NSFOCUS SA2000-07 : Microsoft IIS 4.0/5.0 CGI File Name Inspection Vulnerability

[Nsfocus Security Team <security () NSFOCUS COM>]

NSFOCUS Security Advisory(SA2000-07)

Topic: Microsoft IIS 4.0/5.0  CGI File Name Inspection Vulnerability

Release Date£º Nov 7th, 2000

CVE Candidate Numbers: CAN-2000-0886
BUGTRAQ ID : 1912

Affected system:
================

 - Microsoft IIS 4.0 (<SP6)
 - Microsoft IIS 5.0

Not affected system:
====================

 - Microsoft IIS 4.0 ( SP6/SP6a )

Impact:
=========

NSFOCUS security team has found a security flaw in Microsoft IIS 4.0/
5.0 when handling a CGI file name. Exploitation of it, attacker can
read system file and run arbitrary system command.

Description£º
============

In CGI application (.exe, .pl, .php etc.) handling, Microsoft IIS 4.0/
5.0 do not present an integrated security inspection of CGI file name,
which may cause IIS to mistakenly open or run a file if a special
character is contained in the file name.

1. Providing a malformed HTTP request that calls IIS to run a ".exe"
or ".com" program under executable directory, IIS will try to load
the program and check file existence and file type first. Attacker can
trap the loading program to check a non-requested file by inserting
a special character in the file name.

If fulfilling these terms:

 (1) Target file exists
 (2) Target file is a batch file
 (3) Target file is a plain text file longer than zero byte

IIS will automatically call "cmd.exe" to interpret it. Other part of
file name requested is pass to "cmd.exe" as parameters of the batch
file. Thus, an attacker can run arbitrary command by inserting some
characters like "&".

2. If some script interpreter(php.exe, perl.exe etc.) and relevant
mapping are installed, IIS will call them to interpret the file name
submitted by user to run the corresponding CGI script. Inserting some
special characters, attacker can trap the interpreter to open some
file outside of WEB directory. Depending on the execution method of
the interpreter, attacker may read part or even the full file content.


Exploit:
==========

[Proof of concept code will be available soon]

Workaround:
===================

  Always remove unnecessary batch files, and keep necessary batch
  files in a different driver of any executable virtual directory.

Vendor Status:
==============

Microsoft has been informed on Oct 20th, 2000.
Microsoft has released one security bulletin concerning this flaw on
Nov 6th, 2000.

The bulletin is live at :

http://www.microsoft.com/technet/security/bulletin/MS00-086.asp

Patches are available at:

. Microsoft IIS 5.0:

* English:
  http://www.microsoft.com/Downloads/Release.asp?ReleaseID=25547
* Simplified Chinese:
  http://www.microsoft.com/Downloads/Release.asp?ReleaseID=25580
* Traditional Chinese:
  http://www.microsoft.com/Downloads/Release.asp?ReleaseID=25581
* German:
  http://www.microsoft.com/Downloads/Release.asp?ReleaseID=25582
* Japanese:
  http://www.microsoft.com/Downloads/Release.asp?ReleaseID=25583



Additional Information:
========================

The Common Vulnerabilities and Exposures (CVE) project has
assigned the name CAN-2000-0886 to this issue. This is a
candidate for inclusion in the CVE list (http://cve.mitre.org),
which standardizes names for security problems.  Candidates
may change significantly before they become official CVE entries.

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==========
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NSFOCUS Security Team <security () nsfocus com>
NSFOCUS INFORMATION TECHNOLOGY CO.,LTD
(http://www.nsfocus.com)