Security Advisory

Microsoft Security Advisory 928604

Exploit Code Published Affecting the Windows Client Server Run-Time Subsystem

Published: November 16, 2006 | Updated: November 16, 2006

Microsoft is aware of published information and proof-of-concept code that attempts to exploit a possible vulnerability in the Client Server Run-Time Subsystem. At this time Microsoft has not seen any indications of active exploitation of the vulnerability. Our initial investigation of the public posting confirms that a local authenticated denial of service is possible on Windows Platforms. Microsoft has activated its emergency response process and is continuing to investigate this public report

Customers in the U.S. and Canada who believe they are affected can receive technical support from Microsoft Product Support Services at 1-866-PCSAFETY. .International customers can receive support from their local Microsoft subsidiaries. There. For more information about how to contact Microsoft for support issues, visit the International Support Web site.

Upon completion of this investigation, Microsoft will take the appropriate action to help protect our customers. This may include providing a security update through our monthly release process or providing an out-of-cycle security update, depending on customer needs.

Mitigating Factors:

  • An attacker must have valid logon credentials and be able to log on locally to exploit this vulnerability locally.


  • No workarounds have been identified for this vulnerability.

General Information


Purpose of Advisory: Notification of public posting of exploit code targeting the Windows Client Server Run-Time Subsystem.

Advisory Status: Advisory published.

Recommendation: Monitor systems for unplanned interruption of operations resulting from a system restart.

References Identification
CVE Reference CVE-2006-5996

This advisory discusses the following software.

Related Software
Microsoft Windows 2000 Service Pack 4
Microsoft XP Service Pack 2
Microsoft Windows XP Professional x64 Edition
Microsoft Windows Server 2003
Microsoft Windows Server 2003 Service Pack 1
Microsoft Windows Server 2003 with Service Pack 1 for Itanium-based Systems
Microsoft Windows Server 2003 64 Edition
Microsoft Windows Vista

Frequently Asked Questions

What is the scope of the advisory?
Microsoft is aware of public posting of exploit code targeting a possible vulnerability in the Client Server Run-Time Subsystem. This affects the software that is listed in the “Overview” section

Is this a security vulnerability that requires Microsoft to issue a security update?
Microsoft is still investigating this issue. Upon completion of this investigation, Microsoft will take the appropriate action to help protect our customers. This may include providing a security update.

How could an attacker exploit the vulnerability?
To exploit this vulnerability, an attacker would first have to log on to the system. An attacker could then run a specially crafted application that could exploit the vulnerability and cause the system to stop responding resulting in a reboot.

What is the Client Server Run-Time Subsystem?
Csrss is the user-mode portion of the Win32 subsystem (with Win32.sys being the kernel-mode portion). Csrss stands for client/server run-time subsystem and is an essential subsystem that must be running at all times. Csrss is responsible for console windows, creating and/or deleting threads.

How does this issue relate to the vulnerability that is corrected by MS06-075?
While both issues deal with Client Server Run-Time Subsystem, the published proof of concept code affects a potential new vulnerability that was not addressed as part of MS06-075. MS06-075 helps protect against the vulnerability that is discussed in that bulletin, but does not address this new potential vulnerability.

Suggested Actions

  • Keep Windows Updated

  • All Windows users should apply the latest Microsoft security updates to help make sure that their computers are as protected as possible. If you are not sure whether your software is up to date, visit the Microsoft Update Web site, scan your computer for available updates, and install any high-priority updates that are offered to you. If you have Automatic Updates enabled, the updates are delivered to you when they are released, but you have to make sure you install them.

  • Protect Your PC
    We continue to encourage customers follow our Protect Your PC guidance of enabling a firewall, getting software updates and installing anti-virus software. Customers can learn more about these steps by visiting Protect Your PC Web site.

  • For more information about staying safe on the Internet, customers can visit the Microsoft Security Home Page.

  • Customers who believe they have been attacked should contact their local FBI office or post their complaint on the Internet Fraud Complaint Center Web site. Customers outside the U.S. should contact the national law enforcement agency in their country.

    All customers should apply the most recent security updates released by Microsoft to help ensure that their systems are protected from attempted exploitation. Customers who have enabled Automatic Updates will automatically receive all Windows updates. For more information about security updates, visit the Microsoft Security Web site.

Other Information



The information provided in this advisory is provided "as is" without warranty of any kind. Microsoft disclaims all warranties, either express or implied, including the warranties of merchantability and fitness for a particular purpose. In no event shall Microsoft Corporation or its suppliers be liable for any damages whatsoever including direct, indirect, incidental, consequential, loss of business profits or special damages, even if Microsoft Corporation or its suppliers have been advised of the possibility of such damages. Some states do not allow the exclusion or limitation of liability for consequential or incidental damages so the foregoing limitation may not apply.


  • December 20, 2006: Advisory published.

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