SKYPE-SB/2006-001: Improper handling of URI arguments

Bulletin title:	Improper handling of URI arguments
Bulletin ID:	SKYPE-SB/2006-001
Bulletin status:	FINAL
Date of announcement:	2006-05-19 08:00:00 +0000
Products affected:	Skype for Windows
Vulnerability type:	Argument handling
CVE references:	CVE-2006-2312
Risk assessment:	MEDIUM
CVSS base score:	3.5 (AV:R/AC:L/Au:NR/C:P/I:N/A:N/B:C)
Cross-references:	None

1. Problem description and brief discussion

Description

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

In some circumstances, a Skype URL can be crafted that, if followed, initiates the transfer of a single named file to another Skype user.

Discussion

An attacker who constructs a Skype URL that is malformed in a specific way can initiate the transfer of a single named file from one Skype user to another, provided that the sender follows the malicious link and that the recipient has previously authorized the sender.

This behavior is due to incorrect parsing of the parameters passed by the URI handler. The vulnerability depends on several factors, including host configuration and the authorization relationship of the sender and the receiver.

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 the initiation of a file transfer, which will be accompanied by the normal Skype file transfer dialogue box. If a file transfer is started, it will be visible to the user and may be cancelled by the sender by selecting "Cancel" in the normal way.

This is tracked by Mitre CVE ID CVE-2006-2312.

2. Impact and affected software

Impact

An attacked Skype user may send a specifically named file to another user without having explicitly consented to the action.

Affected software

The following Skype clients are vulnerable to this attack:

Skype for Windows: All releases prior to and including 2.0.*.104 Release 2.5.*.0 to and including 2.5.*.78

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: Skype 2.5, release 2.5.*.79 or later Skype 2.0, release 2.0.*.105 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/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:

- 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) ... Partial
Integrity Impact (I) ......... None
Availability Impact (A) ...... None
Impact Bias (B) .............. Confidentiality

Computed CVSS base score: 3.5

Temporal metrics as of 2006-05-19

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

Computed CVSS temporal score: 2.9

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 Brett Moore of Security- Assessment.com Ltd of New Zealand for having referred this problem to Skype.

9. Bulletin release history

2006-05-19 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.