what you don't know can hurt you


Posted Jun 18, 1999


MD5 | bdbbb3bd3b777f6cf3ebd1a19c9127b0


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From xforce@iss.net Fri Jun 18 03:22:40 1999
From: X-Force <xforce@iss.net>
To: alert@iss.net
Cc: X-Force <xforce@iss.net>
Date: Thu, 17 Jun 1999 23:24:51 -0400 (EDT)
Subject: ISSalert: ISS Vulnerability Alert: Malformed HTR File Vulnerability in Microsoft Internet Information Server 4.0

TO UNSUBSCRIBE: email "unsubscribe alert" in the body of your message to
majordomo@iss.net Contact alert-owner@iss.net for help with any problems!


ISS Vulnerability Alert
June 17, 1999

Malformed HTR File Vulnerability in Microsoft Internet Information Server 4.0


Microsoft Internet Information Server (IIS) 4.0 includes the ability to
remotely administer user passwords via a web browser. IIS is a popular
web server application for Windows NT, and comprises the majority of
Windows NT based web servers. On June 16th it was reported to several
security mailing lists that excessively long requests for .HTR files will
overflow an internal buffer and allow the remote execution of arbitrary

Affected Versions

Microsoft Internet Information Server (IIS) 4.0 as installed with Option
Pack 4 on all Windows NT 4.0 systems through Service Pack 5 are vulnerable
to this exploit.


IIS 4.0 contains the ability to remotely administer user passwords via
"HTR" files. Requests for files ending with ".HTR" are passed to an
external DLL for processing. The DLL responsible for parsing HTR
requests, ISM.DLL, contains unchecked buffers thatcould allow a long
request to overflow these buffers and crash the IIS service. This hole
has also been demonstrated to allow remote execution of arbitrary code and
exploits have been made widely available.

The remote password administration feature is implemented through the
"/iisadmpwd/" directory in the server's document root via ".HTR" files.
HTTP requests made for ".HTR" files are mapped to be redirected to an
ISAPI handler DLL which will further process the request.

This vulnerability is being actively exploited on the Internet due to the
wide availability of exploitation code. Programs to remotely exploit this
vulnerability have been made available for both Unix and Windows
environments. In addition to these programs, preliminary information shows
that worms that actively exploit this hole and seek out other vulnerable
hosts have been or are currently being developed. The development of these
programs before a patch was available from Microsoft was expedited by the
decision of the vulnerability discoverers to release exploitation code.


If your computers or websites run IIS 4.0, ISS X-Force strongly suggests
removing the HTR functionality until such time as a patch is available
from Microsoft. If you see a Dr. Watson error or other exception in
ISM.DLL, someone is probably trying to attack your host, and it should be
assumed that the intrusion was successful.

Microsoft Security is developing a patch for this issue and has
recommended the following workaround until it becomes available, from
Microsoft Security Bulletin MS99-019:

1. 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.

2. Double-click "Internet Information Server."

3. Right-click the computer name and select Properties.

4. In the Master Properties drop-down box, select "WWW Service," then
click the "Edit" button.

5. Click the "Home Directory" tab, then click the "Configuration" button

6. Highlight the line in the extension mappings that contains ".HTR",
then click the "Remove" button.

7. Respond "Yes" to "Remove selected script mapping?", click OK 3 times,
and close Internet Service Manager.


This security vulnerability was discovered by the eEye Digital Security
Team and subsequently reported to Microsoft Security a week prior to its
dissemination on public mailing lists.


eEye Digital Security Team Alert AD06081999: "Retina vs. IIS4, Round 2"
at: http://www.eeye.com/database/advisories/ad06081999/ad06081999.html

Microsoft Security Bulletin (MS99-019): "Workaround Available for
"Malformed HTR Request" Vulnerability" at:

Version: 2.6.3a
Charset: noconv

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