// back

Skype File URI Security Bypass Code Execution Vulnerability

06.04.08

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/

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.

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.

DETECTION

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

WORKAROUND

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

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

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.

DISCLOSURE TIMELINE

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

CREDIT

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

Get paid for vulnerability research
http://labs.idefense.com/methodology/vulnerability/vcp.php

Free tools, research and upcoming events
http://labs.idefense.com/

LEGAL NOTICES

Copyright © 2008 Verisign, Inc.

Permission is granted for the redistribution of this alert electronically. It may not be edited in any way without the express written consent of iDefense. If you wish to reprint the whole or any part of this alert in any other medium other than electronically, please e-mail customer service for permission.

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.