iDefense Security Advisory 06.04.08: Skype File URI Security Bypass Code Execution Vulnerability



iDefense Security Advisory 06.04.08
http://labs.idefense.com/intelligence/vulnerabilities/
Jun 04, 2008

I. BACKGROUND

Skype is a freely available VOIP client that allows access to chat and
video conference with other Skype users and traditional telephone
numbers. More information is available at the vendor's site at the
following URL.

http://www.skype.com/

II. DESCRIPTION

Remote exploitation of a security policy bypass in Skype could allow an
attacker to execute arbitrary code in the context of the user.

The "file:" URI handler in Skype performs checks upon the URL to verify
that the link does not contain certain file extensions related to
executable file formats. If the link is found to contain a blacklisted
file extension, a security warning dialog is shown to the user. The
following file extensions are checked and considered dangerous by
Skype; .ade, .adp, .asd, .bas, .bat, .cab, .chm, .cmd, .com, .cpl,
.crt, .dll, .eml, .exe, .hlp, .hta, .inf, .ins, .isp, .js.

Due to improper logic when performing these checks, it is possible to
bypass the security warning and execute the program. First of all,
checking is performed using a case sensitive comparison. The second
flaw in this check is that the blacklist fails to mention all potential
executable file formats. By using at least one upper case character, or
using an executable file type that is not covered in the list, an
attacker can bypass the security warning.

III. ANALYSIS

Exploitation of this issue allows an attacker to execute arbitrary code
on the targeted user's machine. An attacker would need to persuade a
targeted user to click a "file:" URI pointing to a malicious
executable.

IV. DETECTION

iDefense confirmed version 3.6.0.248 of Skype to be vulnerable. Previous
versions are also suspected to be vulnerable.

V. WORKAROUND

iDefense is currently unaware of any effective workaround for this
issue.

VI. VENDOR RESPONSE

Skype has addressed this vulnerability by releasing version 3.8.0.139.
For more information consult their advisory at the following URL.

http://www.skype.com/security/skype-sb-2008-003.html

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2008-1805 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

05/16/2008 Initial vendor notification
05/17/2008 Initial vendor response
06/04/2008 Coordinated public disclosure

IX. CREDIT

This vulnerability was reported to iDefense by Ismael Briones
(Inkatel.com).

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X. LEGAL NOTICES

Copyright © 2008 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.
There are no warranties with regard to this information. Neither the
author nor the publisher accepts any liability for any direct,
indirect, or consequential loss or damage arising from use of, or
reliance on, this information.



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