Skype Client for Mac Chat Unicode Denial of Service

                                                scip AG Vulnerability ID 4142 (06/22/2010)
http://www.scip.ch/?vuldb.4142

I. INTRODUCTION

Skype is a very popular proprietary voice-over-ip client available for
multiple platforms.

More information is available on the official web site at the following URL:

http://www.skype.com

II. DESCRIPTION

Marc Ruef at scip AG found a denial of service vulnerability in the
current release for Apple MacOS X (version 2.8).

The application provides the possibility of sending messages to other
Skype users via the embedded chat feature.

If a vulnerable client receives a malicious message, the message and all
further messages will be received but not displayed.

It was not possible to reproduce this behavior on different version of
the Skype client for Windows. On the iPhone (Version 1.3.0.275 on iPhone
3gs) the behavior is different. A received message containing the
malicious string is shown but the content not displayed. Instead the
message box contains the hint that the message has been deleted. No
further impact could be determined.

III. EXPLOITATION

Information on how to exploit this vulnerability is included in the
initial bug report.

An attacker has to include Unicode characters in the text message sent
to the victim. The characters used for the proof-of-concept are out of
Mathematical Alphanumeric Symbols (1D400-1D7FF)

An automated toolkit to exploit this vulnerability is available but has
not been disclosed so far.

IV. IMPACT

After receiving a malicious message the attacked client is not able to
use the chat feature anymore.

Furthermore, the handling of some other elements of the application are
not possible anymore (e.g. review the chat history).

V. DETECTION

For a common environment the identification of the attack is possible
after a successful exploitation only.

Deep inspection of received data communication over the Skype channel
may be able to determine an ongoing attack attempt.

VI. SOLUTION

No workaround or solution known at the moment.

It is suggested to allow incoming chat messages from approved friends only.

VII. VENDOR RESPONSE

This issue has been posted on Jira the public issue tracking and
reporting system of Skype.

The vendor verified the existence of the issue. No further information
about bugfixing were published.

VIII. SOURCES

scip AG - Security is our Business (german)
http://www.scip.ch/

scip AG - Vulnerability Database (german)
http://www.scip.ch/?vuldb.4142

computec.ch Document Database (german)
http://www.computec.ch

Skype Jira - Original Bug Report
http://developer.skype.com/jira/browse/SCM-681

IX. DISCLOSURE TIMELINE

2010/05/09 Identification of the vulnerability
2010/05/10 Notification of Skype via Jira (bug tracking)
2010/05/11 Acknowledgement of the issue via Jira
2010/05/11 Providing step-by-step exploit procedure
2010/06/22 Public disclosure of the advisory

X. CREDITS

The vulnerability has been discovered by Marc Ruef.

Marc Ruef, scip AG, Zuerich, Switzerland
maru-at-scip.ch
http://www.scip.ch