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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
Operating System: Windows NT
Impact: Execute Arbitrary Code/Commands
Access Required: Remote
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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
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Any material furnished by Carnegie Mellon University and the Software
Engineering Institute is furnished on an "as is" basis. Carnegie
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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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