Multiple Vendor ImageMagick Off-By-One Vulnerability

iDefense Security Advisory 09.19.07
http://labs.idefense.com/intelligence/vulnerabilities/
Sep 19, 2007

I. BACKGROUND

ImageMagick is a suite of image manipulation tools (animate, composite,
conjure, convert, display, identify, import, mogrify and montage) that
are sometimes used by other applications for processing image files.
For more information about ImageMagick, visit the vendor's site at the
following URL.

http://www.imagemagick.org/

II. DESCRIPTION

Remote exploitation of an off-by-one vulnerability in ImageMagick, as
included in various vendors' operating system distributions, allows
attackers to execute arbitrary code.

This vulnerability specifically exists in the ReadBlobString() function
in magick/blob.c as shown below.

  3110    for (i=0; i < (long) MaxTextExtent; i++)
  3111    {
  3112      p=ReadBlobStream(image,1,buffer,&count);
  ...
  3119      string[i]=(char) (*p);
  3120      if ((string[i] == '\n') || (string[i] == '\r'))
  3121        break;
  3122    }
  3123    string[i]='\0';

The variable "string" is a character array of length "MaxTextExtent". An
off-by-one buffer overflow will occur on line 3123 when "i" is exactly
"MaxTextExtent". This function is called from several image file
processing routines. Most of the buffers involved are stack based,
although some are on the heap.

III. ANALYSIS

Exploitation of this vulnerability allows an attacker to execute
arbitrary code in the context of the user.

One way of exploiting this vulnerability is to persuade a targeted user
to open a malicious image file with a program that utilizes the
ImageMagick library.

As the tools that are part of ImageMagick are sometimes used as helper
tools by other applications, this user may be the same as the web
server user. This scenario is somewhat more severe than the previously
described attack vector since the image processing can occur
automatically.

Exploitation in stack-based scenarios depends on the stack layout, which
depends on the compiler and compiler options used to build the library.

IV. DETECTION

iDefense Labs confirmed that ImageMagick version 6.3.4 is vulnerable. It
is suspected that other versions of ImageMagick are also vulnerable.

V. WORKAROUND

iDefense is unaware of any effective workaround for this vulnerability.

VI. VENDOR RESPONSE

The ImageMagick maintainers have addressed this vulnerability with the
release of version 6.3.5-9. More information is available from the
following URL.

http://studio.imagemagick.org/pipermail/magick-announce/2007-September/000037.html

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2007-4987 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org/), which standardizes names for
security problems.

VIII. DISCLOSURE TIMELINE

09/04/2007  Initial vendor notification
09/05/2007  Initial vendor response
09/19/2007  Coordinated public disclosure

IX. CREDIT

This vulnerability was reported to iDefense by regenrecht.

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X. LEGAL NOTICES

Copyright © 2007 iDefense, Inc.

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Disclaimer: The information in the advisory is believed to be accurate
at the time of publishing based on currently available information. Use
of the information constitutes acceptance for use in an AS IS condition.
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