Archive of CERT general posting, CERT Advisory CA-2001-12

16/05/01, CERT Advisory CA-2001-12
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Subject: CERT Advisory CA-2001-12
From: CERT Advisory <>
Date: Tue, 15 May 2001 11:07:35 -0400 (EDT)
Mail-from: From Wed May 16 00:48:20 2001
Organization: CERT(R) Coordination Center - +1 412-268-7090


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

          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 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.

Using encryption

   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 Please include in the body of your

   subscribe cert-advisory

   * "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
   fitness for a particular purpose or merchantability, exclusivity or
   results obtained from use of the material. Carnegie Mellon University
   does not make any warranty of any kind with respect to freedom from
   patent, trademark, or copyright infringement.

   Conditions for use, disclaimers, and sponsorship information

   Copyright 2001 Carnegie Mellon University.

   Revision History
May 15, 2001: Initial Release

Version: PGPfreeware 5.0i for non-commercial use
Charset: noconv


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