TUCoPS :: Web :: IIS :: ca200112.txt

Superfluous Decoding Vulnerability in IIS

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
          Phone: +1 412-268-7090 (24-hour hotline)
          Fax: +1 412-268-6989
          Postal address:
          CERT Coordination Center
          Software Engineering Institute
          Carnegie Mellon University
          Pittsburgh PA 15213-3890
   CERT/CC 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 hours, on U.S. holidays, and on weekends.
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   Copyright 2001 Carnegie Mellon University.
   Revision History
May 15, 2001: Initial Release

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