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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
Overview
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
vulnerability.
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:
A%20Filename%20With%20Spaces
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
http://www.microsoft.com/technet/security/bulletin/MS01-026.asp
http://www.nsfocus.com/english/homepage/sa01-02.htm
http://www.kb.cert.org/vuls/id/789543
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
in
http://www.microsoft.com/technet/security/iis5chk.asp
http://www.microsoft.com/technet/security/iischk.asp
http://www.microsoft.com/technet/security/tools.asp
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
vulnerability:
http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2001-0333
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
http://www.kb.cert.org/vuls/id/111677
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
http://www.microsoft.com/technet/security/bulletin/MS01-026.asp
Additional advice on securing IIS web servers is available from
http://www.microsoft.com/technet/security/iis5chk.asp
http://www.microsoft.com/technet/security/tools.asp
Appendix A. Vendor Information
Microsoft Corporation
The following documents regarding this vulnerability are available
from Microsoft:
http://www.microsoft.com/technet/security/bulletin/MS01-026.asp
Authors: Shawn Hernan.
______________________________________________________________________
This document is available from:
http://www.cert.org/advisories/CA-2001-12.html
______________________________________________________________________
CERT/CC Contact Information
Email: This email address is being protected from spambots. You need JavaScript enabled to view it.
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
U.S.A.
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
http://www.cert.org/CERT_PGP.key
If you prefer to use DES, please call the CERT hotline for more
information.
Getting security information
CERT publications and other security information are available from
our web site
http://www.cert.org/
To subscribe to the CERT mailing list for advisories and bulletins,
send email to This email address is being protected from spambots. You need JavaScript enabled to view it.. Please include in the body of your
message
subscribe cert-advisory
* "CERT" and "CERT Coordination Center" are registered in the U.S.
Patent and Trademark Office.
______________________________________________________________________
NO WARRANTY
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
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