Re: Preventing XSS Attacks

On Wed, Jun 10, 2009 at 2:26 PM, Ashley Sheridan<ash@ashleysheridan.co.uk> wrote: > On Wed, 2009-06-10 at 14:14 -0400, Eddie Drapkin wrote: >> On Wed, Jun 10, 2009 at 2:08 PM, Ashley Sheridan >> <ash@ashleysheridan.co.uk>wrote: >> >> > On Wed, 2009-06-10 at 19:03 +0100, Ashley Sheridan wrote: >> > > On Wed, 2009-06-10 at 23:17 +0530, Sudheer Satyanarayana wrote: >> > > > Ashley Sheridan wrote: >> > > > > On Wed, 2009-06-10 at 23:05 +0530, Sudheer Satyanarayana wrote: >> > > > > >> > > > >>> I've been doing a bit of reading, and I can't really >> > > > >>> understand why >> > XSS >> > > > >>> is such an issue. Sure, if a user can insert a <script> >> > > > >>> tag, what >> > > > >>> difference will that make to anyone else, as it is only on >> > > > >>> their >> > own >> > > > >>> browser. >> > > > >>> >> > > > >>> >> > > > >> 1. User 1 logs on to the application. Fills up the form with >> > malicious >> > > > >> JS code in it. The server accepts the input, is stored in the >> > database. >> > > > >> 2. User 2 logs on to the application. Goes to the view the >> > information >> > > > >> stored in the database. The JS gets executed on user 2's >> > > > >> browser. >> > User >> > > > >> is attacked by XSS. >> > > > >> >> > > > >> I hope that clarifies the question. >> > > > >> >> > > > >> >> > > > >> >> > > > > It does to a degree. So I shouldn't really worry about it in this >> > case, >> > > > > as input from one user will never be displayed to any other user.. If >> > it >> > > > > was a forum or something, it would, but the search string is only >> > ever >> > > > > shown to the user who entered it, and never stored for later display. >> > > > > >> > > > > >> > > > It is easy to slip by. I recall a website was hacked using XSS on the >> > > > page the admin views the log entries. Just in case, you or somebody >> > else >> > > > tries to add the search log feature in the future, keep this at the >> > back >> > > > of your mind. Having the user to click on a harmful URI is ridiculously >> > > > easy. >> > > > >> > > > >> > > > >> > > > >> > > > -- >> > > > >> > > > With warm regards, >> > > > Sudheer. S >> > > > Business: http://binaryvibes.co.in, Tech >> > > > stuff: http://techchorus.net, >> > Personal: http://sudheer.net >> > > > >> > > > >> > > Yeah, I never realised what a minefield it could be, but I've been doing >> > > a lot of reading today! >> > > >> > > Thanks >> > > Ash >> > > www.ashleysheridan.co.uk >> > > >> > > >> > So something like this would be acceptable?: >> > >> > $searchTerms = >> > (isset($_REQUEST['q']))?$_REQUEST['q']:''; >> > $searchTerms = htmlentities($searchTerms); >> > $dbSearchTerms = mysql_real_escape_string($searchTerms); >> > >> > Giving me two variables, one for display output to user, the other for >> > use in the database? >> > >> > Thanks >> > Ash >> > www.ashleysheridan.co.uk >> > >> >> >> You wouldn't want to insert htmlentity escaped information into your >> database. >> >> This method has always worked well for me: >> >> Accept input -> db escape -> store; >> Retrieve output from db -> html escape -> display; >> >> So, I'm actually storing (in at least one case that I've seen), human >> readable XSS in the database, but I have a consistent approach to escaping >> before outputting so that it never gets displayed as XSS and I never >> accidentally escape it twice, which depending on a few factors, can have >> some pretty ugly results.  You wouldn't want to see &amp;amp; anywhere, >> would you? Alternatively though, if you are storing it html-escaped in the >> database, make sure you don't ever escape it before you output, but I find >> that approach a lot less flexible, has problems with searches, isn't easy to >> read from the mysql cli console, etc. etc. > > OK, so I just swapped those last two lines over like so: > > $searchTerms = (isset($_REQUEST['q']))?trim($_REQUEST['q']):''; > $dbSearchTerms = mysql_real_escape_string($searchTerms); > $searchTerms = htmlentities($searchTerms); > > > Thanks > Ash > www.ashleysheridan.co.uk > I wouldn't self-assign the output of htmlentities to $searchTerms at all. <?php $searchTerms = (isset($_REQUEST['q']))?trim($_REQUEST['q']):''; // Rather than this: $searchTerms = htmlspecialchars($searchTerms); echo $searchTerms; // I prefer this: echo htmlspecialchars($searchTerms); ?> Escape sequences are not part of the data, so I don't store them. Andrew