîýòîìó îñíîâíîé èñïîëüçóåìûé çäåñü ïîäõîä - ïîäõîä ðàçóìíîãî ðèñêà. Ê ïðèìåðó, åñëè êàæäàÿ ïåðåìåííàÿ, ïåðåäàííàÿ ïîëüçîâàòåëåì, áóäåò òðåáîâàòü äâóõ áèîìåòðè÷åñêèõ ïîäòâåðæäåíèé (âðîäå ñêàíèðîâàíèÿ ñåò÷àòêè è îòïå÷àòêà ïàëüöà), ó âàñ áóäåò î÷åíü âûñîêèé óðîâåíü êîíòðîëÿ. Ñ äðóãîé ñòîðîíû, çàïîëíåíèå äëèííûõ è ñëîæíûõ ôîðì ìîæåò çàíèìàòü äîñòàòî÷íî äëèòåëüíîå âðåìÿ, ÷òî îáû÷íî çàñòàâëÿåò ïîëüçîâàòåëåé èñêàòü ïóòè îáõîäà ñèñòåìû áåçîïàñíîñòè. </simpara> <simpara> Íàèëó÷øèé âàðèàíò îáû÷íî ñîîòâåòñòâóåò òðåáîâàíèÿì áåçîïàñíîñòè, íî íå ìåøàåò ðàáîòå ïîëüçîâàòåëåé, à òàêæå íå ïåðåãðóæàåò ïðîãðàììíûé êîä èçëèøíåé ñëîæíîñòüþ. ×òî êàñàåòñÿ ñëîæíîñòè, òî íåêîòîðûå àòàêè êàê ðàç è ÿâëÿþòñÿ ñëåäñòâèåì ÷ðåçìåðíîãî "íàâîðà÷èâàíèÿ" çàùèòû. Êàê èçâåñòíî, ÷åì ñëîæíåå çàùèòà, òåì áîëüøå â íåé íå ñîâñåì î÷åâèäíûõ "äûð". </simpara> <simpara> Âñåãäà íåîáõîäèìî ïîìíèòü: ñèñòåìà áåçîïàñíîñòè íàñòîëüêî õîðîøà, íàñêîëüêî áåçîïàñíî åå ñàìîå ñëàáîå ìåñòî. Åñëè, ê ïðèìåðó, âåäóòñÿ îò÷åòû ïî âñåì îïåðàöèÿì, âðåìåíè, ìåñòó è ò.ä., íî ïðîâåðêà ïîëüçîâàòåëåé îñóùåñòâëÿåòñÿ ñ èñïîëüçîâàíèåì îäíîãî cookie, òî ñîîòâåòñòâèå èìåí ïîëüçîâàòåëåé â îò÷åòàõ ñ ðåàëüíûìè ïîëüçîâàòåëÿìè âåñüìà è âåñüìà ñîìíèòåëüíî. </simpara> <simpara> Ïðè îòëàäêå ïîìíèòå, ÷òî íåâîçìîæíî ïðåäóãàäàòü âñå âàðèàíòû ðàçâèòèÿ ñîáûòèé äàæå äëÿ îäíîé åäèíñòâåííîé ñòðàíè÷êè. Îæèäàåìûå âõîäíûå äàííûå ìîãóò îòëè÷àòü îò ââåäåííûõ íåäîáðîñîâåñòíûì ðàáîòíèêîì, õàêåðîì ñ ìíîãîëåòíèì ñòàæåì èëè ïðîñòî êîøêîé, ïðîøåäøåéñÿ ïî êëàâèàòóðå. Ïîýòîìó âñåãäà óäîáíåå ïîäõîäèòü ê ïðîãðàììå ñ òî÷êè çðåíèÿ ëîãèêè, ÷òîáû ïðîñëåäèòü ìåñòà âîçìîæíîãî ïîÿâëåíèÿ íåîæèäàííûõ äàííûõ, à òàêæå èõ èñïîëüçîâàíèÿ. </simpara> <simpara> Internet ïîëîí ëþäåé, êîòîðûå õîòÿò ñäåëàòü ñåáå èìÿ ïóòåì âçëîìà âàøåãî êîäà, îñòàíîâà âàøåãî ñàéòà, ðàçìåùåíèÿ íåïîäõîäÿùèõ äàííûõ, â îáùåì, âñåãî, ÷òî íå äàåò âàì æèòü ñïîêîéíî. Íåâàæíî, êàêîãî ìàñøòàáà âàø ñàéò, âû íàõîäèòåñü ïîä ïðèöåëîì ïðîñòî ïîòîìó, ÷òî âû â ñåòè, ïîòîìó, ÷òî ó âàñ åñòü ñåðâåð, ê êîòîðîìó ìîæíî ïîäêëþ÷èòüñÿ. Ìíîæåñòâî ïðîãðàìì äëÿ âçëîìà íå ðàçëè÷àþò ðàçìåðîâ, îíè ïðîñòî ïðîêà÷èâàþò îãðîìíîå êîëè÷åñòâî IP-òðàôôèêà â ïîèñêàõ æåðòâû. Ïîñòàðàéòåñü íå ñòàòü òàêîâîé. </simpara> </sect1> <sect1 id="security.cgi-bin"> <title>Ïðè èíñòàëëÿöèè â âèäå ïðèëîæåíèÿ CGI</title> <sect2 id="security.cgi-bin.attacks"> <title>Âîçìîæíûå àòàêè</title> <simpara> Èñïîëüçîâàíèå PHP â âèäå ïðèëîæåíèÿ <acronym>CGI</acronym> - âàðèàíò äëÿ òåõ, êòî ïî òåì èëè èíûì ïðè÷èíàì íå õî÷åò âñòðàèâàòü PHP â ïðèëîæåíèå ñåðâåðà (ê ïðèìåðó, Apache), â âèäå ìîäóëÿ, èëè äëÿ òåõ, òî õî÷åò èñïîëüçîâàòü PHP ñîâìåñòíî ñ CGI-îáîëî÷êàìè, ïîìîãàþùèìè ñîçäàòü áåçîïàñíóþ ñðåäó äëÿ âûïîëíåíèÿ ïðîãðàìì. Òàêàÿ íàñòðîéêà îáû÷íî âêëþ÷àåò â ñåáÿ èíñòàëëÿöèþ ïðèëîæåíèÿ PHP â äèðåêòîðèþ cgi-bin íà ñåðâåðå. Ðåêîìåíäàöèÿ CERT<ulink url="&url.cert;">CA-96.11</ulink> âûñòóïàåò ïðîòèâ ïîìåùåíèÿ èíòåðïðåòàòîðîâ â cgi-bin. Äàæå ïðè òîì, ÷òî PHP ìîæåò áûòü èñïîëüçîâàí, êàê íåçàâèñèìûé èíòåðïðåòàòîð, îí ðàçðàáàòàâûëñÿ äëÿ ïðåäîòâðàùåíèÿ àòàê, ñâÿçàííûõ ñ äàííûì âàðèàíòîì íàñòðîéêè: </simpara> <itemizedlist> <listitem> <simpara> Ïîëó÷åíèå äîñòóïà ê ñèñòåìíûì ôàéëàì: <filename role="url">http://my.host/cgi-bin/php?/etc/passwd</filename> </simpara> <simpara> Èíôîðìàöèÿ çàïðîñà â URL ïîñëå çíàêà âîïðîñà (?) ïåðåäàåòñÿ ñ ïîìîùüþ CGI èíòåðïðåòàòîðó â âèäå êîìàíäíîé ñòðîêè. Îáû÷íî èíòåðïðåòàòîðû îòêðûâàþò è âûïîëíÿþò ôàéë, çàäàííûé ïåðâûì àðãóìåíòîì â äàííîé ñòðîêå. </simpara> <simpara> Ïðè âûçîâå â ðîëè ïðèëîæåíèÿ CGI, PHP íå èíòåðïðåòèðóåò àðãóìåíòû êîìàíäíîé ñòðîêè. </simpara> </listitem> <listitem> <simpara> Ïîëó÷åíèå äîñòóïà ê ëþáîìó äîêóìåíòó íà ñåðâåðå: <filename role="url">http://my.host/cgi-bin/php/secret/doc.html</filename> </simpara> <simpara> Èíôîðìàöèîííàñÿ ÷àñòü URL ïîñëå íàçâàíèÿ ïðèëîæåíèÿ PHP <filename role="uri">/secret/doc.html</filename> îáû÷íî èñïîëüçóåòñÿ äëÿ çàäàíèÿ ôàéëà, êîòîðûé áóäåò îòêðûò è èíòåðïðåòèðîâàí ïðèëîæåíèå <acronym>CGI</acronym>.  îáùåì ñëó÷àå äëÿ ïåðåíàïðàâëåíèÿ çàïðîñîâ âèäà <filename role="url">http://my.host/secret/script.php</filename> èíòåðïðåòàòîðó PHP èñïîëüçóþòñÿ äèðåêòèâû êîíôèãóðàöèè Web-ñåðâåðà (ê ïðèìåðó, Action â Apache). Ïðè òàêîé íàñòðîéêå, ñåðâåð ñíà÷àëà ïðîâåðÿåò ðàçðåøåíèÿ íà äîñòóï ê äàííîé äèðåêòîðèè <filename role="uri">/secret</filename>, è òîëüêî ïîñëå ñîçäàåò ïåðåíàïðàâëåííûé çàïðîñ <filename role="url">http://my.host/cgi-bin/php/secret/script.php</filename>. Ê ñîæàëåíèþ, åñëè çàïðîñ óæå äàí â âûøåïðèâåäåííîé ôîðìå, ñåðâåð íå âûïîëíÿåò íèêàêèõ ïðîâåðîê íà ïðàâî äîñòóïà ê ôàéëó <filename role="uri">/secret/script.php</filename>, âûïîëíÿÿ äàííóþ ïðîâåðêó òîëüêî äëÿ ôàéëà <filename role="uri">/cgi-bin/php</filename>. Òàêèì îáðàçîì, ëþáîé ïîëüçîâàòåëü c äîñòóïîì ê <filename role="uri">/cgi-bin/php</filename> ìîæåò ïîëó÷èòü äîñòóï ê ëþáîìó çàùèùåííîìó äîêóìåíòó íà ñåðâåðå. </simpara> <simpara>  PHP, îïöèÿ êîíôèãóðàöèè êîìïèëÿòîðà<link linkend="install.configure.enable-force-cgi-redirect">--enable-force-cgi-redirect</link> è îïöèè êîíôèãóðàöèè âðåìåíè âûïîëíåíèÿ<link linkend="ini.doc-root">doc_root</link> è <link linkend="ini.user-dir">user_dir</link> ìîãóò áûòü èñïîëüçîâàíû äëÿ ïðåäîòâðàùåíèÿ ïîäîáíûõ àòàê, åñëè äåðåâî äîêóìåíòîâ ñåðâåðà èìååò êàòàëîãè ñ îãðàíè÷åíèåì äîñòóïà. Äàëåå ïðèâîäÿòñÿ ïîäðîáíûå ïîÿñíåíèÿ ê ðàçëè÷íûì êîìáèíàöèÿì îïöèé. </simpara> </listitem> </itemizedlist> </sect2> <sect2 id="security.cgi-bin.default"> <title>Âàðèàíò 1: ïðåäîñòàâëåíèå òîëüêî îáùåãî ñîäåðæèìîãî</title> <simpara> Åñëè âàø ñåðâåð íå ñîäåðæèò íè÷åãî çàùèùåííîãî ïàðîëåì èëè ñïèñêîì IP-àäðåñîâ, òî â âûøåïåðå÷èñëåííûõ îïöèÿõ íåò íåîáõîäèìîñòè. Åñëè âàø Web-ñåðâåð íå äîïóñêàåò ïåðåíàïðàâëåíèé, èëè åñëè ñåðâåð íå èìååò âîçìîæíîñòè áåçîïàñíî ïåðåíàïðàâèòü çàïðîñ ïðèëîæåíèþ PHP, âû ìîæåòå óñòàíîâèòü îïöèþ <link linkend="install.configure.enable-force-cgi-redirect">--enable-force-cgi-redirect</link> êîíôèãóðèðóþùåé ïðîãðàììå. Îäíàêî, âàì âñå ðàâíî ïðèäåòñÿ ñëåäèòü çà òåì, ÷òîáû âàøè ïðîãðàììû íå çàâèñåëè îò ìåòîäà èõ âûçîâà, êàê ïðÿìîãî <filename role="php">http://my.host/cgi-bin/php/dir/script.php</filename> òàê è ñ èñïîëüçîâàíèåì ïåðåíàïðàâëåíèÿ <filename role="php">http://my.host/dir/script.php</filename>. </simpara> <simpara> Ïåðåíàïðàâëåíèå ìîæåò áûòü íàñòðîåíî â ñåðâåðå Apache ñ èñïîëüçîâàíèåì äèðåêòèâ AddHandler è Action directives (ñì. íèæå). </simpara> </sect2> <sect2 id="security.cgi-bin.force-redirect"> <title>Âàðèàíò 2: èñïîëüçîâàíèå --enable-force-cgi-redirect</title> <simpara> Ýòà îïöèÿ êîìïèëÿöèè íå äàåò íèêîìó èñïîëüçîâàòü PHP íàïðÿìóþ, ñ ïðèìåíåíèåì URL âðîäå <filename role="php">http://my.host/cgi-bin/php/secretdir/script.php</filename>. PHP áóäåò òðàíñëèðîâàòü äàííûå òîëüêî â òîì ñëó÷àå, åñëè îíè óæå ïðîøëè ÷åðåç ïåðåíàïðàâëåíèå Web-ñåðâåðà. </simpara> <simpara> Îáû÷íî ïåðåíàïðàâëåíèå â ñåðâåðå Apache êîíôèãóðèðóåòñÿ ñ èñïîëüçîâàíèåì ñëåäóþùèõ êîìàíä: </simpara> <programlisting role="apache-conf"> <![CDATA[ Action php-script /cgi-bin/php AddHandler php-script .php ]]> </programlisting> <simpara> Ýòà îïöèÿ áûëà ïðîâåðåíà òîëüêî äëÿ Web-ñåðâåðà Apache è îñíîâûâàåòñÿ íà óñòàíîâêå èì íå-CGI-ñòàíäàðòíîé ïåðåìåííîé îêðóæåíèÿ <envar>REDIRECT_STATUS</envar> äëÿ ïåðåíàïðàâëåííûõ çàïðîñîâ. Åñëè âàø Web-ñåðâåð íå ïîääåðæèâàåò íèêàêèõ ïóòåé óêàçàíèÿ òîãî, îñóùåñòâëÿëîñü ëè ïåðåíàïðàâëåíèå çàïðîñà, âû íå ìîæåòå èñïîëüçîâàòü ýòó îïöèþ è äîëæíû èñïîëüçîâàòü äðóãèå âàðèàíòû èñïîëüçîâàíèÿ ïðèëîæåíèÿ CGI, îïèñàííûå íèæå. </simpara> </sect2> <sect2 id="security.cgi-bin.doc-root"> <title>Âàðèàíò 3: óñòàíîâêà doc_root èëè user_dir</title> <simpara> To include active content, like scripts and executables, in the web server document directories is sometimes consider an insecure practice. If, because of some configuration mistake, the scripts are not executed but displayed as regular HTML documents, this may result in leakage of intellectual property or security information like passwords. Therefore many sysadmins will prefer setting up another directory structure for scripts that are accessible only through the PHP CGI, and therefore always interpreted and not displayed as such. </simpara> <simpara> Also if the method for making sure the requests are not redirected, as described in the previous section, is not available, it is necessary to set up a script doc_root that is different from web document root. </simpara> <simpara> You can set the PHP script document root by the configuration directive <link linkend="ini.doc-root">doc_root</link> in the <link linkend="configuration.file">configuration file</link>, or you can set the environment variable <envar>PHP_DOCUMENT_ROOT</envar>. If it is set, the CGI version of PHP will always construct the file name to open with this <parameter>doc_root</parameter> and the path information in the request, so you can be sure no script is executed outside this directory (except for <parameter>user_dir</parameter> below). </simpara> <simpara> Another option usable here is <link linkend="ini.user-dir">user_dir</link>. When user_dir is unset, only thing controlling the opened file name is <parameter>doc_root</parameter>. Opening an url like <filename role="url">http://my.host/~user/doc.php</filename> does not result in opening a file under users home directory, but a file called <filename role="uri">~user/doc.php</filename> under doc_root (yes, a directory name starting with a tilde [<literal>~</literal>]). </simpara> <simpara> If user_dir is set to for example <filename role="dir">public_php</filename>, a request like <filename role="url">http://my.host/~user/doc.php</filename> will open a file called <filename>doc.php</filename> under the directory named <filename role="dir">public_php</filename> under the home directory of the user. If the home of the user is <filename role="dir">/home/user</filename>, the file executed is <filename>/home/user/public_php/doc.php</filename>. </simpara> <simpara> <parameter>user_dir</parameter> expansion happens regardless of the <parameter>doc_root</parameter> setting, so you can control the document root and user directory access separately. </simpara> </sect2> <sect2 id="security.cgi-bin.shell"> <title>Case 4: PHP parser outside of web tree</title> <para> A very secure option is to put the PHP parser binary somewhere outside of the web tree of files. In <filename role="dir">/usr/local/bin</filename>, for example. The only real downside to this option is that you will now have to put a line similar to: <informalexample> <programlisting> <![CDATA[ #!/usr/local/bin/php ]]> </programlisting> </informalexample> as the first line of any file containing PHP tags. You will also need to make the file executable. That is, treat it exactly as you would treat any other CGI script written in Perl or sh or any other common scripting language which uses the <literal>#!</literal> shell-escape mechanism for launching itself. </para> <para> To get PHP to handle <envar>PATH_INFO</envar> and <envar>PATH_TRANSLATED</envar> information correctly with this setup, the php parser should be compiled with the <link linkend="install.configure.enable-discard-path">--enable-discard-path</link> configure option. </para> </sect2> </sect1> <sect1 id="security.apache"> <title>Installed as an Apache module</title> <simpara> When PHP is used as an Apache module it inherits Apache's user permissions (typically those of the "nobody" user). This has several impacts on security and authorization. For example, if you are using PHP to access a database, unless that database has built-in access control, you will have to make the database accessable to the "nobody" user. This means a malicious script could access and modify the database, even without a username and password. It's entirely possible that a web spider could stumble across a database administrator's web page, and drop all of your databases. You can protect against this with Apache authorization, or you can design your own access model using LDAP, .htaccess files, etc. and include that code as part of your PHP scripts. </simpara> <simpara> Often, once security is established to the point where the PHP user (in this case, the apache user) has very little risk attached to it, it is discovered that PHP is now prevented from writing any files to user directories. Or perhaps it has been prevented from accessing or changing databases. It has equally been secured from writing good and bad files, or entering good and bad database transactions. </simpara> <simpara> A frequent security mistake made at this point is to allow apache root permissions, or to escalate apache's abilitites in some other way. </simpara> <simpara> Escalating the Apache user's permissions to root is extremely dangerous and may compromise the entire system, so sudo'ing, chroot'ing, or otherwise running as root should not be considered by those who are not security professionals. </simpara> <simpara> There are some simpler solutions. By using <link linkend="ini.open-basedir">open_basedir</link> you can control and restrict what directories are allowed to be used for PHP. You can also set up apache-only areas, to restrict all web based activity to non-user, or non-system, files. </simpara> </sect1> <sect1 id="security.filesystem"> <title>Filesystem Security</title> <simpara> PHP is subject to the security built into most server systems with respect to permissions on a file and directory basis. This allows you to control which files in the filesystem may be read. Care should be taken with any files which are world readable to ensure that they are safe for reading by all users who have access to that filesystem. </simpara> <simpara> Since PHP was designed to allow user level access to the filesystem, it's entirely possible to write a PHP script that will allow you to read system files such as /etc/passwd, modify your ethernet connections, send massive printer jobs out, etc. This has some obvious implications, in that you need to ensure that the files that you read from and write to are the appropriate ones. </simpara> <simpara> Consider the following script, where a user indicates that they'd like to delete a file in their home directory. This assumes a situation where a PHP web interface is regularly used for file management, so the Apache user is allowed to delete files in the user home directories. </simpara> <para> <example> <title>Poor variable checking leads to....</title> <programlisting role="php"> <![CDATA[ <?php // remove a file from the user's home directory $username = $_POST['user_submitted_name']; $homedir = "/home/$username"; $file_to_delete = "$userfile"; unlink ($homedir/$userfile); echo "$file_to_delete has been deleted!"; ?> ]]> </programlisting> </example> Since the username is postable from a user form, they can submit a username and file belonging to someone else, and delete files. In this case, you'd want to use some other form of authentication. Consider what could happen if the variables submitted were "../etc/" and "passwd". The code would then effectively read: <example> <title>... A filesystem attack</title> <programlisting role="php"> <![CDATA[ <?php // removes a file from anywhere on the hard drive that // the PHP user has access to. If PHP has root access: $username = "../etc/"; $homedir = "/home/../etc/"; $file_to_delete = "passwd"; unlink ("/home/../etc/passwd"); echo "/home/../etc/passwd has been deleted!"; ?> ]]> </programlisting> </example> There are two important measures you should take to prevent these issues. <itemizedlist> <listitem> <simpara> Only allow limited permissions to the PHP web user binary. </simpara> </listitem> <listitem> <simpara> Check all variables which are submitted. </simpara> </listitem> </itemizedlist> Here is an improved script: <example> <title>More secure file name checking</title> <programlisting role="php"> <![CDATA[ <?php // removes a file from the hard drive that // the PHP user has access to. $username = $_SERVER['REMOTE_USER']; // using an authentication mechanisim $homedir = "/home/$username"; $file_to_delete = basename("$userfile"); // strip paths unlink ($homedir/$file_to_delete); $fp = fopen("/home/logging/filedelete.log","+a"); //log the deletion $logstring = "$username $homedir $file_to_delete"; fputs ($fp, $logstring); fclose($fp); echo "$file_to_delete has been deleted!"; ?> ]]> </programlisting> </example> However, even this is not without it's flaws. If your authentication system allowed users to create their own user logins, and a user chose the login "../etc/", the system is once again exposed. For this reason, you may prefer to write a more customized check: <example> <title>More secure file name checking</title> <programlisting role="php"> <![CDATA[ <?php $username = $_SERVER['REMOTE_USER']; // using an authentication mechanisim $homedir = "/home/$username"; if (!ereg('^[^./][^/]*$', $userfile)) die('bad filename'); //die, do not process if (!ereg('^[^./][^/]*$', $username)) die('bad username'); //die, do not process //etc... ?> ]]> </programlisting> </example> </para> <para> Depending on your operating system, there are a wide variety of files which you should be concerned about, including device entries (/dev/ or COM1), configuration files (/etc/ files and the .ini files), well known file storage areas (/home/, My Documents), etc. For this reason, it's usually easier to create a policy where you forbid everything except for what you explicitly allow. </para> </sect1> <sect1 id="security.database"> <title>Database Security</title> <simpara> Nowadays, databases are cardinal components of any web based application by enabling websites to provide varying dynamic content. Since very sensitive or secret informations can be stored in such database, you should strongly consider to protect them somehow. </simpara> <simpara> To retrieve or to store any information you need to connect to the database, send a legitimate query, fetch the result, and close the connecion. Nowadays, the commonly used query language in this interaction is the Structured Query Language (SQL). See how an attacker can <link linkend="security.database.sql-injection">tamper with an SQL query</link>. </simpara> <simpara> As you can realize, PHP cannot protect your database by itself. The following sections aim to be an introduction into the very basics of how to access and manipulate databases within PHP scripts. </simpara> <simpara> Keep in mind this simple rule: defence in depth. In the more place you take the more action to increase the protection of your database, the less probability of that an attacker succeeds, and exposes or abuse any stored secret information. Good design of the database schema and the application deals with your greatest fears. </simpara> <sect2 id="security.database.design"> <title>Designing Databases</title> <simpara> The first step is always to create the database, unless you want to use an existing third party's one. When a database is created, it is assigned to an owner, who executed the creation statement. Usually, only the owner (or a superuser) can do anything with the objects in that database, and in order to allow other users to use it, privileges must be granted. </simpara> <simpara> Applications should never connect to the database as its owner or a superuser, because these users can execute any query at will, for example, modifying the schema (e.g. dropping tables) or deleting its entire content. </simpara> <simpara> You may create different database users for every aspect of your application with very limited rights to database objects. The most required privileges should be granted only, and avoid that the same user can interact with the database in different use cases. This means that if intruders gain access to your database using one of these credentials, they can only effect as many changes as your application can. </simpara> <simpara> You are encouraged not to implement all the business logic in the web application (i.e. your script), instead to do it in the database schema using views, triggers or rules. If the system evolves, new ports will be intended to open to the database, and you have to reimplement the logic in each separate database client. Over and above, triggers can be used to transparently and automatically handle fields, which often provides insight when debugging problems with your application or tracing back transactions. </simpara> </sect2> <sect2 id="security.database.connection"> <title>Connecting to Database</title> <simpara> You may want to estabilish the connections over SSL to encrypt client/server communications for increased security, or you can use ssh to encrypt the network connection between clients and the database server. If either of them is done, then monitoring your traffic and gaining informations in this way will be a hard work. </simpara> <!--simpara> If your database server native SSL support, consider to use <link linkend="ref.openssl">OpenSSL functions</link> in communication between PHP and database via SSL. </simpara--> </sect2> <sect2 id="security.database.storage"> <title>Encrypted Storage Model</title> <simpara> SSL/SSH protects data travelling from the client to the server, SSL/SSH does not protect the persistent data stored in a database. SSL is an on-the-wire protocol. </simpara> <simpara> Once an attacker gains access to your database directly (bypassing the webserver), the stored sensitive data may be exposed or misused, unless the information is protected by the database itself. Encrypting the data is a good way to mitigate this threat, but very few databases offer this type of data encryption. </simpara> <simpara> The easiest way to work around this problem is to first create your own encryption package, and then use it from within your PHP scripts. PHP can assist you in this case with its several extensions, such as <link linkend="ref.mcrypt">Mcrypt</link> and <link linkend="ref.mhash">Mhash</link>, covering a wide variety of encryption algorithms. The script encrypts the data be stored first, and decrypts it when retrieving. See the references for further examples how encryption works. </simpara> <simpara> In case of truly hidden data, if its raw representation is not needed (i.e. not be displayed), hashing may be also taken into consideration. The well-known example for the hashing is storing the MD5 hash of a password in a database, instead of the password itself. See also <function>crypt</function> and <function>md5</function>. </simpara> <example> <title>Using hashed password field</title> <programlisting role="php"> <![CDATA[ // storing password hash $query = sprintf("INSERT INTO users(name,pwd) VALUES('%s','%s');", addslashes($username), md5($password)); $result = pg_exec($connection, $query); // querying if user submitted the right password $query = sprintf("SELECT 1 FROM users WHERE name='%s' AND pwd='%s';", addslashes($username), md5($password)); $result = pg_exec($connection, $query); if (pg_numrows($result) > 0) { echo "Welcome, $username!"; } else { echo "Authentication failed for $username."; } ]]> </programlisting> </example> </sect2> <sect2 id="security.database.sql-injection"> <title>SQL Injection</title> <simpara> Many web developers are unaware of how SQL queries can be tampered with, and assume that an SQL query is a trusted command. It means that SQL queries are able to circumvent access controls, thereby bypassing standard authentication and authorization checks, and sometimes SQL queries even may allow access to host operating system level commands. </simpara> <simpara> Direct SQL Command Injection is a technique where an attacker creates or alters existing SQL commands to expose hidden data, or to override valuable ones, or even to execute dangerous system level commands on the database host. This is accomplished by the application taking user input and combining it with static parameters to build a SQL query. The following examples are based on true stories, unfortunately. </simpara> <para> Owing to the lack of input validation and connecting to the database on behalf of a superuser or the one who can create users, the attacker may create a superuser in your database. <example> <title> Splitting the result set into pages ... and making superusers (PostgreSQL and MySQL) </title> <programlisting role="php"> <![CDATA[ $offset = argv[0]; // beware, no input validation! $query = "SELECT id, name FROM products ORDER BY name LIMIT 20 OFFSET $offset;"; // with PostgreSQL $result = pg_exec($conn, $query); // with MySQL $result = mysql_query($query); ]]> </programlisting> </example> Normal users click on the 'next', 'prev' links where the <varname>$offset</varname> is encoded into the URL. The script expects that the incoming <varname>$offset</varname> is decimal number. However, someone tries to break in with appending <function>urlencode</function>'d form of the following to the URL <informalexample> <programlisting> <![CDATA[ // in case of PostgreSQL 0; insert into pg_shadow(usename,usesysid,usesuper,usecatupd,passwd) select 'crack', usesysid, 't','t','crack' from pg_shadow where usename='postgres'; -- // in case of MySQL 0; UPDATE user SET Password=PASSWORD('crack') WHERE user='root'; FLUSH PRIVILEGES; ]]> </programlisting> </informalexample> If it happened, then the script would present a superuser access to him. Note that <literal>0;</literal> is to supply a valid offset to the original query and to terminate it. </para> <note> <para> It is common technique to force the SQL parser to ignore the rest of the query written by the developer with <literal>--</literal> which is the comment sign in SQL. </para> </note> <para> A feasible way to gain passwords is to circumvent your search result pages. What the attacker needs only is to try if there is any submitted variable used in SQL statement which is not handled properly. These filters can be set commonly in a preceding form to customize <literal>WHERE, ORDER BY, LIMIT</literal> and <literal>OFFSET</literal> clauses in <literal>SELECT</literal> statements. If your database supports the <literal>UNION</literal> construct, the attacker may try to append an entire query to the original one to list passwords from an arbitrary table. Using encrypted password fields is strongly encouraged. <example> <title> Listing out articles ... and some passwords (any database server) </title> <programlisting role="php"> <![CDATA[ $query = "SELECT id, name, inserted, size FROM products WHERE size = '$size' ORDER BY $order LIMIT $limit, $offset;"; $result = odbc_exec($conn, $query); ]]> </programlisting> </example> The static part of the query can be combined with another <literal>SELECT</literal> statement which reveals all passwords: <informalexample> <programlisting> <![CDATA[ ' union select '1', concat(uname||'-'||passwd) as name, '1971-01-01', '0' from usertable; -- ]]> </programlisting> </informalexample> If this query (playing with the <literal>'</literal> and <literal>--</literal>) were assigned to one of the variables used in <varname>$query</varname>, the query beast awakened. </para> <para> SQL UPDATEs are also subject to attacking your database. These queries are also threatened by chopping and appending an entirely new query to it. But the attacker might fiddle with the <literal>SET</literal> clause. In this case some schema information must be possessed to manipulate the query successfully. This can be acquired by examing the form variable names, or just simply brute forcing. There are not so many naming convention for fields storing passwords or usernames. <example> <title> From resetting a password ... to gaining more privileges (any database server) </title> <programlisting role="php"> <![CDATA[ $query = "UPDATE usertable SET pwd='$pwd' WHERE uid='$uid';"; ]]> </programlisting> </example> But a malicious user sumbits the value <literal>' or uid like'%admin%'; --</literal> to <varname>$uid</varname> to change the admin's password, or simply sets <varname>$pwd</varname> to <literal>"hehehe', admin='yes', trusted=100 "</literal> (with a trailing space) to gain more privileges. Then, the query will be twisted: <informalexample> <programlisting role="php"> <![CDATA[ // $uid == ' or uid like'%admin%'; -- $query = "UPDATE usertable SET pwd='...' WHERE uid='' or uid like '%admin%'; --"; // $pwd == "hehehe', admin='yes', trusted=100 " $query = "UPDATE usertable SET pwd='hehehe', admin='yes', trusted=100 WHERE ...;" ]]> </programlisting> </informalexample> </para> <para> A frightening example how operating system level commands can be accessed on some database hosts. <example> <title>Attacking the database host's operating system (MSSQL Server)</title> <programlisting role="php"> <![CDATA[ $query = "SELECT * FROM products WHERE id LIKE '%$prod%'"; $result = mssql_query($query); ]]> </programlisting> </example> If attacker submits the value <literal>a%' exec master..xp_cmdshell 'net user test testpass /ADD' --</literal> to <varname>$prod</varname>, then the <varname>$query</varname> will be: <informalexample> <programlisting role="php"> <![CDATA[ $query = "SELECT * FROM products WHERE id LIKE '%a%' exec master..xp_cmdshell 'net user test testpass /ADD'--"; $result = mssql_query($query); ]]> </programlisting> </informalexample> MSSQL Server executes the SQL statements in the batch including a command to add a new user to the local accounts database. If this application were running as <literal>sa</literal> and the MSSQLSERVER service is running with sufficient privileges, the attacker would now have an account with which to access this machine. </para> <note> <para> Some of the examples above is tied to a specific database server. This does not mean that a similar attack is impossible against other products. Your database server may be so vulnerable in other manner. </para> </note> <sect3 id="security.database.avoiding"> <title>Avoiding techniques</title> <simpara> You may plead that the attacker must possess a piece of information about the database schema in most examples. You are right, but you never know when and how it can be taken out, and if it happens, your database may be exposed. If you are using an open source, or publicly available database handling package, which may belong to a content management system or forum, the intruders easily produce a copy of a piece of your code. It may be also a security risk if it is a poorly designed one. </simpara> <simpara> These attacks are mainly based on exploiting the code not being written with security in mind. Never trust on any kind of input, especially which comes from the client side, even though it comes from a select box, a hidden input field or a cookie. The first example shows that such a blameless query can cause disasters. </simpara> <itemizedlist> <listitem> <simpara> Never connect to the database as a superuser or as the database owner. Use always customized users with very limited privileges. </simpara> </listitem> <listitem> <simpara> Check if the given input has the expected data type. PHP has a wide range of input validating functions, from the simplest ones found in <link linkend="ref.variables">Variable Functions</link> and in <link linkend="ref.ctype">Character Type Functions</link> (e.g. <function>is_numeric</function>, <function>ctype_digit</function> respectively) onwards the <link linkend="ref.pcre">Perl compatible Regular Expressions</link> support. </simpara> </listitem> <listitem> <para> If the application waits for numerical input, consider to verify data with <function>is_numeric</function>, or silently change its type using <function>settype</function>, or use its numeric representation by <function>sprintf</function>. <example> <title>A more secure way to compose a query for paging</title> <programlisting role="php"> <![CDATA[ settype($offset, 'integer'); $query = "SELECT id, name FROM products ORDER BY name LIMIT 20 OFFSET $offset;"; // please note %d in the format string, using %s would be meaningless $query = sprintf("SELECT id, name FROM products ORDER BY name LIMIT 20 OFFSET %d;", $offset); ]]> </programlisting> </example> </para> </listitem> <listitem> <simpara> Quote each non numeric user input which is passed to the database with <function>addslashes</function> or <function>addcslashes</function>. See <link linkend="security.database.storage">the first example</link>. As the examples shows, quotes burnt into the static part of the query is not enough, and can be easily hacked. </simpara> </listitem> <listitem> <simpara> Do not print out any database specific information, especially about the schema, by fair means or foul. See also <link linkend="security.errors">Error Reporting</link> and <link linkend="ref.errorfunc">Error Handling and Logging Functions</link>. </simpara> </listitem> <listitem> <simpara> You may use stored procedures and previously defined cursors to abstract data access so that users do not directly access tables or views, but this solution has another impacts. </simpara> </listitem> </itemizedlist> <simpara> Besides these, you benefit from logging queries either within your script or by the database itself, if it supports. Obviously, the logging is unable to prevent any harmful attempt, but it can be helpful to trace back which application has been circumvented. The log is not useful by itself, but through the information it contains. The more detail is generally better. </simpara> </sect3> </sect2> </sect1> <sect1 id="security.errors"> <title>Error Reporting</title> <para> With PHP security, there are two sides to error reporting. One is beneficial to increasing security, the other is detrimental. </para> <para> A standard attack tactic involves profiling a system by feeding it improper data, and checking for the kinds, and contexts, of the errors which are returned. This allows the system cracker to probe for information about the server, to determine possible weaknesses. For example, if an attacker had gleaned information about a page based on a prior form submission, they may attempt to override variables, or modify them: <example> <title>Attacking Variables with a custom HTML page</title> <programlisting role="php"> <![CDATA[ <form method="post" action="attacktarget?username=badfoo&password=badfoo"> <input type="hidden" name="username" value="badfoo"> <input type="hidden" name="password" value="badfoo"> </form> ]]> </programlisting> </example> </para> <para> The PHP errors which are normally returned can be quite helpful to a developer who is trying to debug a script, indicating such things as the function or file that failed, the PHP file it failed in, and the line number which the failure occured in. This is all information that can be exploited. It is not uncommon for a php developer to use <function>show_source</function>, <function>highlight_string</function>, or <function>highlight_file</function> as a debugging measure, but in a live site, this can expose hidden variables, unchecked syntax, and other dangerous information. Especially dangerous is running code from known sources with built-in debugging handlers, or using common debugging techniques. If the attacker can determine what general technique you are using, they may try to brute-force a page, by sending various common debugging strings: <example> <title>Exploiting common debugging variables</title> <programlisting role="php"> <![CDATA[ <form method="post" action="attacktarget?errors=Y&showerrors=1"&debug=1"> <input type="hidden" name="errors" value="Y"> <input type="hidden" name="showerrors" value="1"> <input type="hidden" name="debug" value="1"> </form> ]]> </programlisting> </example> </para> <para> Regardless of the method of error handling, the ability to probe a system for errors leads to providing an attacker with more information. </para> <para> For example, the very style of a generic PHP error indicates a system is running PHP. If the attacker was looking at an .html page, and wanted to probe for the back-end (to look for known weaknesses in the system), by feeding it the wrong data they may be able to determine that a system was built with PHP. </para> <para> A function error can indicate whether a system may be running a specific database engine, or give clues as to how a web page or programmed or designed. This allows for deeper investigation into open database ports, or to look for specific bugs or weaknesses in a web page. By feeding different pieces of bad data, for example, an attacker can determine the order of authentication in a script, (from the line number errors) as well as probe for exploits that may be exploited in different locations in the script. </para> <para> A filesystem or general PHP error can indicate what permissions the webserver has, as well as the structure and organization of files on the web server. Developer written error code can aggravate this problem, leading to easy exploitation of formerly "hidden" information. </para> <para> There are three major solutions to this issue. The first is to scrutinize all functions, and attempt to compensate for the bulk of the errors. The second is to disable error reporting entirely on the running code. The third is to use PHP's custom error handling functions to create your own error handler. Depending on your security policy, you may find all three to be applicable to your situation. </para> <para> One way of catching this issue ahead of time is to make use of PHP's own <function>error_reporting</function>, to help you secure your code and find variable usage that may be dangerous. By testing your code, prior to deployment, with E_ALL, you can quickly find areas where your variables may be open to poisoning or modification in other ways. Once you are ready for deployment, by using E_NONE, you insulate your code from probing. <example> <title>Finding dangerous variables with E_ALL</title> <programlisting role="php"> <![CDATA[ <?php if ($username) { // Not initialized or checked before usage $good_login = 1; } if ($good_login == 1) { // If above test fails, not initialized or checked before usage fpassthru ("/highly/sensitive/data/index.html"); } ?> ]]> </programlisting> </example> </para> </sect1> <sect1 id="security.registerglobals"> <title>Using Register Globals</title> <para> One feature of PHP that can be used to enhance security is configuring PHP with <link linkend="ini.register-globals">register_globals</link> = off. By turning off the ability for any user-submitted variable to be injected into PHP code, you can reduce the amount of variable poisoning a potential attacker may inflict. They will have to take the additional time to forge submissions, and your internal variables are effectively isolated from user submitted data. </para> <para> While it does slightly increase the amount of effort required to work with PHP, it has been argued that the benefits far outweigh the effort. <example> <title>Working with register_globals=on</title> <programlisting role="php"> <![CDATA[ <?php if ($username) { // can be forged by a user in get/post/cookies $good_login = 1; } if ($good_login == 1) { // can be forged by a user in get/post/cookies, fpassthru ("/highly/sensitive/data/index.html"); } ?> ]]> </programlisting> </example> <example> <title>Working with register_globals = off</title> <programlisting role="php"> <![CDATA[ <?php if($_COOKIE['username']){ // can only come from a cookie, forged or otherwise $good_login = 1; fpassthru ("/highly/sensitive/data/index.html"); } ?> ]]> </programlisting> </example> By using this wisely, it's even possible to take preventative measures to warn when forging is being attempted. If you know ahead of time exactly where a variable should be coming from, you can check to see if submitted data is coming from an inappropriate kind of submission. While it doesn't guarantee that data has not been forged, it does require an attacker to guess the right kind of forging. <example> <title>Detecting simple variable poisoning</title> <programlisting role="php"> <![CDATA[ <?php if ($_COOKIE['username'] && !$_POST['username'] && !$_GET['username'] ) { // Perform other checks to validate the user name... $good_login = 1; fpassthru ("/highly/sensitive/data/index.html"); } else { mail("admin@example.com", "Possible breakin attempt", $_SERVER['REMOTE_ADDR']); echo "Security violation, admin has been alerted."; exit; } ?> ]]> </programlisting> </example> Of course, simply turning off register_globals does not mean code is secure. For every piece of data that is submitted, it should also be checked in other ways. </para> </sect1> <sect1 id="security.variables"> <title>User Submitted Data</title> <para> The greatest weakness in many PHP programs is not inherent in the language itself, but merely an issue of code not being written with security in mind. For this reason, you should always take the time to consider the implications of a given piece of code, to ascertain the possible damage if an unexpected variable is submitted to it. <example> <title>Dangerous Variable Usage</title> <programlisting role="php"> <![CDATA[ <?php // remove a file from the user's home directory... or maybe // somebody else's? unlink ($evil_var); // Write logging of their access... or maybe an /etc/passwd entry? fputs ($fp, $evil_var); // Execute something trivial.. or rm -rf *? system ($evil_var); exec ($evil_var); ?> ]]> </programlisting> </example> You should always carefully examine your code to make sure that any variables being submitted from a web browser are being properly checked, and ask yourself the following questions: <itemizedlist> <listitem> <simpara> Will this script only affect the intended files? </simpara> </listitem> <listitem> <simpara> Can unusual or undesirable data be acted upon? </simpara> </listitem> <listitem> <simpara> Can this script be used in unintended ways? </simpara> </listitem> <listitem> <simpara> Can this be used in conjunction with other scripts in a negative manner? </simpara> </listitem> <listitem> <simpara> Will any transactions be adequately logged? </simpara> </listitem> </itemizedlist> By adequately asking these questions while writing the script, rather than later, you prevent an unfortunate re-write when you need to increase your security. By starting out with this mindset, you won't guarantee the security of your system, but you can help improve it. </para> <para> You may also want to consider turning off register_globals, magic_quotes, or other convenience settings which may confuse you as to the validity, source, or value of a given variable. Working with PHP in error_reporting(E_ALL) mode can also help warn you about variables being used before they are checked or initialized (so you can prevent unusual data from being operated upon). </para> </sect1> <sect1 id="security.hiding"> <title>Hiding PHP</title> <para> In general, security by obscurity is one of the weakest forms of security. But in some cases, every little bit of extra security is desirable. </para> <para> A few simple techniques can help to hide PHP, possibly slowing down an attacker who is attempting to discover weaknesses in your system. By setting expose_php = off in your &php.ini; file, you reduce the amount of information available to them. </para> <para> Another tactic is to configure web servers such as apache to parse different filetypes through PHP, either with an .htaccess directive, or in the apache configuration file itself. You can then use misleading file extensions: <example> <title>Hiding PHP as another language</title> <programlisting role="apache-conf"> <![CDATA[ # Make PHP code look like other code types AddType application/x-httpd-php .asp .py .pl ]]> </programlisting> </example> Or obscure it completely: <example> <title>Using unknown types for PHP extensions</title> <programlisting role="apache-conf"> <![CDATA[ # Make PHP code look like unknown types AddType application/x-httpd-php .bop .foo .133t ]]> </programlisting> </example> Or hide it as html code, which has a slight performance hit because all html will be parsed through the PHP engine: <example> <title>Using html types for PHP extensions</title> <programlisting role="apache-conf"> <![CDATA[ # Make all PHP code look like html AddType application/x-httpd-php .htm .html ]]> </programlisting> </example> For this to work effectively, you must rename your PHP files with the above extensions. While it is a form of security through obscurity, it's a minor preventative measure with few drawbacks. </para> </sect1> <sect1 id="security.current"> <title>Keeping Current</title> <simpara> PHP, like any other large system, is under constant scrutiny and improvement. Each new version will often include both major and minor changes to enhance and repair security flaws, configuration mishaps, and other issues that will affect the overall security and stability of your system. </simpara> <simpara> Like other system-level scripting languages and programs, the best approach is to update often, and maintain awareness of the latest versions and their changes. </simpara> </sect1> </chapter> <!-- Keep this comment at the end of the file Local variables: mode: sgml sgml-omittag:t sgml-shorttag:t sgml-minimize-attributes:nil sgml-always-quote-attributes:t sgml-indent-step:1 sgml-indent-data:t indent-tabs-mode:nil sgml-parent-document:nil sgml-default-dtd-file:"../../manual.ced" sgml-exposed-tags:nil sgml-local-catalogs:nil sgml-local-ecat-files:nil End: vim600: syn=xml fen fdm=syntax fdl=2 si vim: et tw=78 syn=sgml vi: ts=1 sw=1 -->
Index: phpdoc-ru/chapters/security.xml +++ phpdoc-ru/chapters/security.xml <?xml version="1.0" encoding="iso-8859-1"?> <!-- EN-Revision: 1.49 Maintainer: alexws Status: in-progress --> <!-- $Revision: 1.1 $ --> <chapter id="security"> <title>Áåçîïàñíîñòü</title> <simpara> PHP - ýòî ìîùíûé ÿçûê ïðîãðàììèðîâàíèÿ è èíòåðïðåòàòîð. Áóäó÷è èíòåãðèðîâàííûì â âèäå ìîäóëÿ â Web-ñåðâåð, èëè âûïîëíÿÿñü êàê îääåëüíîå <acronym>CGI</acronym>-ïðèëîæåíèå, îí ìîæåò îáðàùàòüñÿ ê ôàéëàì, èñïîëíÿòü êîìàíäû è îòêðûâàòü ñåòåâûå ñîåäèíåíèÿ íà ñåðâåðå. Ýòè îñîáåííîñòè äåëàþò âñå, ÷òî âûïîëíÿåòñÿ íà Web-ñåðâåðå, íåáåçîïàñíûì. PHP ñïåöèàëüíî ðàçðàáàòûâàëñÿ äëÿ òîãî, ÷òîáû áûòü áîëåå áåçîïàñíûì ÿçûêîì äëÿ íàïèñàíèÿ CGI-ñîâìåñòèìûõ ïðîãðàìì, íåæåëè Perl èëè Ñ, è, åñëè ïðàâèëüíî âûñòàâëåíû íàñòðîéêè êîìïèëÿöèè è âûïîëíåíèÿ, à òàêæå ïðè íàëè÷èè äîñòàòî÷íîãî îïûòà ïðîãðàììèðîâàíèÿ, PHP äàñò âàì íåîáõîäèìîå ñîîòíîøåíèå ãèáêîñòè è áåçîïàñíîñòè. </simpara> <simpara> Ïîñêîëüêó ñóùåñòâóåò ìíîæåñòâî ðàçëè÷íûõ âàðèàíòîâ èñïîëüçîâàíèÿ PHP, ñóùåñòâóåò î÷åíü ìíîãî íàñòðîåê, êîíòðîëèðóþùèõ åãî ïîâåäåíèå. Áîëüøîé âûáîð îïöèé îçíà÷àåò, ÷òî âû ñìîæåòå èñïîëüçîâàòü PHP äëÿ ìíîãèõ öåëåé, îäíàêî òàêæå çíà÷èò, ÷òî åñòü òàêèå êîìáèíàöèè ýòèõ îïöèé è íàñòðîåê ñåðâåðà, êîòîðûå ïðèâåäóò ê ïðîáëåìàì áåçîïàñíîñòè. </simpara> <simpara> Ãèáêîñòü íàñòðîéêè PHP êîíêóðèðóåò ñ ãèáêîñòüþ ïðîãðàììèðîâàíèÿ. PHP äëÿ ñîçäàíèÿ ñåðâåðíûõ ïðèëîæåíèé ñî âñåìè âîçìîæíîñòÿìè ðÿäîâîãî ïîëüçîâàòåëÿ ñèñòåìû, èëè äëÿ ïðîñòîãî âêëþ÷åíèÿ ôàéëîâ íà ñòîðîíå ñåðâåðà, êîòîðîå íå ïðåäñòàâëÿåò îïàñíîñòè â õîðîøî êîíòðîëèðóåìîé ñèñòåìå. Òî, êàê ïîñòðîåíà ñèñòåìà, è íàñêîëüêî îíà áåçîïàñíà, ïðàêòè÷åñêè ïîëíîñòüþ çàâèñèò îò ðàçðàáîò÷èêà, èñïîëüçóþùåãî PHP. </simpara> <simpara>  íà÷àëå ãëàâû äàþòñÿ îñíîâíûå ñîâåòû ïî ðåàëèçàöèè áåçîïàñíîñòè, îáúÿñíÿþòñÿ ðàçëè÷íûå êîìáèíàöèè îïöèé íàñòðîéêè è ñèòóàöèè, â êîòîðûõ îíè ìîãóò áûòü áåçîïàñíî èñïîëüçîâàíû. Òàêæå ïðèâîäÿòñÿ ðàçëè÷íûå ñîîáðàæåíèÿ ïî ÷àñòè ïðîãðàììèðîâàíèÿ äëÿ ðàçíûõ óðîâíåé áåçîïàñíîñòè. </simpara> <sect1 id="security.general"> <title>Îñíîâíûå ïîëîæåíèÿ</title> <simpara> Ïîëíîñòüþ áåçîïàñíàÿ ñèñòåìà ïðàêòè÷åñêè íå ìîæåò ñóùåñòâîâàòü, ïîýòîìó îñíîâíîé èñïîëüçóåìûé çäåñü ïîäõîä - ïîäõîä ðàçóìíîãî ðèñêà. Ê ïðèìåðó, åñëè êàæäàÿ ïåðåìåííàÿ, ïåðåäàííàÿ ïîëüçîâàòåëåì, áóäåò òðåáîâàòü äâóõ áèîìåòðè÷åñêèõ ïîäòâåðæäåíèé (âðîäå ñêàíèðîâàíèÿ ñåò÷àòêè è îòïå÷àòêà ïàëüöà), ó âàñ áóäåò î÷åíü âûñîêèé óðîâåíü êîíòðîëÿ. Ñ äðóãîé ñòîðîíû, çàïîëíåíèå äëèííûõ è ñëîæíûõ ôîðì ìîæåò çàíèìàòü äîñòàòî÷íî äëèòåëüíîå âðåìÿ, ÷òî îáû÷íî çàñòàâëÿåò ïîëüçîâàòåëåé èñêàòü ïóòè îáõîäà ñèñòåìû áåçîïàñíîñòè. </simpara> <simpara> Íàèëó÷øèé âàðèàíò îáû÷íî ñîîòâåòñòâóåò òðåáîâàíèÿì áåçîïàñíîñòè, íî íå ìåøàåò ðàáîòå ïîëüçîâàòåëåé, à òàêæå íå ïåðåãðóæàåò ïðîãðàììíûé êîä èçëèøíåé ñëîæíîñòüþ. ×òî êàñàåòñÿ ñëîæíîñòè, òî íåêîòîðûå àòàêè êàê ðàç è ÿâëÿþòñÿ ñëåäñòâèåì ÷ðåçìåðíîãî "íàâîðà÷èâàíèÿ" çàùèòû. Êàê èçâåñòíî, ÷åì ñëîæíåå çàùèòà, òåì áîëüøå â íåé íå ñîâñåì î÷åâèäíûõ "äûð". </simpara> <simpara> Âñåãäà íåîáõîäèìî ïîìíèòü: ñèñòåìà áåçîïàñíîñòè íàñòîëüêî õîðîøà, íàñêîëüêî áåçîïàñíî åå ñàìîå ñëàáîå ìåñòî. Åñëè, ê ïðèìåðó, âåäóòñÿ îò÷åòû ïî âñåì îïåðàöèÿì, âðåìåíè, ìåñòó è ò.ä., íî ïðîâåðêà ïîëüçîâàòåëåé îñóùåñòâëÿåòñÿ ñ èñïîëüçîâàíèåì îäíîãî cookie, òî ñîîòâåòñòâèå èìåí ïîëüçîâàòåëåé â îò÷åòàõ ñ ðåàëüíûìè ïîëüçîâàòåëÿìè âåñüìà è âåñüìà ñîìíèòåëüíî. </simpara> <simpara> Ïðè îòëàäêå ïîìíèòå, ÷òî íåâîçìîæíî ïðåäóãàäàòü âñå âàðèàíòû ðàçâèòèÿ ñîáûòèé äàæå äëÿ îäíîé åäèíñòâåííîé ñòðàíè÷êè. Îæèäàåìûå âõîäíûå äàííûå ìîãóò îòëè÷àòü îò ââåäåííûõ íåäîáðîñîâåñòíûì ðàáîòíèêîì, õàêåðîì ñ ìíîãîëåòíèì ñòàæåì èëè ïðîñòî êîøêîé, ïðîøåäøåéñÿ ïî êëàâèàòóðå. Ïîýòîìó âñåãäà óäîáíåå ïîäõîäèòü ê ïðîãðàììå ñ òî÷êè çðåíèÿ ëîãèêè, ÷òîáû ïðîñëåäèòü ìåñòà âîçìîæíîãî ïîÿâëåíèÿ íåîæèäàííûõ äàííûõ, à òàêæå èõ èñïîëüçîâàíèÿ. </simpara> <simpara> Internet ïîëîí ëþäåé, êîòîðûå õîòÿò ñäåëàòü ñåáå èìÿ ïóòåì âçëîìà âàøåãî êîäà, îñòàíîâà âàøåãî ñàéòà, ðàçìåùåíèÿ íåïîäõîäÿùèõ äàííûõ, â îáùåì, âñåãî, ÷òî íå äàåò âàì æèòü ñïîêîéíî. Íåâàæíî, êàêîãî ìàñøòàáà âàø ñàéò, âû íàõîäèòåñü ïîä ïðèöåëîì ïðîñòî ïîòîìó, ÷òî âû â ñåòè, ïîòîìó, ÷òî ó âàñ åñòü ñåðâåð, ê êîòîðîìó ìîæíî ïîäêëþ÷èòüñÿ. Ìíîæåñòâî ïðîãðàìì äëÿ âçëîìà íå ðàçëè÷àþò ðàçìåðîâ, îíè ïðîñòî ïðîêà÷èâàþò îãðîìíîå êîëè÷åñòâî IP-òðàôôèêà â ïîèñêàõ æåðòâû. Ïîñòàðàéòåñü íå ñòàòü òàêîâîé. </simpara> </sect1> <sect1 id="security.cgi-bin"> <title>Ïðè èíñòàëëÿöèè â âèäå ïðèëîæåíèÿ CGI</title> <sect2 id="security.cgi-bin.attacks"> <title>Âîçìîæíûå àòàêè</title> <simpara> Èñïîëüçîâàíèå PHP â âèäå ïðèëîæåíèÿ <acronym>CGI</acronym> - âàðèàíò äëÿ òåõ, êòî ïî òåì èëè èíûì ïðè÷èíàì íå õî÷åò âñòðàèâàòü PHP â ïðèëîæåíèå ñåðâåðà (ê ïðèìåðó, Apache), â âèäå ìîäóëÿ, èëè äëÿ òåõ, òî õî÷åò èñïîëüçîâàòü PHP ñîâìåñòíî ñ CGI-îáîëî÷êàìè, ïîìîãàþùèìè ñîçäàòü áåçîïàñíóþ ñðåäó äëÿ âûïîëíåíèÿ ïðîãðàìì. Òàêàÿ íàñòðîéêà îáû÷íî âêëþ÷àåò â ñåáÿ èíñòàëëÿöèþ ïðèëîæåíèÿ PHP â äèðåêòîðèþ cgi-bin íà ñåðâåðå. Ðåêîìåíäàöèÿ CERT<ulink url="&url.cert;">CA-96.11</ulink> âûñòóïàåò ïðîòèâ ïîìåùåíèÿ èíòåðïðåòàòîðîâ â cgi-bin. Äàæå ïðè òîì, ÷òî PHP ìîæåò áûòü èñïîëüçîâàí, êàê íåçàâèñèìûé èíòåðïðåòàòîð, îí ðàçðàáàòàâûëñÿ äëÿ ïðåäîòâðàùåíèÿ àòàê, ñâÿçàííûõ ñ äàííûì âàðèàíòîì íàñòðîéêè: </simpara> <itemizedlist> <listitem> <simpara> Ïîëó÷åíèå äîñòóïà ê ñèñòåìíûì ôàéëàì: <filename role="url">http://my.host/cgi-bin/php?/etc/passwd</filename> </simpara> <simpara> Èíôîðìàöèÿ çàïðîñà â URL ïîñëå çíàêà âîïðîñà (?) ïåðåäàåòñÿ ñ ïîìîùüþ CGI èíòåðïðåòàòîðó â âèäå êîìàíäíîé ñòðîêè. Îáû÷íî èíòåðïðåòàòîðû îòêðûâàþò è âûïîëíÿþò ôàéë, çàäàííûé ïåðâûì àðãóìåíòîì â äàííîé ñòðîêå. </simpara> <simpara> Ïðè âûçîâå â ðîëè ïðèëîæåíèÿ CGI, PHP íå èíòåðïðåòèðóåò àðãóìåíòû êîìàíäíîé ñòðîêè. </simpara> </listitem> <listitem> <simpara> Ïîëó÷åíèå äîñòóïà ê ëþáîìó äîêóìåíòó íà ñåðâåðå: <filename role="url">http://my.host/cgi-bin/php/secret/doc.html</filename> </simpara> <simpara> Èíôîðìàöèîííàñÿ ÷àñòü URL ïîñëå íàçâàíèÿ ïðèëîæåíèÿ PHP <filename role="uri">/secret/doc.html</filename> îáû÷íî èñïîëüçóåòñÿ äëÿ çàäàíèÿ ôàéëà, êîòîðûé áóäåò îòêðûò è èíòåðïðåòèðîâàí ïðèëîæåíèå <acronym>CGI</acronym>.  îáùåì ñëó÷àå äëÿ ïåðåíàïðàâëåíèÿ çàïðîñîâ âèäà <filename role="url">http://my.host/secret/script.php</filename> èíòåðïðåòàòîðó PHP èñïîëüçóþòñÿ äèðåêòèâû êîíôèãóðàöèè Web-ñåðâåðà (ê ïðèìåðó, Action â Apache). Ïðè òàêîé íàñòðîéêå, ñåðâåð ñíà÷àëà ïðîâåðÿåò ðàçðåøåíèÿ íà äîñòóï ê äàííîé äèðåêòîðèè <filename role="uri">/secret</filename>, è òîëüêî ïîñëå ñîçäàåò ïåðåíàïðàâëåííûé çàïðîñ <filename role="url">http://my.host/cgi-bin/php/secret/script.php</filename>. Ê ñîæàëåíèþ, åñëè çàïðîñ óæå äàí â âûøåïðèâåäåííîé ôîðìå, ñåðâåð íå âûïîëíÿåò íèêàêèõ ïðîâåðîê íà ïðàâî äîñòóïà ê ôàéëó <filename role="uri">/secret/script.php</filename>, âûïîëíÿÿ äàííóþ ïðîâåðêó òîëüêî äëÿ ôàéëà <filename role="uri">/cgi-bin/php</filename>. Òàêèì îáðàçîì, ëþáîé ïîëüçîâàòåëü c äîñòóïîì ê <filename role="uri">/cgi-bin/php</filename> ìîæåò ïîëó÷èòü äîñòóï ê ëþáîìó çàùèùåííîìó äîêóìåíòó íà ñåðâåðå. </simpara> <simpara>  PHP, îïöèÿ êîíôèãóðàöèè êîìïèëÿòîðà<link linkend="install.configure.enable-force-cgi-redirect">--enable-force-cgi-redirect</link> è îïöèè êîíôèãóðàöèè âðåìåíè âûïîëíåíèÿ<link linkend="ini.doc-root">doc_root</link> è <link linkend="ini.user-dir">user_dir</link> ìîãóò áûòü èñïîëüçîâàíû äëÿ ïðåäîòâðàùåíèÿ ïîäîáíûõ àòàê, åñëè äåðåâî äîêóìåíòîâ ñåðâåðà èìååò êàòàëîãè ñ îãðàíè÷åíèåì äîñòóïà. Äàëåå ïðèâîäÿòñÿ ïîäðîáíûå ïîÿñíåíèÿ ê ðàçëè÷íûì êîìáèíàöèÿì îïöèé. </simpara> </listitem> </itemizedlist> </sect2> <sect2 id="security.cgi-bin.default"> <title>Âàðèàíò 1: ïðåäîñòàâëåíèå òîëüêî îáùåãî ñîäåðæèìîãî</title> <simpara> Åñëè âàø ñåðâåð íå ñîäåðæèò íè÷åãî çàùèùåííîãî ïàðîëåì èëè ñïèñêîì IP-àäðåñîâ, òî â âûøåïåðå÷èñëåííûõ îïöèÿõ íåò íåîáõîäèìîñòè. Åñëè âàø Web-ñåðâåð íå äîïóñêàåò ïåðåíàïðàâëåíèé, èëè åñëè ñåðâåð íå èìååò âîçìîæíîñòè áåçîïàñíî ïåðåíàïðàâèòü çàïðîñ ïðèëîæåíèþ PHP, âû ìîæåòå óñòàíîâèòü îïöèþ <link linkend="install.configure.enable-force-cgi-redirect">--enable-force-cgi-redirect</link> êîíôèãóðèðóþùåé ïðîãðàììå. Îäíàêî, âàì âñå ðàâíî ïðèäåòñÿ ñëåäèòü çà òåì, ÷òîáû âàøè ïðîãðàììû íå çàâèñåëè îò ìåòîäà èõ âûçîâà, êàê ïðÿìîãî <filename role="php">http://my.host/cgi-bin/php/dir/script.php</filename> òàê è ñ èñïîëüçîâàíèåì ïåðåíàïðàâëåíèÿ <filename role="php">http://my.host/dir/script.php</filename>. </simpara> <simpara> Ïåðåíàïðàâëåíèå ìîæåò áûòü íàñòðîåíî â ñåðâåðå Apache ñ èñïîëüçîâàíèåì äèðåêòèâ AddHandler è Action directives (ñì. íèæå). </simpara> </sect2> <sect2 id="security.cgi-bin.force-redirect"> <title>Âàðèàíò 2: èñïîëüçîâàíèå --enable-force-cgi-redirect</title> <simpara> Ýòà îïöèÿ êîìïèëÿöèè íå äàåò íèêîìó èñïîëüçîâàòü PHP íàïðÿìóþ, ñ ïðèìåíåíèåì URL âðîäå <filename role="php">http://my.host/cgi-bin/php/secretdir/script.php</filename>. PHP áóäåò òðàíñëèðîâàòü äàííûå òîëüêî â òîì ñëó÷àå, åñëè îíè óæå ïðîøëè ÷åðåç ïåðåíàïðàâëåíèå Web-ñåðâåðà. </simpara> <simpara> Îáû÷íî ïåðåíàïðàâëåíèå â ñåðâåðå Apache êîíôèãóðèðóåòñÿ ñ èñïîëüçîâàíèåì ñëåäóþùèõ êîìàíä: </simpara> <programlisting role="apache-conf"> <![CDATA[ Action php-script /cgi-bin/php AddHandler php-script .php ]]> </programlisting> <simpara> Ýòà îïöèÿ áûëà ïðîâåðåíà òîëüêî äëÿ Web-ñåðâåðà Apache è îñíîâûâàåòñÿ íà óñòàíîâêå èì íå-CGI-ñòàíäàðòíîé ïåðåìåííîé îêðóæåíèÿ <envar>REDIRECT_STATUS</envar> äëÿ ïåðåíàïðàâëåííûõ çàïðîñîâ. Åñëè âàø Web-ñåðâåð íå ïîääåðæèâàåò íèêàêèõ ïóòåé óêàçàíèÿ òîãî, îñóùåñòâëÿëîñü ëè ïåðåíàïðàâëåíèå çàïðîñà, âû íå ìîæåòå èñïîëüçîâàòü ýòó îïöèþ è äîëæíû èñïîëüçîâàòü äðóãèå âàðèàíòû èñïîëüçîâàíèÿ ïðèëîæåíèÿ CGI, îïèñàííûå íèæå. </simpara> </sect2> <sect2 id="security.cgi-bin.doc-root"> <title>Âàðèàíò 3: óñòàíîâêà doc_root èëè user_dir</title> <simpara> To include active content, like scripts and executables, in the web server document directories is sometimes consider an insecure practice. If, because of some configuration mistake, the scripts are not executed but displayed as regular HTML documents, this may result in leakage of intellectual property or security information like passwords. Therefore many sysadmins will prefer setting up another directory structure for scripts that are accessible only through the PHP CGI, and therefore always interpreted and not displayed as such. </simpara> <simpara> Also if the method for making sure the requests are not redirected, as described in the previous section, is not available, it is necessary to set up a script doc_root that is different from web document root. </simpara> <simpara> You can set the PHP script document root by the configuration directive <link linkend="ini.doc-root">doc_root</link> in the <link linkend="configuration.file">configuration file</link>, or you can set the environment variable <envar>PHP_DOCUMENT_ROOT</envar>. If it is set, the CGI version of PHP will always construct the file name to open with this <parameter>doc_root</parameter> and the path information in the request, so you can be sure no script is executed outside this directory (except for <parameter>user_dir</parameter> below). </simpara> <simpara> Another option usable here is <link linkend="ini.user-dir">user_dir</link>. When user_dir is unset, only thing controlling the opened file name is <parameter>doc_root</parameter>. Opening an url like <filename role="url">http://my.host/~user/doc.php</filename> does not result in opening a file under users home directory, but a file called <filename role="uri">~user/doc.php</filename> under doc_root (yes, a directory name starting with a tilde [<literal>~</literal>]). </simpara> <simpara> If user_dir is set to for example <filename role="dir">public_php</filename>, a request like <filename role="url">http://my.host/~user/doc.php</filename> will open a file called <filename>doc.php</filename> under the directory named <filename role="dir">public_php</filename> under the home directory of the user. If the home of the user is <filename role="dir">/home/user</filename>, the file executed is <filename>/home/user/public_php/doc.php</filename>. </simpara> <simpara> <parameter>user_dir</parameter> expansion happens regardless of the <parameter>doc_root</parameter> setting, so you can control the document root and user directory access separately. </simpara> </sect2> <sect2 id="security.cgi-bin.shell"> <title>Case 4: PHP parser outside of web tree</title> <para> A very secure option is to put the PHP parser binary somewhere outside of the web tree of files. In <filename role="dir">/usr/local/bin</filename>, for example. The only real downside to this option is that you will now have to put a line similar to: <informalexample> <programlisting> <![CDATA[ #!/usr/local/bin/php ]]> </programlisting> </informalexample> as the first line of any file containing PHP tags. You will also need to make the file executable. That is, treat it exactly as you would treat any other CGI script written in Perl or sh or any other common scripting language which uses the <literal>#!</literal> shell-escape mechanism for launching itself. </para> <para> To get PHP to handle <envar>PATH_INFO</envar> and <envar>PATH_TRANSLATED</envar> information correctly with this setup, the php parser should be compiled with the <link linkend="install.configure.enable-discard-path">--enable-discard-path</link> configure option. </para> </sect2> </sect1> <sect1 id="security.apache"> <title>Installed as an Apache module</title> <simpara> When PHP is used as an Apache module it inherits Apache's user permissions (typically those of the "nobody" user). This has several impacts on security and authorization. For example, if you are using PHP to access a database, unless that database has built-in access control, you will have to make the database accessable to the "nobody" user. This means a malicious script could access and modify the database, even without a username and password. It's entirely possible that a web spider could stumble across a database administrator's web page, and drop all of your databases. You can protect against this with Apache authorization, or you can design your own access model using LDAP, .htaccess files, etc. and include that code as part of your PHP scripts. </simpara> <simpara> Often, once security is established to the point where the PHP user (in this case, the apache user) has very little risk attached to it, it is discovered that PHP is now prevented from writing any files to user directories. Or perhaps it has been prevented from accessing or changing databases. It has equally been secured from writing good and bad files, or entering good and bad database transactions. </simpara> <simpara> A frequent security mistake made at this point is to allow apache root permissions, or to escalate apache's abilitites in some other way. </simpara> <simpara> Escalating the Apache user's permissions to root is extremely dangerous and may compromise the entire system, so sudo'ing, chroot'ing, or otherwise running as root should not be considered by those who are not security professionals. </simpara> <simpara> There are some simpler solutions. By using <link linkend="ini.open-basedir">open_basedir</link> you can control and restrict what directories are allowed to be used for PHP. You can also set up apache-only areas, to restrict all web based activity to non-user, or non-system, files. </simpara> </sect1> <sect1 id="security.filesystem"> <title>Filesystem Security</title> <simpara> PHP is subject to the security built into most server systems with respect to permissions on a file and directory basis. This allows you to control which files in the filesystem may be read. Care should be taken with any files which are world readable to ensure that they are safe for reading by all users who have access to that filesystem. </simpara> <simpara> Since PHP was designed to allow user level access to the filesystem, it's entirely possible to write a PHP script that will allow you to read system files such as /etc/passwd, modify your ethernet connections, send massive printer jobs out, etc. This has some obvious implications, in that you need to ensure that the files that you read from and write to are the appropriate ones. </simpara> <simpara> Consider the following script, where a user indicates that they'd like to delete a file in their home directory. This assumes a situation where a PHP web interface is regularly used for file management, so the Apache user is allowed to delete files in the user home directories. </simpara> <para> <example> <title>Poor variable checking leads to....</title> <programlisting role="php"> <![CDATA[ <?php // remove a file from the user's home directory $username = $_POST['user_submitted_name']; $homedir = "/home/$username"; $file_to_delete = "$userfile"; unlink ($homedir/$userfile); echo "$file_to_delete has been deleted!"; ?> ]]> </programlisting> </example> Since the username is postable from a user form, they can submit a username and file belonging to someone else, and delete files. In this case, you'd want to use some other form of authentication. Consider what could happen if the variables submitted were "../etc/" and "passwd". The code would then effectively read: <example> <title>... A filesystem attack</title> <programlisting role="php"> <![CDATA[ <?php // removes a file from anywhere on the hard drive that // the PHP user has access to. If PHP has root access: $username = "../etc/"; $homedir = "/home/../etc/"; $file_to_delete = "passwd"; unlink ("/home/../etc/passwd"); echo "/home/../etc/passwd has been deleted!"; ?> ]]> </programlisting> </example> There are two important measures you should take to prevent these issues. <itemizedlist> <listitem> <simpara> Only allow limited permissions to the PHP web user binary. </simpara> </listitem> <listitem> <simpara> Check all variables which are submitted. </simpara> </listitem> </itemizedlist> Here is an improved script: <example> <title>More secure file name checking</title> <programlisting role="php"> <![CDATA[ <?php // removes a file from the hard drive that // the PHP user has access to. $username = $_SERVER['REMOTE_USER']; // using an authentication mechanisim $homedir = "/home/$username"; $file_to_delete = basename("$userfile"); // strip paths unlink ($homedir/$file_to_delete); $fp = fopen("/home/logging/filedelete.log","+a"); //log the deletion $logstring = "$username $homedir $file_to_delete"; fputs ($fp, $logstring); fclose($fp); echo "$file_to_delete has been deleted!"; ?> ]]> </programlisting> </example> However, even this is not without it's flaws. If your authentication system allowed users to create their own user logins, and a user chose the login "../etc/", the system is once again exposed. For this reason, you may prefer to write a more customized check: <example> <title>More secure file name checking</title> <programlisting role="php"> <![CDATA[ <?php $username = $_SERVER['REMOTE_USER']; // using an authentication mechanisim $homedir = "/home/$username"; if (!ereg('^[^./][^/]*$', $userfile)) die('bad filename'); //die, do not process if (!ereg('^[^./][^/]*$', $username)) die('bad username'); //die, do not process //etc... ?> ]]> </programlisting> </example> </para> <para> Depending on your operating system, there are a wide variety of files which you should be concerned about, including device entries (/dev/ or COM1), configuration files (/etc/ files and the .ini files), well known file storage areas (/home/, My Documents), etc. For this reason, it's usually easier to create a policy where you forbid everything except for what you explicitly allow. </para> </sect1> <sect1 id="security.database"> <title>Database Security</title> <simpara> Nowadays, databases are cardinal components of any web based application by enabling websites to provide varying dynamic content. Since very sensitive or secret informations can be stored in such database, you should strongly consider to protect them somehow. </simpara> <simpara> To retrieve or to store any information you need to connect to the database, send a legitimate query, fetch the result, and close the connecion. Nowadays, the commonly used query language in this interaction is the Structured Query Language (SQL). See how an attacker can <link linkend="security.database.sql-injection">tamper with an SQL query</link>. </simpara> <simpara> As you can realize, PHP cannot protect your database by itself. The following sections aim to be an introduction into the very basics of how to access and manipulate databases within PHP scripts. </simpara> <simpara> Keep in mind this simple rule: defence in depth. In the more place you take the more action to increase the protection of your database, the less probability of that an attacker succeeds, and exposes or abuse any stored secret information. Good design of the database schema and the application deals with your greatest fears. </simpara> <sect2 id="security.database.design"> <title>Designing Databases</title> <simpara> The first step is always to create the database, unless you want to use an existing third party's one. When a database is created, it is assigned to an owner, who executed the creation statement. Usually, only the owner (or a superuser) can do anything with the objects in that database, and in order to allow other users to use it, privileges must be granted. </simpara> <simpara> Applications should never connect to the database as its owner or a superuser, because these users can execute any query at will, for example, modifying the schema (e.g. dropping tables) or deleting its entire content. </simpara> <simpara> You may create different database users for every aspect of your application with very limited rights to database objects. The most required privileges should be granted only, and avoid that the same user can interact with the database in different use cases. This means that if intruders gain access to your database using one of these credentials, they can only effect as many changes as your application can. </simpara> <simpara> You are encouraged not to implement all the business logic in the web application (i.e. your script), instead to do it in the database schema using views, triggers or rules. If the system evolves, new ports will be intended to open to the database, and you have to reimplement the logic in each separate database client. Over and above, triggers can be used to transparently and automatically handle fields, which often provides insight when debugging problems with your application or tracing back transactions. </simpara> </sect2> <sect2 id="security.database.connection"> <title>Connecting to Database</title> <simpara> You may want to estabilish the connections over SSL to encrypt client/server communications for increased security, or you can use ssh to encrypt the network connection between clients and the database server. If either of them is done, then monitoring your traffic and gaining informations in this way will be a hard work. </simpara> <!--simpara> If your database server native SSL support, consider to use <link linkend="ref.openssl">OpenSSL functions</link> in communication between PHP and database via SSL. </simpara--> </sect2> <sect2 id="security.database.storage"> <title>Encrypted Storage Model</title> <simpara> SSL/SSH protects data travelling from the client to the server, SSL/SSH does not protect the persistent data stored in a database. SSL is an on-the-wire protocol. </simpara> <simpara> Once an attacker gains access to your database directly (bypassing the webserver), the stored sensitive data may be exposed or misused, unless the information is protected by the database itself. Encrypting the data is a good way to mitigate this threat, but very few databases offer this type of data encryption. </simpara> <simpara> The easiest way to work around this problem is to first create your own encryption package, and then use it from within your PHP scripts. PHP can assist you in this case with its several extensions, such as <link linkend="ref.mcrypt">Mcrypt</link> and <link linkend="ref.mhash">Mhash</link>, covering a wide variety of encryption algorithms. The script encrypts the data be stored first, and decrypts it when retrieving. See the references for further examples how encryption works. </simpara> <simpara> In case of truly hidden data, if its raw representation is not needed (i.e. not be displayed), hashing may be also taken into consideration. The well-known example for the hashing is storing the MD5 hash of a password in a database, instead of the password itself. See also <function>crypt</function> and <function>md5</function>. </simpara> <example> <title>Using hashed password field</title> <programlisting role="php"> <![CDATA[ // storing password hash $query = sprintf("INSERT INTO users(name,pwd) VALUES('%s','%s');", addslashes($username), md5($password)); $result = pg_exec($connection, $query); // querying if user submitted the right password $query = sprintf("SELECT 1 FROM users WHERE name='%s' AND pwd='%s';", addslashes($username), md5($password)); $result = pg_exec($connection, $query); if (pg_numrows($result) > 0) { echo "Welcome, $username!"; } else { echo "Authentication failed for $username."; } ]]> </programlisting> </example> </sect2> <sect2 id="security.database.sql-injection"> <title>SQL Injection</title> <simpara> Many web developers are unaware of how SQL queries can be tampered with, and assume that an SQL query is a trusted command. It means that SQL queries are able to circumvent access controls, thereby bypassing standard authentication and authorization checks, and sometimes SQL queries even may allow access to host operating system level commands. </simpara> <simpara> Direct SQL Command Injection is a technique where an attacker creates or alters existing SQL commands to expose hidden data, or to override valuable ones, or even to execute dangerous system level commands on the database host. This is accomplished by the application taking user input and combining it with static parameters to build a SQL query. The following examples are based on true stories, unfortunately. </simpara> <para> Owing to the lack of input validation and connecting to the database on behalf of a superuser or the one who can create users, the attacker may create a superuser in your database. <example> <title> Splitting the result set into pages ... and making superusers (PostgreSQL and MySQL) </title> <programlisting role="php"> <![CDATA[ $offset = argv[0]; // beware, no input validation! $query = "SELECT id, name FROM products ORDER BY name LIMIT 20 OFFSET $offset;"; // with PostgreSQL $result = pg_exec($conn, $query); // with MySQL $result = mysql_query($query); ]]> </programlisting> </example> Normal users click on the 'next', 'prev' links where the <varname>$offset</varname> is encoded into the URL. The script expects that the incoming <varname>$offset</varname> is decimal number. However, someone tries to break in with appending <function>urlencode</function>'d form of the following to the URL <informalexample> <programlisting> <![CDATA[ // in case of PostgreSQL 0; insert into pg_shadow(usename,usesysid,usesuper,usecatupd,passwd) select 'crack', usesysid, 't','t','crack' from pg_shadow where usename='postgres'; -- // in case of MySQL 0; UPDATE user SET Password=PASSWORD('crack') WHERE user='root'; FLUSH PRIVILEGES; ]]> </programlisting> </informalexample> If it happened, then the script would present a superuser access to him. Note that <literal>0;</literal> is to supply a valid offset to the original query and to terminate it. </para> <note> <para> It is common technique to force the SQL parser to ignore the rest of the query written by the developer with <literal>--</literal> which is the comment sign in SQL. </para> </note> <para> A feasible way to gain passwords is to circumvent your search result pages. What the attacker needs only is to try if there is any submitted variable used in SQL statement which is not handled properly. These filters can be set commonly in a preceding form to customize <literal>WHERE, ORDER BY, LIMIT</literal> and <literal>OFFSET</literal> clauses in <literal>SELECT</literal> statements. If your database supports the <literal>UNION</literal> construct, the attacker may try to append an entire query to the original one to list passwords from an arbitrary table. Using encrypted password fields is strongly encouraged. <example> <title> Listing out articles ... and some passwords (any database server) </title> <programlisting role="php"> <![CDATA[ $query = "SELECT id, name, inserted, size FROM products WHERE size = '$size' ORDER BY $order LIMIT $limit, $offset;"; $result = odbc_exec($conn, $query); ]]> </programlisting> </example> The static part of the query can be combined with another <literal>SELECT</literal> statement which reveals all passwords: <informalexample> <programlisting> <![CDATA[ ' union select '1', concat(uname||'-'||passwd) as name, '1971-01-01', '0' from usertable; -- ]]> </programlisting> </informalexample> If this query (playing with the <literal>'</literal> and <literal>--</literal>) were assigned to one of the variables used in <varname>$query</varname>, the query beast awakened. </para> <para> SQL UPDATEs are also subject to attacking your database. These queries are also threatened by chopping and appending an entirely new query to it. But the attacker might fiddle with the <literal>SET</literal> clause. In this case some schema information must be possessed to manipulate the query successfully. This can be acquired by examing the form variable names, or just simply brute forcing. There are not so many naming convention for fields storing passwords or usernames. <example> <title> From resetting a password ... to gaining more privileges (any database server) </title> <programlisting role="php"> <![CDATA[ $query = "UPDATE usertable SET pwd='$pwd' WHERE uid='$uid';"; ]]> </programlisting> </example> But a malicious user sumbits the value <literal>' or uid like'%admin%'; --</literal> to <varname>$uid</varname> to change the admin's password, or simply sets <varname>$pwd</varname> to <literal>"hehehe', admin='yes', trusted=100 "</literal> (with a trailing space) to gain more privileges. Then, the query will be twisted: <informalexample> <programlisting role="php"> <![CDATA[ // $uid == ' or uid like'%admin%'; -- $query = "UPDATE usertable SET pwd='...' WHERE uid='' or uid like '%admin%'; --"; // $pwd == "hehehe', admin='yes', trusted=100 " $query = "UPDATE usertable SET pwd='hehehe', admin='yes', trusted=100 WHERE ...;" ]]> </programlisting> </informalexample> </para> <para> A frightening example how operating system level commands can be accessed on some database hosts. <example> <title>Attacking the database host's operating system (MSSQL Server)</title> <programlisting role="php"> <![CDATA[ $query = "SELECT * FROM products WHERE id LIKE '%$prod%'"; $result = mssql_query($query); ]]> </programlisting> </example> If attacker submits the value <literal>a%' exec master..xp_cmdshell 'net user test testpass /ADD' --</literal> to <varname>$prod</varname>, then the <varname>$query</varname> will be: <informalexample> <programlisting role="php"> <![CDATA[ $query = "SELECT * FROM products WHERE id LIKE '%a%' exec master..xp_cmdshell 'net user test testpass /ADD'--"; $result = mssql_query($query); ]]> </programlisting> </informalexample> MSSQL Server executes the SQL statements in the batch including a command to add a new user to the local accounts database. If this application were running as <literal>sa</literal> and the MSSQLSERVER service is running with sufficient privileges, the attacker would now have an account with which to access this machine. </para> <note> <para> Some of the examples above is tied to a specific database server. This does not mean that a similar attack is impossible against other products. Your database server may be so vulnerable in other manner. </para> </note> <sect3 id="security.database.avoiding"> <title>Avoiding techniques</title> <simpara> You may plead that the attacker must possess a piece of information about the database schema in most examples. You are right, but you never know when and how it can be taken out, and if it happens, your database may be exposed. If you are using an open source, or publicly available database handling package, which may belong to a content management system or forum, the intruders easily produce a copy of a piece of your code. It may be also a security risk if it is a poorly designed one. </simpara> <simpara> These attacks are mainly based on exploiting the code not being written with security in mind. Never trust on any kind of input, especially which comes from the client side, even though it comes from a select box, a hidden input field or a cookie. The first example shows that such a blameless query can cause disasters. </simpara> <itemizedlist> <listitem> <simpara> Never connect to the database as a superuser or as the database owner. Use always customized users with very limited privileges. </simpara> </listitem> <listitem> <simpara> Check if the given input has the expected data type. PHP has a wide range of input validating functions, from the simplest ones found in <link linkend="ref.variables">Variable Functions</link> and in <link linkend="ref.ctype">Character Type Functions</link> (e.g. <function>is_numeric</function>, <function>ctype_digit</function> respectively) onwards the <link linkend="ref.pcre">Perl compatible Regular Expressions</link> support. </simpara> </listitem> <listitem> <para> If the application waits for numerical input, consider to verify data with <function>is_numeric</function>, or silently change its type using <function>settype</function>, or use its numeric representation by <function>sprintf</function>. <example> <title>A more secure way to compose a query for paging</title> <programlisting role="php"> <![CDATA[ settype($offset, 'integer'); $query = "SELECT id, name FROM products ORDER BY name LIMIT 20 OFFSET $offset;"; // please note %d in the format string, using %s would be meaningless $query = sprintf("SELECT id, name FROM products ORDER BY name LIMIT 20 OFFSET %d;", $offset); ]]> </programlisting> </example> </para> </listitem> <listitem> <simpara> Quote each non numeric user input which is passed to the database with <function>addslashes</function> or <function>addcslashes</function>. See <link linkend="security.database.storage">the first example</link>. As the examples shows, quotes burnt into the static part of the query is not enough, and can be easily hacked. </simpara> </listitem> <listitem> <simpara> Do not print out any database specific information, especially about the schema, by fair means or foul. See also <link linkend="security.errors">Error Reporting</link> and <link linkend="ref.errorfunc">Error Handling and Logging Functions</link>. </simpara> </listitem> <listitem> <simpara> You may use stored procedures and previously defined cursors to abstract data access so that users do not directly access tables or views, but this solution has another impacts. </simpara> </listitem> </itemizedlist> <simpara> Besides these, you benefit from logging queries either within your script or by the database itself, if it supports. Obviously, the logging is unable to prevent any harmful attempt, but it can be helpful to trace back which application has been circumvented. The log is not useful by itself, but through the information it contains. The more detail is generally better. </simpara> </sect3> </sect2> </sect1> <sect1 id="security.errors"> <title>Error Reporting</title> <para> With PHP security, there are two sides to error reporting. One is beneficial to increasing security, the other is detrimental. </para> <para> A standard attack tactic involves profiling a system by feeding it improper data, and checking for the kinds, and contexts, of the errors which are returned. This allows the system cracker to probe for information about the server, to determine possible weaknesses. For example, if an attacker had gleaned information about a page based on a prior form submission, they may attempt to override variables, or modify them: <example> <title>Attacking Variables with a custom HTML page</title> <programlisting role="php"> <![CDATA[ <form method="post" action="attacktarget?username=badfoo&password=badfoo"> <input type="hidden" name="username" value="badfoo"> <input type="hidden" name="password" value="badfoo"> </form> ]]> </programlisting> </example> </para> <para> The PHP errors which are normally returned can be quite helpful to a developer who is trying to debug a script, indicating such things as the function or file that failed, the PHP file it failed in, and the line number which the failure occured in. This is all information that can be exploited. It is not uncommon for a php developer to use <function>show_source</function>, <function>highlight_string</function>, or <function>highlight_file</function> as a debugging measure, but in a live site, this can expose hidden variables, unchecked syntax, and other dangerous information. Especially dangerous is running code from known sources with built-in debugging handlers, or using common debugging techniques. If the attacker can determine what general technique you are using, they may try to brute-force a page, by sending various common debugging strings: <example> <title>Exploiting common debugging variables</title> <programlisting role="php"> <![CDATA[ <form method="post" action="attacktarget?errors=Y&showerrors=1"&debug=1"> <input type="hidden" name="errors" value="Y"> <input type="hidden" name="showerrors" value="1"> <input type="hidden" name="debug" value="1"> </form> ]]> </programlisting> </example> </para> <para> Regardless of the method of error handling, the ability to probe a system for errors leads to providing an attacker with more information. </para> <para> For example, the very style of a generic PHP error indicates a system is running PHP. If the attacker was looking at an .html page, and wanted to probe for the back-end (to look for known weaknesses in the system), by feeding it the wrong data they may be able to determine that a system was built with PHP. </para> <para> A function error can indicate whether a system may be running a specific database engine, or give clues as to how a web page or programmed or designed. This allows for deeper investigation into open database ports, or to look for specific bugs or weaknesses in a web page. By feeding different pieces of bad data, for example, an attacker can determine the order of authentication in a script, (from the line number errors) as well as probe for exploits that may be exploited in different locations in the script. </para> <para> A filesystem or general PHP error can indicate what permissions the webserver has, as well as the structure and organization of files on the web server. Developer written error code can aggravate this problem, leading to easy exploitation of formerly "hidden" information. </para> <para> There are three major solutions to this issue. The first is to scrutinize all functions, and attempt to compensate for the bulk of the errors. The second is to disable error reporting entirely on the running code. The third is to use PHP's custom error handling functions to create your own error handler. Depending on your security policy, you may find all three to be applicable to your situation. </para> <para> One way of catching this issue ahead of time is to make use of PHP's own <function>error_reporting</function>, to help you secure your code and find variable usage that may be dangerous. By testing your code, prior to deployment, with E_ALL, you can quickly find areas where your variables may be open to poisoning or modification in other ways. Once you are ready for deployment, by using E_NONE, you insulate your code from probing. <example> <title>Finding dangerous variables with E_ALL</title> <programlisting role="php"> <![CDATA[ <?php if ($username) { // Not initialized or checked before usage $good_login = 1; } if ($good_login == 1) { // If above test fails, not initialized or checked before usage fpassthru ("/highly/sensitive/data/index.html"); } ?> ]]> </programlisting> </example> </para> </sect1> <sect1 id="security.registerglobals"> <title>Using Register Globals</title> <para> One feature of PHP that can be used to enhance security is configuring PHP with <link linkend="ini.register-globals">register_globals</link> = off. By turning off the ability for any user-submitted variable to be injected into PHP code, you can reduce the amount of variable poisoning a potential attacker may inflict. They will have to take the additional time to forge submissions, and your internal variables are effectively isolated from user submitted data. </para> <para> While it does slightly increase the amount of effort required to work with PHP, it has been argued that the benefits far outweigh the effort. <example> <title>Working with register_globals=on</title> <programlisting role="php"> <![CDATA[ <?php if ($username) { // can be forged by a user in get/post/cookies $good_login = 1; } if ($good_login == 1) { // can be forged by a user in get/post/cookies, fpassthru ("/highly/sensitive/data/index.html"); } ?> ]]> </programlisting> </example> <example> <title>Working with register_globals = off</title> <programlisting role="php"> <![CDATA[ <?php if($_COOKIE['username']){ // can only come from a cookie, forged or otherwise $good_login = 1; fpassthru ("/highly/sensitive/data/index.html"); } ?> ]]> </programlisting> </example> By using this wisely, it's even possible to take preventative measures to warn when forging is being attempted. If you know ahead of time exactly where a variable should be coming from, you can check to see if submitted data is coming from an inappropriate kind of submission. While it doesn't guarantee that data has not been forged, it does require an attacker to guess the right kind of forging. <example> <title>Detecting simple variable poisoning</title> <programlisting role="php"> <![CDATA[ <?php if ($_COOKIE['username'] && !$_POST['username'] && !$_GET['username'] ) { // Perform other checks to validate the user name... $good_login = 1; fpassthru ("/highly/sensitive/data/index.html"); } else { mail("admin@example.com", "Possible breakin attempt", $_SERVER['REMOTE_ADDR']); echo "Security violation, admin has been alerted."; exit; } ?> ]]> </programlisting> </example> Of course, simply turning off register_globals does not mean code is secure. For every piece of data that is submitted, it should also be checked in other ways. </para> </sect1> <sect1 id="security.variables"> <title>User Submitted Data</title> <para> The greatest weakness in many PHP programs is not inherent in the language itself, but merely an issue of code not being written with security in mind. For this reason, you should always take the time to consider the implications of a given piece of code, to ascertain the possible damage if an unexpected variable is submitted to it. <example> <title>Dangerous Variable Usage</title> <programlisting role="php"> <![CDATA[ <?php // remove a file from the user's home directory... or maybe // somebody else's? unlink ($evil_var); // Write logging of their access... or maybe an /etc/passwd entry? fputs ($fp, $evil_var); // Execute something trivial.. or rm -rf *? system ($evil_var); exec ($evil_var); ?> ]]> </programlisting> </example> You should always carefully examine your code to make sure that any variables being submitted from a web browser are being properly checked, and ask yourself the following questions: <itemizedlist> <listitem> <simpara> Will this script only affect the intended files? </simpara> </listitem> <listitem> <simpara> Can unusual or undesirable data be acted upon? </simpara> </listitem> <listitem> <simpara> Can this script be used in unintended ways? </simpara> </listitem> <listitem> <simpara> Can this be used in conjunction with other scripts in a negative manner? </simpara> </listitem> <listitem> <simpara> Will any transactions be adequately logged? </simpara> </listitem> </itemizedlist> By adequately asking these questions while writing the script, rather than later, you prevent an unfortunate re-write when you need to increase your security. By starting out with this mindset, you won't guarantee the security of your system, but you can help improve it. </para> <para> You may also want to consider turning off register_globals, magic_quotes, or other convenience settings which may confuse you as to the validity, source, or value of a given variable. Working with PHP in error_reporting(E_ALL) mode can also help warn you about variables being used before they are checked or initialized (so you can prevent unusual data from being operated upon). </para> </sect1> <sect1 id="security.hiding"> <title>Hiding PHP</title> <para> In general, security by obscurity is one of the weakest forms of security. But in some cases, every little bit of extra security is desirable. </para> <para> A few simple techniques can help to hide PHP, possibly slowing down an attacker who is attempting to discover weaknesses in your system. By setting expose_php = off in your &php.ini; file, you reduce the amount of information available to them. </para> <para> Another tactic is to configure web servers such as apache to parse different filetypes through PHP, either with an .htaccess directive, or in the apache configuration file itself. You can then use misleading file extensions: <example> <title>Hiding PHP as another language</title> <programlisting role="apache-conf"> <![CDATA[ # Make PHP code look like other code types AddType application/x-httpd-php .asp .py .pl ]]> </programlisting> </example> Or obscure it completely: <example> <title>Using unknown types for PHP extensions</title> <programlisting role="apache-conf"> <![CDATA[ # Make PHP code look like unknown types AddType application/x-httpd-php .bop .foo .133t ]]> </programlisting> </example> Or hide it as html code, which has a slight performance hit because all html will be parsed through the PHP engine: <example> <title>Using html types for PHP extensions</title> <programlisting role="apache-conf"> <![CDATA[ # Make all PHP code look like html AddType application/x-httpd-php .htm .html ]]> </programlisting> </example> For this to work effectively, you must rename your PHP files with the above extensions. While it is a form of security through obscurity, it's a minor preventative measure with few drawbacks. </para> </sect1> <sect1 id="security.current"> <title>Keeping Current</title> <simpara> PHP, like any other large system, is under constant scrutiny and improvement. Each new version will often include both major and minor changes to enhance and repair security flaws, configuration mishaps, and other issues that will affect the overall security and stability of your system. </simpara> <simpara> Like other system-level scripting languages and programs, the best approach is to update often, and maintain awareness of the latest versions and their changes. </simpara> </sect1> </chapter> <!-- Keep this comment at the end of the file Local variables: mode: sgml sgml-omittag:t sgml-shorttag:t sgml-minimize-attributes:nil sgml-always-quote-attributes:t sgml-indent-step:1 sgml-indent-data:t indent-tabs-mode:nil sgml-parent-document:nil sgml-default-dtd-file:"../../manual.ced" sgml-exposed-tags:nil sgml-local-catalogs:nil sgml-local-ecat-files:nil End: vim600: syn=xml fen fdm=syntax fdl=2 si vim: et tw=78 syn=sgml vi: ts=1 sw=1 -->