GEN:SID 1:1226
Message X11 xopen
Summary This event is generated when an attempt to launch an X windows application on an X windows server is made.
Impact Possible theft of data and control of the targeted machine leading to a compromise of all resources the machine is connected to.
Detailed Information Implementations of the X windows system from the X consortium may use weak authentication methods when allowing remote machines to connect to a host running X windows.

XDM is used to allow remote users access to the remote X window server. When configured incorrectly, this may allow an unathorised user to connect to the display.

This event is generated when a user outside the protected network opens an X windows application on the remote X server.
Affected Systems  
Attack Scenarios The remote attacker may scan the host for listening X window servers, then connect to the remote host using XDM. The attacker may then execute an X windows application on the remote host.
Ease of Attack Simple.
Corrective Action Disallow connections to X servers from hosts outside the protected network.

Apply the appropriate vendor patches.

Upgrade to the latest version of the software.
Additional References Whitehats arachNIDS
http://www.whitehats.com/info/IDS396
Rule References arachnids: 395