DOCUMENT:Q238600 TITLE :Multiple Connection Requests Promote Denial of Service PRODUCT :Windows NT Server, Terminal Server Edition PROD/VER:4.0 OPER/SYS:WINDOWS NT KEYWORD :kbbug4.00 kbfix4.00 ------------------------------------------------------------------------------- The information in this article applies to: - Microsoft Windows NT Server version 4.0, Terminal Server Edition ------------------------------------------------------------------------------- SYMPTOMS ======== When a request to open a new terminal connection is received by a Terminal Server, the server undertakes a resource-intensive series of operations to prepare for the connection. It does this before authenticating the request. This would allow an attacker to mount a denial of service attack by levying a large number of connection requests and consuming all memory on the Terminal Server. This vulnerability could be exploited remotely if connection requests are not filtered. In extreme cases, the server could crash in the face of such an attack; in other cases, normal processing would return when the attack ceased. The patch works by causing the server to require authentication before processing the connection request. CAUSE ===== During the connection setup there is no control over CPU resource utilization. Simultaneous multiple connection requests can prevent the server from responding to other connection requests. 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://www.microsoft.com/support/supportnet/overview/overview.asp The English version of this fix should have the following file attributes or later: Date Time Size File name Platform ---------------------------------------------------- 07/22/99 03:53p 134,928 Termsrv.exe x86 07/22/99 03:58p 242,448 Termsrv.exe Alpha This hotfix has been posted to the following Internet location as Tsememfxi.exe (x86) and Tsememfxa.exe (Alpha): ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/NT40tse/ hotfixes-postSP4/Flood-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 WORKAROUND ========== To work around this issue, you can filter Transmission Control Protocol (TCP) packets. Terminal Server listens for connection requests on port 3389. If you create a filter that allows only specific TCP/IP addresses or networks to access the Terminal Server, it may be possible to prevent this condition from occurring. For additional information about TCP filters, click the article numbers below to view the articles in the Microsoft Knowledge Base: Q169548 Using Proxy Server with Routing and Remote Access Q166371 NT 4.0 Does Not Filter Ports Destined for Remote Segments Q187628 Using Telnet to Test Port 3389 Functionality Q191146 How to Create a DMZ Network with Proxy Server 2.0 STATUS ====== Microsoft has confirmed this to be a problem in Windows NT Server 4.0, Terminal Server Edition. MORE INFORMATION ================ For more information concerning Windows NT and security issues, please visit the following Microsoft Web site: http://www.microsoft.com/security/ Additional query words: tse wts ============================================================================ THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. 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