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(CERT Issues Advisory) Re: Microsoft IIS Web Server Allows Remote Users to Execute Commands on the Server Due to CGI Decoding Error
SecurityTracker Alert ID: 1001544|
SecurityTracker URL: http://securitytracker.com/id/1001544
(Links to External Site)
Date: May 15 2001
Disclosure of system information, Disclosure of user information, Execution of arbitrary code via network|
Fix Available: Yes Vendor Confirmed: Yes |
Version(s): IIS 5.0, IIS 4.0 (except when on NT 4 with SP6/SP6a[without any new hotfix])|
NSFOCUS announced discovery of a vulnerability in Microsoft Internet Information Server that allows remote users to execute commands on the server.|
NSFOCUS reports that when loading an executable CGI program, the IIS web server will perform two successive decode operations. The first is to decode the CGI filename and determine if it is an executable file. The second is to determine CGI parameters. However, the web server will (improperly) decode the file name on the second pass. As a result, a remote user can create a malformed CGI filename to circumvent normal IIS filename security filtering (such as ".." filtering) and traverse directories.
For example, NSFOCUS reports that the following URL, if the target host has a virtual executable directory called scripts, will provide a directory listing of the C:\ directory:
Malformed URLs can be used to run commands with the privileges of the IUSER_machinename account.
A remote user can execute commands on and retrieve files from the server.|
The vendor has prepared a fix. See the Vendor URL.|
Vendor URL: www.microsoft.com/technet/security/bulletin/MS01-026.asp (Links to External Site)
Windows (NT), Windows (2000)|
This archive entry is a follow-up to the message listed below.|
Source Message Contents
Date: Tue, 15 May 2001 11:06:37 -0400 (EDT)|
Subject: CERT Advisory CA-2001-12
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CERT Advisory CA-2001-12 Superfluous Decoding Vulnerability in IIS
Original release date: May 15, 2001
Last revised: --
A complete revision history is at the end of this file.
* Systems running Microsoft IIS
A serious vulnerability in Microsoft IIS may allow remote intruders to
execute commands on an IIS web server. This vulnerability closely
resembles a previous vulnerability in IIS that was widely exploited.
The CERT/CC urges IIS administrators to take action to correct this
URIs may be encoded according to RFC 2396. Among other things, this
RFC provides an encoding for arbitrary octets using the percent sign
(%) and hexadecimal characters.
Quoting from RFC 2396:
An escaped octet is encoded as a character triplet, consisting of the
percent character "%" followed by the two hexadecimal digits
representing the octet code. For example, "%20" is the escaped
encoding for the US-ASCII space character.
escaped = "%" hex hex
hex = digit | "A" | "B" | "C" | "D" | "E" | "F"
Like all web servers, Microsoft IIS decodes input URIs to a canonical
format. Thus, the following encoded string:
will get decoded to
A Filename With Spaces
Unfortunately, IIS decodes some of the input twice. The second
decoding is superfluous. Security checks are applied to the results of
the first decoding, but IIS utilizes the results of the second
decoding. If the results of the first decoding pass the security
checks and the results of the second decoding refer to a valid file,
access will be granted to the file even if it should not be. More
information is available at
Note that this does not permit intruders to bypass ACLs enforced by
the filesystem, only security checks performed by IIS. We encourage
you to configure your web server according to the guidelines provided
Theses guidelines can help you reduce your exposure to this problem,
and possibly to problems that have not yet been discovered.
This issue was discovered by NSFocus.
The CVE Project has assigned the following identifier to this
This vulnerability has many similarities to the Web Server Folder
Directory Traversal Vulnerability, which has been widely exploited.
For more information on that vulnerability, see
Intruders can run arbitrary commands with the privileges of the
Apply a patch from your vendor
Information on patches from Microsoft is available at
Additional advice on securing IIS web servers is available from
Appendix A. Vendor Information
The following documents regarding this vulnerability are available
Authors: Shawn Hernan.
This document is available from:
CERT/CC Contact Information
Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890
CERT personnel answer the hotline 08:00-17:00 EST(GMT-5) / EDT(GMT-4)
Monday through Friday; they are on call for emergencies during other
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We strongly urge you to encrypt sensitive information sent by email.
Our public PGP key is available from
If you prefer to use DES, please call the CERT hotline for more
Getting security information
CERT publications and other security information are available from
our web site
To subscribe to the CERT mailing list for advisories and bulletins,
send email to email@example.com. Please include in the body of your
* "CERT" and "CERT Coordination Center" are registered in the U.S.
Patent and Trademark Office.
Any material furnished by Carnegie Mellon University and the Software
Engineering Institute is furnished on an "as is" basis. Carnegie
Mellon University makes no warranties of any kind, either expressed or
implied as to any matter including, but not limited to, warranty of
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Conditions for use, disclaimers, and sponsorship information
Copyright 2001 Carnegie Mellon University.
May 15, 2001: Initial Release
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