From chris@scary.beasts.org Sat Sep 18 11:47:16 2004 From: chris@scary.beasts.org To: bugtraq@securityfocus.com Date: Wed, 15 Sep 2004 20:57:43 +0100 (BST) Subject: CESA-2004-005: gtk+ XPM decoder CESA-2004-005 - rev 1 http://scary.beasts.org/security/CESA-2004-005.txt gtk+-2.4.4 XPM image decoder parsing flaws ========================================== Programs: gtk+, and any programs which use gtk+ to decode XPM files. For example, Evolution. Severity: Compromise of account used to browse malicious XPM file. CAN identifier(s): CAN-2004-0782, CAN-2004-0783 This advisory lists code flaws discovered by inspection of the XPM parser within the gtk+ code. Specifically, gtk+-2.4.4 was investigated. Flaw 1. Heap-based overflow in pixbuf_create_from_xpm (io-xpm.c) CAN-2004-0782 name_buf = g_new (gchar, n_col * (cpp + 1)); colors = g_new (XPMColor, n_col); Here, n_col is an arbitrary integer value from the XPM. cpp is an integer value ranging from 1 to 31 from the XPM. By careful choice of values of n_col and cpp, integer overflow can occur on integer multiplication. This leads to heap buffers being allocated that cannot hold n_col elements, so a subsequent heap overflow occurs. Demo XPM: http://scary.beasts.org/misc/gdk1.xpm Flaw 2. Subtle stack-based overflow in xpm_extract_color (io-xpm.c) CAN-2004-0783 gint space = 128; gchar word[128], color[128], current_color[128]; ... if (color[0] != '\0') { strcat (color, " "); [*] space--; } strncat (color, word, space); space -= MIN (space, strlen (word)); Here, an attempt is actually made to prevent overflow of the stack buffers. However, a logic error means one of the buffers can still be made to overflow. When "space" reaches 0, "space" can be sent to -1 by the line marked with [*], if the color string is broken up by whitespace. When "space" is -1, the strncat() call is effectively morphed to a strcat() call, allowing overflow of the "color" buffer (probably into the "word" buffer, which may cause a minor inconvenience to exploitation. Note use of the word "minor" :-) Demo XPM: http://scary.beasts.org/misc/gdk2.xpm CESA-2004-005 - rev 1 Chris Evans chris@scary.beasts.org