Multiple Vendor libchm Page Block Length Memory Corruption Vulnerability

iDefense Security Advisory 01.26.07
https://labs.idefense.com/intelligence/vulnerabilities/
Jan 26, 2007

I. BACKGROUND

CHMlib is an open source library used to read Microsoft CHM, compressed
HTML, files. CHM files were originally designed for use by Microsoft as
help files, but are now commonly used to store e-books. The 'xchm' program
uses CHMlib to display CHM files. More information can be found on the
package's website:

https://www.jedrea.com/chmlib/

II. DESCRIPTION

CHM files contain various tables and objects stored in "pages." When
parsing a page of objects, CHMlib passes an unsanitized value from the file
to the alloca() function. This allows an attacker to shift the stack
pointer to point to arbitrary locations in memory. Consequently it is
possible to write arbitrary data from the file to arbitrary memory
locations.

III. ANALYSIS

Successful exploitation of this vulnerability allows an attacker to execute
arbitrary code with the permissions of the user viewing the file. An
attacker would have to first convince the user to view the CHM file
through some type of social engineering.

IV. DETECTION

iDefense has confirmed the existence of this vulnerability in CHMlib
version 0.38.

V. WORKAROUND

iDefense is currently unaware of any workarounds for this issue.

VI. VENDOR RESPONSE

The CHMlib maintainer has addressed this issue in version 0.39.

VII. CVE INFORMATION

A Mitre Corp. Common Vulnerabilities and Exposures (CVE) number has not
been assigned yet.

VIII. DISCLOSURE TIMELINE

01/24/2007  Initial vendor notification
01/24/2007  Initial vendor response
01/26/2007  Coordinated public disclosure

IX. CREDIT

Sean Larsson (iDefense Labs) is credited with the discovery of this
vulnerability.

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X. LEGAL NOTICES

Copyright © 2006 iDefense, Inc.

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