Multiple Connection Requests Promote Denial of Service Attack

• 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 computer, the server undertakes a resource-intensive series of operations to prepare for the connection. The server performs these operations before authenticating the request, thereby 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

This problem occurs because during the connection setup, there is no control over CPU resource usage. 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

   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 

ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/NT40tse/hotfixes-postSP4/Flood-fix/

WORKAROUND

To work around this problem, you can filter Transmission Control Protocol (TCP) packets. Terminal Server monitors connection requests on port 3389. If you create a filter that allows only specific TCP/IP addresses or networks to gain access to 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:

Keywords : kbnetwork ntsecurity ntsp kbbug4.00 kbfix4.00
Version : winnt:4.0
Platform : winnt
Issue type : kbbug