Exceeding MaxRequestThreads May Cause Windows NT to Hang

• Microsoft Windows NT Workstation version 4.0
• Microsoft Windows NT Server version 4.0
• Microsoft Windows NT Server, Enterprise Edition version 4.0

SYMPTOMS

On a computer running Windows NT, a denial of service condition exists if a malicious process is run locally or if an improperly written service is installed. If more than the maximum number (16 by default) of processes that require user input are started without receiving the corresponding input, the server may stop responding (hang).

CAUSE

This vulnerability affects Client Server Runtime Subsystem (CSRSS), which is otherwise known as the Win32 subsystem. CSRSS provides Windows NT services to client processes running on the local computer. When a client process requests a Win32 service, CSRSS generates a worker thread to service the request. If all worker threads are occupied, the request is queued until a thread completes its work and becomes available.

The underlying vulnerability in this issue is how the CSRSS service in Windows NT handles requests that require user input. A worker thread that needs user input remains occupied until it receives input. If all worker threads in CSRSS are servicing requests that require user input, all threads are occupied but idle until they receive the required input, effectively causing the computer to hang until the user input is provided. After the user input is received, processing returns to normal.

The patch listed below eliminates this vulnerability by ensuring that the last available worker thread is never used to process a request that requires user input.

RESOLUTION

To resolve this problem, obtain the latest service pack for Windows NT 4.0 or the individual software update. For information on obtaining the latest service pack, please go to:

• http://www.microsoft.com/windows/servicepacks/


-or-


• Q152734 How to Obtain the Latest Windows NT 4.0 Service Pack

http://www.microsoft.com/support/supportnet/overview/overview.asp

ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/NT40/hotfixes-postSP5/Csrss-fix/

STATUS

Microsoft has confirmed this to be a problem in the Microsoft products listed at the beginning of this article. This problem was first corrected in Windows NT 4.0 Service Pack 6.

MORE INFORMATION

It is important to understand that CSRSS only services requests made by other processes on the same local computer, so the computers primarily at risk are those that allow users to log on interactively with assigned rights to run a program or remotely install a service. Specifically, servers would be the most likely targets of such an attack.

Windows NT Workstation may not be affected, because the user can always provide the information that's requested, and prevent the supply of worker threads from being exhausted. Windows NT 4.0, Terminal Server Edition, is not affected by this vulnerability as each user session has its own pool of worker threads and this issue would only affect individual Terminal Server clients. A Terminal Server client (like NT Workstation) can clear the requests for user input and prevent the denial of service condition. The enforcement of recommended security practices can prevent anyone but administrators from interactively logging on to servers or installing a service that can cause this problem.

For additional security-related information about Microsoft products, please visit the following Microsoft Web site:

http://www.microsoft.com/security/

Additional query words:

Keywords : kbenv kbprg ntsecurity kbbug4.00 NT4SP6Fix
Version : winnt:4.0
Platform : winnt
Issue type : kbbug