CERT^® Advisory CA-2001-12 Superfluous Decoding Vulnerability in IIS

A complete revision history is at the end of this file.

Systems Affected

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

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

Intruders can run arbitrary commands with the privileges of the IUSR_machinename account.

III. Solutions

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

http://www.microsoft.com/technet/security/bulletin/MS01-026.asp

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
U.S.A.

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.

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/

* "CERT" and "CERT Coordination Center" are registered in the U.S. Patent and Trademark Office.

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Copyright 2001 Carnegie Mellon University.

Revision History

May 15, 2001: Initial Release