Vulnerability Note VU#118892

Older SSH clients do not allow users to disable X11 forwarding

Overview

This vulnerability may allow an attacker to make unauthorized connections to affected client machines.

I. Description

Older versions of the SSH client do not allow the user to disable X11 forwarding. As a result, if the client connects to a malicious server, the server can open an X11 connection to the client's display without notifying the client. The attacker would then have access to the client's X11 session, which would permit activities such as keystroke monitoring.

It is also possible for an attacker to make X11 connections to the client by taking over the victim's TCP connection; however, this should generate a client-side warning about changed host keys. If the client responds to the prompt by breaking the connection, no keystrokes or sensitive data will be transmitted.

II. Impact

Attackers can make unauthorized connections to victim machines, potentially accessing sensitive data.

III. Solution

Upgrade to a version of SSH that allows X11 forwarding to be disabled.

Systems Affected

Vendor	Status	Date Updated
OpenSSH	Not Vulnerable	29-Oct-2001
SSH Communications Security	Vulnerable	3-Nov-2000

References

http://www.ssh.com/products/ssh/cert/

Credit

The CERT/CC thanks Antti Huima, Tuomas Aura, and Janne Salmi for their analysis and Tatu Ylonen for bringing this vulnerability to our attention.

This document was written by Jeffrey P. Lanza.

Other Information

Date Public	01/18/2001
Date First Published	10/25/2001 08:43:43 PM
Date Last Updated	03/05/2002
CERT Advisory
CVE Name
US-CERT Technical Alerts
Metric	0.42
Document Revision	6

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