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iDefense Security Advisory 06.04.08: Skype File URI Security Bypass Code Execution Vulnerability
iDefense Security Advisory 06.04.08
Jun 04, 2008
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
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.
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
iDefense confirmed version 22.214.171.124 of Skype to be vulnerable. Previous
versions are also suspected to be vulnerable.
iDefense is currently unaware of any effective workaround for this
VI. VENDOR RESPONSE
Skype has addressed this vulnerability by releasing version 126.96.36.199.
For more information consult their advisory at the following URL.
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
VIII. DISCLOSURE TIMELINE
05/16/2008 Initial vendor notification
05/17/2008 Initial vendor response
06/04/2008 Coordinated public disclosure
This vulnerability was reported to iDefense by Ismael Briones
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Copyright © 2008 iDefense, Inc.
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