ISSalert: ISS Security Advisory: Citrix MetaFrame Remote Denial of Service Vulnerability

Type securityvulns
Reporter Securityvulns
Modified 2001-10-17T00:00:00


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Internet Security Systems Security Advisory October 16, 2001

Citrix MetaFrame Remote Denial of Service Vulnerability


ISS has discovered a remote Denial of Service (DoS) vulnerability in Citrix MetaFrame. Citrix MetaFrame is an application server that works with Windows Terminal Services. This vulnerability causes a MetaFrame installation to crash or "blue screen" and requires an affected system to be restarted manually. No local access is needed to exploit this vulnerability.

Affected Versions:

Citrix MetaFrame 1.8 Server with Service Pack 3 Citrix MetaFrame XP Server Citrix MetaFrame XP Server Service Pack 1


Citrix MetaFrame works with Windows Terminal Services to provide application server capabilities.

This vulnerability is caused by the improper handling of the establishment of multiple sessions in the Citrix product. An attacker can initiate multiple fake sessions with the target server by simulating the protocol used between the MetaFrame client and server. These sessions pass file name and other information from client to server before encrypted channels are established. The server allows a maximum of approximately 52 sessions to be started. After the sessions time out, new sessions that are initiated will cause the server to crash. The new sessions cause an exception that results in a blue screen. This exception, which is usually a page fault, can occur in various processes.


ISS X-Force recommends that all vulnerable MetaFrame customers download and apply the appropriate Citrix hotfix for their server platform from the Citrix Web site. To obtain this hotfix, connect to the Citrix Web site (, click the link to the Solution Knowledge Base, and select Hotfixes and Service Packs from the Additional Resources menu.

The hotfixes that apply to the vulnerability described in this Security Advisory are:

MetaFrame 1.8 with Service Pack 3: Windows NT 4.0 Terminal Server: English: ME183T012 German: MG183T012 Windows 2000: English: ME183W018 German: MG183W018 MetaFrame XP:
Hotfixes will be made available for MetaFrame XP and MetaFrame XP Service Pack 1/Feature Pack 1

Refer to the Hotfixes and Service Packs section of the Citrix Knowledge Base Web site for more information on patching your system.

ISS X-Force will provide detection and assessment support for this vulnerability in upcoming X-Press Updates for RealSecure Network Sensor and Internet Scanner.

Additional Information: The Common Vulnerabilities and Exposures (CVE) project has assigned the name CAN-2001-0716 to this issue. This is a candidate for inclusion in the CVE list (, which standardizes names for security problems.

Credits: This vulnerability was discovered and researched by Justine Bone from the ISS Consulting Solutions Group (USA) as well as Glyn Geoghegan and Paul Davies of ISS UK Security Assessment Services (SAS). Internet Security Systems would like to thank Citrix Systems for their response and handling of this vulnerability.

About Internet Security Systems (ISS) Internet Security Systems is a leading global provider of security management solutions for the Internet, protecting digital assets and ensuring safe and uninterrupted e-business. With its industry-leading intrusion detection and vulnerability assessment, remote managed security services, and strategic consulting and education offerings, ISS is a trusted security provider to more than 8,000 customers worldwide including 21 of the 25 largest U.S. commercial banks and the top 10 U.S. telecommunications companies. Founded in 1994, ISS is headquartered in Atlanta, GA, with additional offices throughout North America and international operations in Asia, Australia, Europe, Latin America and the Middle East. For more information, visit the Internet Security Systems web site at or call 888-901-7477.

Copyright (c) 2001 Internet Security Systems, Inc.

Permission is hereby granted for the redistribution of this Alert electronically. It is not to be edited in any way without express consent of the X-Force. If you wish to reprint the whole or any part of this Alert in any other medium excluding electronic medium, please e-mail for permission.


The information within this paper may change without notice. Use of this information constitutes acceptance for use in an AS IS condition. There are NO warranties with regard to this information. In no event shall the author be liable for any damages whatsoever arising out of or in connection with the use or spread of this information. Any use of this information is at the user's own risk.

X-Force PGP Key available at: as well as on MIT's PGP key server and's key server.

Please send suggestions, updates, and comments to: X-Force of Internet Security Systems, Inc.

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