#41630 [Opn]: Non-exploitable (DoS) heap corruption with GIF

ID: 41630 Updated by: pajoye@php.net Reported By: wccoder at gmail dot com Status: Open Bug Type: GD related Operating System: Linux PHP Version: 5.2.3 -Assigned To: +Assigned To: pajoye New Comment: > I have a sample .gif file. Email me for it. done, I sent you a mail. Previous Comments: ------------------------------------------------------------------------ [2007-06-07 19:37:36] wccoder at gmail dot com Description: ------------ The corruption actually happens at ext/gd/libgd/gd_gif_in.c:590-594 590 while ((v = LWZReadByte(fd, &sd, FALSE, c, ZeroDataBlockP)) >= 0 ) { 591 /* This how we recognize which colors are actually used. */ 592 if (im->open[v]) { 593 im->open[v] = 0; 594 } Values read from the data stream are not bounds-checked. An attacker can create values of v (type int) which are larger than the array im->open (256 bytes). If the value of im->open[v] happens to be non-zero, it is overwritten with a zero on line 593. This corrupts the heap. The attacker can't control what value is overwritten (always 0), but they could control which bytes are overwritten. libgd should clamp values of v to be between acceptable values [0,255]. Note that values of v are clamped to 4096, since this is the maximum number of bits per codeword that gd will accept, so it isn't an arbitrary overwrite. It is enough to corrupt the heap and cause a DoS though. Reproduce code: --------------- I have a sample .gif file. Email me for it. ------------------------------------------------------------------------ -- Edit this bug report at http://bugs.php.net/?id=41630&edit=1