===================================================================== CERT-Renater Note d'Information No. 2008/VULN207 _____________________________________________________________________ DATE : 05/06/2008 HARDWARE PLATFORM(S) : / OPERATING SYSTEM(S) : Systems running Skype. ====================================================================== http://www.skype.com/security/skype-sb-2008-003.html ______________________________________________________________________ SKYPE-SB/2008-003: Skype File URI Security Bypass Code Execution Vulnerability Bulletin title: Skype File URI Security Bypass Code Execution Vulnerability Bulletin ID: SKYPE-SB/2008-003 Bulletin status: PRELIMINARY Date of announcement: 2008-06-04 14:00:00 +0000 Products affected: Skype for Windows Vulnerability type: Original Vulnerability CVE references: Risk assessment: MEDIUM CVSS base score: 5.6 (AV:N/AC:M/Au:N/C:N/I:C/A:N/E:PoC/RL:OF/RC:C) Cross-references: Table of contents: 1. Problem description and brief discussion 2. Impact and affected software 3. Solution or work-around 4. Special instructions and notes 5. Software download location 6. Authenticity verification 7. Common Vulnerability Scoring System (CVSS) assessment 8. Credits and additional information 9. Bulletin release history 10. Notices 1. Problem description and brief discussion Description Remote exploitation of a security policy bypass in Windows Skype versions could allow an attacker to execute arbitrary code. 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 executable file extension a security warning dialog is shown to the user. This check is flawed in two ways. The check is performed using the case sensitive comparison. The second flaw in this check is that the blacklist fails to mention all potential executable file formats. This allows an attacker to bypass this security policy and execute arbitrary code if a victim clicks an attacker supplied URL. 2. Impact and affected software Impact Exploitation of this issue allows an attacker to execute arbitrary code on the targeted victim's machine. An attacker would need to construct a malicious file: URI and send it to the intended victim. Upon clicking the link execution of arbitrary code on the victim's machine will be possible. Affected software The following Skype clients are vulnerable to this attack: Skype for Windows: All releases prior to and including 3.8.*.115 3. Solution or work-around Skype has fixed the vulnerability in version 3.8.0.139 4. Special instructions and notes None. 5. Software download location The preferred method for installing security updates is to download the software directly from Skype's website, from the website of Skype's authorized partners, or from a reliable mirror site. Skype may also be safely downloaded from other locations, but in this case it is particularly important that you verify the authenticity of the download. We recommend that once you download any Skype software that you verify its integrity by the methods listed in Section 6 of this Bulletin. x86 platform, Microsoft Windows 2000 or Microsoft Windows XP: http://www.skype.com/download/skype/windows/ x86 platform, Linux: http://www.skype.com/download/skype/linux/ PPC and x86 platforms, Mac OS X v10.3.9 or later: http://www.skype.com/download/skype/macosx/ Pocket PC platform, Microsoft Windows Mobile 2003: http://www.skype.com/download/skype/pocketpc/ 6. Authenticity verification - Bulletin authenticity verification: Skype security bulletins are published on Skype's web site and via mailing lists. The authenticity and integrity of a Skype security bulletins may be determined by inspecting the crypto- graphic signature that is attached to each bulletin. All Skype security bulletins are published with a valid digital signature produced by PGP. - Software authenticity verification: Both the Skype installer program and the Skype program that is installed by the installer are digitally signed. For Skype software built for Microsoft Windows and Mac OSX operating environments, the digital certificate used by Skype to sign software packages is signed by "VeriSign Class 3 Code Signing 2004 CA". For Skype software built for Linux platforms, all packages are signed by PGP key ID 0xD66B746E, the public component of which may be downloaded from http://www.skype.com/download/skype/linux/. - For general information about Skype security, please visit the Skype Security Resource Center at http://www.skype.com/security/. 7. Common Vulnerability Assessment System (CVSS) assessment Skype has rated the issue covered by this Security Bulletin under the CVSS scheme as follows: Base metrics as of 2008-06-04: Access Vector (AV) ........... Network Access Complexity (AC) ....... Medium Authentication (Au) .....,.... None Confidentiality Impact (C) ... None Integrity Impact (I) ......... Complete Availability Impact (A) ...... None Computed CVSS base score: 7.1 Temporal metrics as of 2008-06-04 Exploitability (E) ........... proof-of-concept Remediation Level (RL) ....... official-fix Report Confidence (RC) ....... confirmed Computed CVSS temporal score: 5.6 Skype participates in the CVSS by rating each identifiable security vulnerability against the CVSS base metrics. In addition, Skype may rate each vulnerability against temporal metrics from time to time. As suggested by the name, temporal metrics for a particular vulnerability may change from time to time. More information about the CVSS may be obtained from the CVSS host website at http://www.first.org/cvss/. 8. Credits and additional information Skype would like to thank Ismael Briones (Inkatel.com) working with the iDefense VCP 9. Bulletin release history 2008-06-04 Initial bulletin release 10. Notices Copyright 2008 Skype Technologies, S.A. All rights reserved. This Skype Security Bulletin may be reproduced and distributed, provided that the Bulletin is not modified in any way and is attributed to Skype Technologies, S.A. and provided that repro- duction and distribution is performed for non-commercial purposes. This Skype Security Bulltin is provided to you on an "AS IS" basis and may contain information provided by third parties. Skype makes no guarantees or warranties as to the information contained herein. ANY AND ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, OR NON-INFRINGEMENT, ARE HEREBY DISCLAIMED. To report a security issue to Skype, please send an e-mail that describes the problem or vulnerability to security@skype.com. Please consider securing any reports that disclose security vulnerabilities by encrypting them using the current PGP key of the Skype Computer Emergency Response Team (SKY-CERT), PGP key ID 0xAD2DF320. ====================================================================== ========================================================= Les serveurs de référence du CERT-Renater http://www.urec.fr/securite http://www.cru.fr/securite http://www.renater.fr ========================================================= + CERT-RENATER | tel : 01-53-94-20-44 + + 151 bd de l'Hopital | fax : 01-53-94-20-41 + + 75013 Paris | email: certsvp@renater.fr + =========================================================