SKYPE-SB/2006-002: Improper handling of URI arguments

Bulletin title:	Improper handling of URI arguments
Bulletin ID:	SKYPE-SB/2006-002
Bulletin status:	FINAL
Date of announcement:	2006-10-03 15:00:00 +0000
Products affected:	Skype for Mac
Vulnerability type:	Argument handling
CVE references:	CVE-2006-5084
Risk assessment:	HIGH
CVSS base score:	10.0 (AV:R/AC:L/Au:NR/C:C/I:C/A:C/B:N)
Cross-references:	http://www.securityfocus.com/bid/20218 http://securitytracker.com/id?1016966

1. Problem description and brief discussion

Description

A security bug in the Skype for Mac user client has been identified and fixed.

In some circumstances, a Skype URL can be crafted that, if followed, could cause the execution of arbitrary code on the platform on which Skype is running.

Discussion

An attacker who constructs a Skype URL that is malformed in a specific way can cause Skype to crash or, in some cases, allow for the execution of arbitrary instructions. For the vulnerability to be triggered, the target must follow a malformed Skype URL.

This behavior is due to incorrect handling of arguments passed to a function in the Skype URI handler that initializes an alert panel. In some cases the function prototype is not respected, allowing for the format parameter to be filled from the URI string itself instead of being of a fixed format. This, in turn, can allow the kinds of format string vulnerabilities outlined in the following document:

https://buildsecurityin.us-cert.gov/daisy/bsi/articles/knowledge/guidelines/340.html

The attack requires the targeted user to manually follow a specially crafted malformed link, such as on a web page. Depending on several factors, doing so may result in an application crash, possibly succeeded by the execution of arbitrary code.

This is tracked by Mitre CVE ID CVE-2006-5084.

2. Impact and affected software

Impact

A user of Skype for Mac who follows a specially-crafted URL may experience a crash of the Skype software and possibly may execute arbitrary code without consent.

Affected software

The following Skype clients are vulnerable to this attack:

Skype for Mac: All releases prior to and including 1.5.*.79

3. Solution or work-around

An official fix to the issue covered by this Security Bulletin has been released. To implement this fix, update to one of the following releases of Skype. (Downloading instructions are shown in Section 5 of this Bulletin.)

Skype for Mac: Skype 1.5, release 1.5.*.80 or later

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/windowsmobile/

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 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/features/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:

Access Vector (AV) ........... Remote
Access Complexity (AC) ....... Low
Authentication (Au) .....,.... Not Required
Confidentiality Impact (C) ... Complete
Integrity Impact (I) ......... Complete
Availability Impact (A) ...... Complete
Impact Bias (B) .............. Normal

Computed CVSS base score: 10.0

Temporal metrics as of 2006-10-03

Exploitability (E) ........... Functional
Remediation Level (RL) ....... Official Fix
Report Confidence (RC) ....... Confirmed

Computed CVSS temporal score: 8.3

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 and credit Tom Ferris of Security- Protocols.com for having referred this problem to Skype.

9. Bulletin release history

2006-10-03 Initial bulletin release

10. Notices

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.