AUSCERT External Security Bulletin Redistribution

                 ESB-2001.204 -- CERT Advisory CA-2001-12
                 Superfluous Decoding Vulnerability in IIS
                                16 May 2001


        AusCERT Security Bulletin Summary

Product:                IIS 4.0
                        IIS 5.0
Vendor:                 Microsoft
Operating System:       Windows NT
                        Windows 2000
Impact:                 Execute Arbitrary Code/Commands
Access Required:        Remote

Ref:                    ESB-2001.203

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CERT Advisory CA-2001-12 Superfluous Decoding Vulnerability in IIS

   Original release date: May 15, 2001
   Last revised: --
   Source: CERT/CC

   A complete revision history is at the end of this file.

Systems Affected

     * 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

I. Description

   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


II. Impact

   Intruders can run arbitrary commands with the privileges of the
   IUSR_machinename account.

III. Solutions

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

Microsoft Corporation

   The following documents regarding this vulnerability are available
   from Microsoft:


   Authors:  Shawn Hernan.

   This document is available from:

CERT/CC Contact Information

   Email: cert@cert.org
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          Fax: +1 412-268-6989
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   Any material furnished by Carnegie Mellon University and the Software
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   Copyright 2001 Carnegie Mellon University.

   Revision History
May 15, 2001: Initial Release

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