DOCUMENT:Q233323 TITLE :Exceeding MaxRequestThreads May Cause Windows NT to Hang PRODUCT :Windows NT PROD/VER:4.0 OPER/SYS:WINDOWS NT KEYWORD :kbbug4.00 kbfix4.00 ------------------------------------------------------------------------------- The information in this article applies to: - 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 ========== A supported fix that corrects this problem is now available from Microsoft, but it has not been fully regression tested and should be applied only to systems experiencing this specific problem. If you are not severely affected by this specific problem, Microsoft recommends that you wait for the next Windows NT 4.0 service pack that contains this fix. To resolve this problem immediately, contact Microsoft Product Support Services to obtain the fix. For a complete list of Microsoft Product Support Services phone numbers and information on support costs, please go to the following address on the World Wide Web: http://support.microsoft.com/support/supportnet/default.asp The English version of this fix should have the following file attributes or later: Date Time Size File name Platform --------------------------------------------------- 05/24/99 07:34p 29,968 Csrsrv.dll x86 05/24/99 07:33p 49,424 Csrsrv.dll Alpha This hotfix has been posted to the following Internet location as Csrssfxi.exe (x86) and Csrssfxa.exe (Alpha): ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/NT40/ hotfixes-postSP5/Csrss-fix/ NOTE: If this product was already installed on your computer when you purchased it from the Original Equipment Manufacturer (OEM) and you need this fix, please call the Pay Per Incident number listed on the above Web site. If you contact Microsoft to obtain this fix, and if it is determined that you only require the fix you requested, no fee will be charged. However, if you request additional technical support, and if your no-charge technical support period has expired, or if you are not eligible for standard no-charge technical support, you may be charged a non-refundable fee. For more information about eligibility for no-charge technical support, see the following article in the Microsoft Knowledge Base: Q154871 Determining If You Are Eligible for No-Charge Technical Support STATUS ====== Microsoft has confirmed this to be a problem in the Microsoft products listed at the beginning of this article. 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: ============================================================================ THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE 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.