NSFBUGTRAQOCUS Security Advisory(SA2001-02)

Topic:  Microsoft IIS CGI Filename Decode Error Vulnerability

Release Date£º 2001-5-15

CVE Candidate Numbers: CAN-2001-0333
BUGTRAQ ID : 2708

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

 - Microsoft IIS 4.0 
 - Microsoft IIS 5.0 

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

 - Microsoft IIS 4.0 
    + Microsoft Windows NT 4 + SP6/SP6a(without any new hotfix)

Impact: 
=========

NSFOCUS Security Team has found a vulnerability in filename processing 
of CGI program in MS IIS4.0/5.0. CGI filename is decoded twice by error. 
Exploitation of this vulnerability, intruder may run arbitrary system 
command.

Description£º
============

When loading executable CGI program, IIS will decode twice. First, CGI 
filename will be decoded to check if it is an executable file (for 
example, '.exe' or '.com' suffix check-up). Successfully passing the 
filename check-up, IIS will run another decode process. 
Normally, only CGI parameters should be decoded in this process. But 
this time IIS mistakenly decodes both CGI parameters and the decoded 
CGI filename. In this way, CGI filename is decoded twice by error.
 
With a malformed CGI filename, attacker can get round IIS filename 
security check-ups like '../' or './' check-up. In some cases, attacker
can run arbitrary system command.

For example, a character '\' will be encoded to "%5c". And the corresponding 
code of these 3 characters is:
'%' = %25
'5' = %35
'c' = %63

encode this 3 characters for another time, we can get many results such as:
%255c
%%35c
%%35%63 
%25%35%63
...

Thereby, '..\' can be represented by '..%255c' and '..%%35c', etc.
After first decoding, '..%255c' is turned into '..%5c'. IIS will take it as a 
legal character string that can pass security check-up. But after a second
decode process, it will be reverted to '..\'. Hence, attacker can use '..\'
to carry out directory traversal and run arbitrary program outside of Web 
directory.


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

For example, TARGET has a virtual executable directory (e.g. "scripts") 
that is located on the same driver of Windows system. Submit request like
this:

http://TARGET/scripts/..%255c..%255cwinnt/system32/cmd.exe?/c+dir+c:\

Directory list of C:\ will be revealed.

Of course, same effect can be achieved by this kind of processing to '/' 
and '.'.
For example: "..%252f", ".%252e/"...

Note: Attacker can run commands of IUSER_machinename account privilege only.


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

  1¡¢If executable CGI is not integrant, delete the executable virtual 
     directory like /scripts etc.
  2¡¢If executable virtual directory is needed, we suggest you to assign a 
     separate local driver for it.
  3¡¢Move all command-line utilities to another directory that could be used 
     by an attacker, and forbid GUEST group access those utilities. 


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

2001.3.27  We informed Microsoft of this vulnerability.
2001.4.01  Microsoft replied that the bug has been reproduced.
2001.4.16  Microsoft supplied private patches for testing and  the 
           problem had been solved. 
2001.4.23  For further testing, Microsoft requested us to hold off the 
           release of the advisory for 2 weeks.
2001.4.30  Microsoft informed us that we should put off the release 
           for another week.
2001.5.14  Microsoft has released one security bulletin(MS01-026) 
           concerning this flaw.

The bulletin is live at :

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

Patches are available at:
 
 Microsoft IIS 4.0:
http://www.microsoft.com/Downloads/Release.asp?ReleaseID=29787 

 Microsoft IIS 5.0:
http://www.microsoft.com/Downloads/Release.asp?ReleaseID=29764 

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

The Common Vulnerabilities and Exposures (CVE) project has 
assigned the name CAN-2001-0333 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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Copyright 1999-2001 NSFOCUS. All Rights Reserved. Terms of use.


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