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Microsoft Security Bulletin (MS00-031)
Patch Available for "Undelimited .HTR Request" and "File Fragment
Reading via .HTR" Vulnerabilities
Originally Posted: May 10, 2000
Microsoft has released a patch that eliminates two security
vulnerabilities in Microsoft(r) Internet Information Server. The
vulnerabilities could, respectively, be used to slow an affected web
server's response or to obtain the source code of certain types of
files under very restricted conditions.
Frequently asked questions regarding these vulnerabilities
and the patch can be found at
This patch eliminates two security vulnerabilities that are unrelated
except by virtue of the fact that both exist in the ISAPI extension
that provides web-based password administration via .HTR scripts.
- The "Undelimited .HTR Request" vulnerability is a denial
of service vulnerability. If a malicious user provided a
password change request that was missing an expected
delimiter, the algorithm would conduct an unbounded search.
This would prevent it from servicing additional .HTR requests,
and could also slow the overall response of the server.
- The ".HTR File Fragment Reading" vulnerability could allow
fragments of certain types of files to be read by providing a
malformed request that would cause the .HTR processing to be
applied to them. However, the vulnerability could only be
exploited under extremely restrictive conditions, and the
most valuable data in the files would be the least likely to
actually appear in the fragments sent to the user.
Neither of these vulnerabilities would allow data to be added, deleted
or changed on the server, nor would they allow any administrative
control on the server to be usurped. Although .HTR files are used to
allow web-based password administration, neither of these
vulnerabilities involve any weakness in password handling. Also, if
security best practices have been followed, and unneeded script
mappings have been removed, many customers will have removed the .HTR
script mapping and thus be unaffected by either vulnerability.
Affected Software Versions
- Internet Information Server 4.0
- Internet Information Server 5.0
- Internet Information Server 4.0:
- Internet Information Server 5.0:
Note: Additional security patches are available at the Microsoft
Please see the following references for more information related to
- Frequently Asked Questions: Microsoft Security Bulletin MS00-031,
- Microsoft Knowledge Base article Q260838 discusses the
"Undelimited .HTR Request" vulnerability and will be available
- Microsoft Knowledge Base article Q260069 discusses the "File
Fragment Reading via .HTR" vulnerabilty and will be available
- How to Change Windows NT Account Passwords Using Internet
Information Server (IIS) 4.0,
- Microsoft TechNet Security web site,
Obtaining Support on this Issue
This is a fully supported patch. Information on contacting Microsoft
Technical Support is available at
Microsoft thanks Internet Security Systems' X-force
(http://xforce.iss.net/), and David Litchfield of Cerberus Information
Security, Ltd (http://www.cerberus-infosec.co.uk/), for reporting
these vulnerabilities to us, and working with us to protect
- May 10, 2000: Bulletin Created.
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.
Last updated May 10, 2000
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