Re: CVE Request -- PHP 5 - 5.2.11

[Tomas Hoger <thoger () redhat com>]

On Tue, 22 Sep 2009 03:24:34 -0400 (EDT) "Steven M. Christey"
<coley () linus mitre org> wrote:

> Name: CVE-2009-3293
>
> Unspecified vulnerability in the imagecolortransparent function in PHP
> before 5.2.11 has unknown impact and attack vectors related to an
> incorrect "sanity check for the color index."

While looking into this one, I spotted few interesting things.

Patch for this is:
- if (color > -1 && color<im->colorsTotal && color<=gdMaxColors) {
+ if (color > -1 && color < im->colorsTotal && color < gdMaxColors) {

Besides "color<=gdMaxColors" check, there is also "color<im->colorsTotal"
check.  GD code also assumes that im->colorsTotal is <= gdMaxColors, as
it is used as an upper bound in multiple cases when accessing arrays of
gdMaxColors size.  You can see "im->colorsTotal<=gdMaxColors" enforced in
e.g.  gdImageColorAllocateAlpha(), which is called for PHP function
imagecolorallocate().

Hence:
  color<im->colorsTotal (from the check)
and
  im->colorsTotal<=gdMaxColors (assumed in the rest of the code)
implies
  color < gdMaxColor

So the change should not really introduce any extra protection for current
PHP versions.

This change is relevant for pre-4.3.5 PHP versions, which do not have
"color<im->colorsTotal" part of the check.  It is possible to trigger
im->alpha[] off-by-one over-write in those versions.  This changes
neighbor member of the gdImageStruct structure - trueColor.  If that
happens, gd will believe that previously non-TrueColor image is now
TrueColor, which can lead to buffer over-reads or over-writes in
subsequent gd operations (due to a different storage space needed for
pixels of TrueColor and non-TrueColor images).

But there is also concern for current PHP versions, as im->colorsTotal may
be initialized with a value greater than gdMaxColors when using
imagecreatefromgd() PHP function on a specially crafted GD file.  Value
read from file is not properly checked in _gdGetColors() (gd_gd.c),
possibly allowing previously mentioned over-reads or over-writes on
various places (e.g. colorsTotal is used in _gdGetColors()
when initializing im->open[] with 0s).  CVE-2009-3546 was assigned to
this problem and the fix is now committed in PHP SVN:
  http://svn.php.net/viewvc?view=revision&revision=289557

-- 
Tomas Hoger / Red Hat Security Response Team