Advisory CA-2001-12

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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.

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This document is available from:
http://www.cert.org/advisories/CA-2001-12.html

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CERT/CC Contact Information

Email: [email protected]
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/

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Revision History
May 15, 2001: Initial Release

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