SKYPE-SB/2005-003: Heap overflow in networking routine

Bulletin title:	Heap overflow in networking routine
Bulletin ID:	SKYPE-SB/2005-003
Bulletin status:	FINAL
Date of announcement:	2005-10-25 08:00:00 +0000
Date of last revision:	2005-10-27 15:06:11 +0000
Products affected:	Skype client (all platforms)
Vulnerability type:	Heap overflow
CVE references:	CVE-2005-3267
Risk assessment:	HIGH
CVSS base score:	8.0 (AV:R/AC:H/Au:NR/C:C/I:C/A:C/B:A)
Cross-references:	None

1. Problem description and brief discussion

Description

A security bug in the Skype user client, for all platforms, has been identified and fixed.

Skype can be remotely forced to crash due to an error in bounds checking in a specific networking routine.

Discussion

An attacker who sends a stream of specifically-crafted network traffic to a Skype client network can cause the client to overwrite part of the heap, including the heap integrity control data. Since the attacker cannot control the address where the data is written, the most likely effect will be that the Skype will abort execution due to an internal error, although other unpredictable behavior is possible.

Such a crash will lead to a loss of availability of the Skype application until it is restarted by the user. Skype has been able to induce Skype clients to crash, but has not been able to cause the client to execute specific instructions.

This is tracked by Mitre CVE ID CVE-2005-3267.

2. Impact and affected software

Impact

An attacked Skype client may crash.

Affected software

The following Skype clients are vulnerable to this attack:

Skype for Windows: All releases prior to and including 1.4.*.83

Skype for Mac OS X: All releases prior to and including 1.3.*.16

Skype for Linux: All releases prior to and including 1.2.*.17

Skype for Pocket PC: All releases prior to and including 1.1.*.6

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 4 of this Bulletin.)

Skype for Windows: Release 1.4.*.84 or later

Skype for Mac OS X: Release 1.3.*.17 or later

Skype for Linux: Release 1.1.*.20 or later

Skype for Pocket PC: No patch is yet available. This bulletin will be updated when it has been made available.

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.

You may install Skype by running the Skype installer using the installation commands displayed under the appropriate operating system listed at http://www.skype.com/go/download/.

x86 platform, Microsoft Windows 2000 or Microsoft Windows XP: http://www.skype.com/features/skype/windows/

x86 platform, Linux: http://www.skype.com/features/skype/linux/

PPC platform, Mac OS X v10.3 (Panther) or later: http://www.skype.com/features/skype/macosx/

Pocket PC platform, Microsoft Windows Mobile 2003: http://www.skype.com/features/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 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) ....... High
Authentication (Au) .....,.... Not Required
Confidentiality Impact (C) ... Complete
Integrity Impact (I) ......... Complete
Availability Impact (A) ...... Complete
Impact Bias (B) .............. Availability

Computed CVSS base score: 8.0

Temporal metrics as of 2005-10-25

Exploitability (E) ........... Proof of Concept
Remediation Level (RL) ....... Official Fix
Report Confidence (RC) ....... Confirmed

Computed CVSS temporal score: 6.3

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

This bug was simultaneously referred to SKY-CERT by two independent sources, one internal to Skype and one external. We would like to thank and acknowledge the external referrer, the EADS Corporate Research Center security lab, for having referred this issue to Skype.

9. Bulletin release history

2005-10-25 Initial bulletin release

2005-10-25 Corrected credit information at the request of Imad Lahoud of the EADS Corporate Research Center

2005-10-27 Updated to reflect release of updated software for Skype for PocketPC

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 Bulletin 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.