bugtraq - CESA-2004-005: gtk+ XPM decoder

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Date: Wed, 15 Sep 2004 20:57:43 +0100 (BST)
From: chris@...ry.beasts.org
To: bugtraq@...urityfocus.com
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@...ry.beasts.org