TUCoPS :: Web :: IIS :: ciach077.txt

Microsoft IIS Boundary Cond

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[   For Public Release  ]
             __________________________________________________________

                       The U.S. Department of Energy
                    Computer Incident Advisory Capability
                           ___  __ __    _     ___
                          /       |     /_\   /
                          \___  __|__  /   \  \___
             __________________________________________________________

                             INFORMATION BULLETIN

                 Microsoft IIS Boundary Condition Vulnerability

June 26, 1997 20:00 GMT                                            Number H-77
______________________________________________________________________________
PROBLEM:       A vulnerability in Microsoft Internet Information Server (IIS)
               allows an outside user to cause an access violation on a server
               ("denial of service").
PLATFORM:      Windows NT servers running Internet Information Server 2.0
               or 3.0.
DAMAGE:        Causes an access violation (i.e., server is unresponsive).
               Unsaved data is lost.
SOLUTION:      Apply the local Microsoft patch described below.
______________________________________________________________________________
VULNERABILITY  Exploit is widely available, and attack can be successfully
ASSESSMENT:    executed remotely.
______________________________________________________________________________

Introduction:

A vulnerability in Microsoft Internet Information Server (IIS) for Windows NT
4.0 allows users to locally or remotely cause an access violation on a  Web
server of this configuration.  The result is the server crashes, and is
therefore rendered unresponsive until rebooted.  To determine whether your IIS
server has been successfully attacked, this access violation may be seen in
Event Viewer's Application log file as a Dr. Watson entry of access violation
on the INETINFO.EXE process.

Problem:

A Web browser sends a request such as a URL to the IIS Web server.  If the
request is a certain length, the sender causes an access violation on the
server. This length varies from server to server, but typically ranges between
4K and 8K.  Code is available which sends requests of varying sizes (lengths)
to the target IIS server.  Through this "trial and error", the code will
eventually send the exact size request needed to render the target server
unresponsive.

Solution:

If your server  has been successfully attacked, it must be rebooted. The
system operates normally once the system is rebooted.

Prevention:

Microsoft has developed a local patch to prevent users from successfully
accomplishing this attack.  This patch detects when the specific URL length 
is reached in IIS.  CIAC recommends that you update your Emergency Repair Disk
before you apply the patch, as the patch has not been regression tested and
therefore may not work as described.

Microsoft's patch location:

Windows NT 4.0 (CIAC recommends that Service Pack 3 is installed first):

ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/nt40/hotfixes-
postSP3/iis-fix/       

===========================================================================

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This document was prepared as an account of work sponsored by an
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