GEN:SID 1:1225
Message X11 MIT Magic Cookie detected
Summary This event is generated when an attempt to exploit a weakness in the authentication mechanism used to connect to 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.
Affected Systems  
Attack Scenarios The remote attacker may scan the host for listening X window servers, then connect to the remote host using XDM.
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 CVE:
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-1999-0241

CIAC:
http://ciac.llnl.gov/ciac/bulletins/g-04.shtml

Whitehats arachNIDS
http://www.whitehats.com/info/IDS396
Rule References arachnids: 396