The following is a Security Bulletin from the Microsoft Product Security
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Microsoft Security Bulletin (MS99-019)
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Patch Available for "Malformed HTR Request" Vulnerability
Originally Posted: June 15, 1999
Updated: June 17, 1999
Summary
=======
This is an update to Microsoft Security Bulletin MS99-019. The purpose of
the update is to advise customers of the availability of a patch that
eliminates the vulnerability originally discussed, as well as variants that
Microsoft has subsequently identified.
Microsoft has released a patch that eliminates a vulnerability in
Microsoft(r) Internet Information Server 4.0. The vulnerability could allow
denial of service attacks against an IIS server or, under certain
conditions, could allow arbitrary code to be run on the server. The patch
is fully supported, and Microsoft recommends that affected customers
download and install it, if appropriate.
Issue
=====
IIS supports several file types that require server-side processing. When a
web site visitor requests a file of one of these types, an appropriate
filter DLL processes it. A vulnerability exists in the way that .HTR, .STM
and .IDC files are processed.
The vulnerability involves an unchecked buffer in the filter DLLs for these
file types. This poses two threats to safe operation. The first is a denial
of service threat. A malformed request for an .HTR, .STM or .IDC file could
overflow the buffer, causing IIS to crash. The server would not need to be
rebooted, but IIS would need to be rebooted in order to resume service. The
second threat is that a carefully-constructed file request could cause
arbitrary code to execute on the server via a classic buffer overrun
technique. Neither attack could occur accidentally.
While there are no reports of customers being adversely affected
by this vulnerability, Microsoft is proactively releasing this
patch to allow customers to take appropriate action to protect
themselves against it. In addition, web sites that do not require
these file types can disable them altogether, as discussed in
Microsoft's IIS Security Checklist
(http://www.microsoft.com/security/products/iis/CheckList.asp)
Affected Software Versions
==========================
- Microsoft Internet Information Server 4.0
What Microsoft is Doing
=======================
Microsoft has released a patch that fixes the problem identified. The patch
is available for download from the site listed below in What Customers
Should Do.
Microsoft also has sent this security bulletin to customers
subscribing to the Microsoft Product Security Notification Service.
See http://www.microsoft.com/security/services/bulletin.asp for
more information about this free customer service.
Microsoft has published the following Knowledge Base (KB) article on this
issue:
- Microsoft Knowledge Base (KB) article Q234905,
An Improperly Formatted HTTP Request Can Cause The Inetinfo
Process To Fail,
http://support.microsoft.com/support/kb/articles/q234/9/05.asp
(Note: It might take 24 hours from the original posting of this
bulletin for the KB article to be visible in the Web-based
Knowledge Base.)
What Customers Should Do
========================
Microsoft highly recommends that customers evaluate the degree of risk that
this vulnerability poses to their systems and determine whether to download
and install the patch. The patch can be found at:
ftp://ftp.microsoft.com/bussys/IIS/iis-public/fixes/usa/ext-fix/.
More Information
================
Please see the following references for more information related to this
issue.
- Microsoft Security Bulletin MS99-019,
Patch Available for "Malformed HTR Request" Vulnerability,
(The Web-posted version of this bulletin),
http://www.microsoft.com/security/bulletins/ms99-019.asp.
- Microsoft Knowledge Base (KB) article Q234905,
An Improperly Formatted HTTP Request Can Cause The Inetinfo
Process To Fail,
http://support.microsoft.com/support/kb/articles/q234/9/05.asp.
- IIS Security Checklist,
http://www.microsoft.com/security/products/iis/CheckList.asp
Obtaining Support on this Issue
===============================
If you require technical assistance with this issue, please contact
Microsoft Technical Support. For information on contacting Microsoft
Technical Support, please see
http://support.microsoft.com/support/contact/default.asp.
Revisions
=========
- June 15, 1999: Bulletin Created.
- June 17, 1999: Bulletin updated to advise availability of patch
for .HTR, .STM, and .IDC files
For additional security-related information about Microsoft products,
please visit http://www.microsoft.com/security
------------------------------------------------------------------------
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(c) 1999 Microsoft Corporation. All rights reserved. Terms of Use.
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