Vulnerability Note VU#118892

Older SSH clients do not allow users to disable X11 forwarding

Original Release date: 25 Oct 2001 | Last revised: 05 Mar 2002


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


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.


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


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

Systems Affected (Learn More)

VendorStatusDate NotifiedDate Updated
SSH Communications SecurityAffected-03 Nov 2000
OpenSSHNot Affected-29 Oct 2001
If you are a vendor and your product is affected, let us know.

CVSS Metrics (Learn More)

Group Score Vector
Base N/A N/A
Temporal N/A N/A
Environmental N/A N/A



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

  • CVE IDs: Unknown
  • Date Public: 18 Jan 2001
  • Date First Published: 25 Oct 2001
  • Date Last Updated: 05 Mar 2002
  • Severity Metric: 0.42
  • Document Revision: 6


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