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.
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 executable.
iDefense confirmed version 220.127.116.11 of Skype to be vulnerable. Previous versions are also suspected to be vulnerable.
iDefense is currently unaware of any effective workaround for this issue.
Skype has addressed this vulnerability by releasing version 18.104.22.168. For more information consult their advisory at the following URL.
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.
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 (Inkatel.com).
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Copyright © 2008 Verisign, Inc.
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