Vulnerability Note VU#789543

IIS decodes filenames superfluously after applying security checks

Overview

Microsoft IIS decodes filenames after applying security checks, allowing an attacker to execute commands.

I. Description

To accomodate complex URIs, RFC 2396 specifies a means to encode arbitrary octets using hexadecimal characters and the percent sign (%). 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"

This makes it possible to express certain characters and strings unambiguosly, but it requires the server to decode the information provided in a URI. Thus, if an HTTP server is provided a reference to a filename encoded as follows:

A%20Filename%20With%20Spaces

It will decode each occurence of %20 to a space, and then it will try to find a file named

A Filename With Spaces

This is a fundamental feature of HTTP servers, and is not specific to IIS in any way.

Any octet can be encoded in this way, including "normal" characters like "." (dot). Thus an intruder could attempt to access files outside of the web folders by encoding relative path references in this scheme. IIS includes checks for "illegal" URIs even after they have been decoded.

However, IIS decodes filenames twice. After the first decoding, IIS applies security checks to see if access to the specified file is permitted. The second decoding, which is superfluous, may result in a reference to a file outside the web folders, and after the security checks (on the results of the first decoding) have been applied.

For more information, see Microsoft Security Bulletin MS01-026.

II. Impact

Remote users can execute arbitrary commands with the privileges of the IUSR_machinename account.

III. Solution

Update your version of IIS according to the instructions in http://www.microsoft.com/Downloads/Release.asp?ReleaseID=29787 (for IIS 4.0) and http://www.microsoft.com/Downloads/Release.asp?ReleaseID=29764 (for IIS 5.0).

Systems Affected

Vendor	Status	Date Updated
Microsoft	Vulnerable	15-May-2001

References

http://www.microsoft.com/technet/security/bulletin/MS01-026.asp
http://www.microsoft.com/Downloads/Release.asp?ReleaseID=29787
http://www.microsoft.com/Downloads/Release.asp?ReleaseID=29764
http://www.nsfocus.com/english/homepage/sa01-02.htm
http://www.microsoft.com/technet/security/mbrsrvcl.asp
http://www.microsoft.com/technet/security/iis5chk.asp
http://www.microsoft.com/technet/security/iischk.asp
http://www.securityfocus.com/bid/2708

Credit

Our thanks to NSfocus who originally discovered this problem. Their original advisory can be found at http://www.nsfocus.com/english/homepage/sa01-02.htm.

This document was written by Shawn V. Hernan.

Other Information

Date Public	05/14/2001
Date First Published	05/15/2001 01:26:07 AM
Date Last Updated	09/18/2001
CERT Advisory
CVE Name	CAN-2001-0333
US-CERT Technical Alerts
Metric	79.31
Document Revision	12

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