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Microsoft Security Bulletin (MS99-019)
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Workaround Available for "Malformed HTR Request" Vulnerability
Originally Posted: June 15, 1999
Summary
=======
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.
Microsoft has issued this bulletin to advise customers of steps they can
take to protect themselves against this vulnerability. A patch to eliminate
this vulnerability is being developed, and an update to this bulletin will
be released to advise customers when it is available.
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 ISM.DLL, the filter DLL
that processes .HTR files. HTR files enable remote administration of user
passwords.
The vulnerability involves an unchecked buffer in ISM.DLL. This poses two
threats to safe operation. The first is a denial of service threat. A
malformed request for an .HTR file could overflow the buffer, causing IIS
to crash. The server would not need to be rebooted, but IIS would need to
be restarted. The second threat would be more difficult to exploit. A
carefully-constructed file request could cause arbitrary code to execute on
the server via a classic buffer overrun technique. Neither scenario could
occur accidentally. This vulnerability does not involve the functionality
of the password administration features of .HTR files.
While there are no reports of customers being adversely affected by this
vulnerability, Microsoft is proactively releasing this bulletin to allow
customers to take appropriate action to protect themselves against it.
Affected Software Versions
==========================
- Microsoft Internet Information Server 4.0
What Microsoft is Doing
=======================
Microsoft has provided a workaround that fixes the problem identified. The
workaround is discussed 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.
What Customers Should Do
========================
Microsoft highly recommends that customers disable the script mapping for
.HTR files as follows:
- From the desktop, start the Internet Service Manager
by clicking Start | Programs | Windows NT 4.0 Option
Pack | Microsoft Internet Information Server | Internet
Service Manager
- Double-click "Internet Information Server"
- Right-click on the computer name and select Properties
- In the Master Properties drop-down box, select "WWW Service",
then click the "Edit" button .
- Click the "Home Directory" tab, then click the "Configuration"
button .
- Highlight the line in the extension mappings that contains ".HTR",
then click the "Remove" button.
- Respond "yes" to "Remove selected script mapping?" say yes,
click OK 3 times, close ISM
A patch will be available shortly to eliminate the vulnerability altogether.
Customers should monitor http://www.microsoft.com/security for an
announcement when the patches are available.
Microsoft recommends that customers review the IIS Security Checklist at
http://www.microsoft.com/security/products/iis/CheckList.asp
More Information
================
Please see the following references for more information related to this
issue.
- Microsoft Security Bulletin MS99-019,
Workaround Available for "Malformed HTR Request" Vulnerability
(The Web-posted version of this bulletin),
http://www.microsoft.com/security/bulletins/ms99-019.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.
For additional security-related information about Microsoft products,
please visit http://www.microsoft.com/security
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