NSFOCUS Security Advisory(SA2001-02)

Topic: Microsoft IIS CGI Filename Decode Error Vulnerability

Release Date: May 15, 2001

CVE Candidate Numbers: CVE-2001-0333

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

Microsoft IIS 4.0
Microsoft IIS 5.0

Non-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:
========

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 drive 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 drive 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 CVE-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.