NSFOCUS Security Advisory(SA2000-08)

Topic: Microsoft IIS for Far East Editions File Disclosure Vulnerability

Release Date£ºDec 13rd, 2000

CVE Candidate Numbers: CAN-2000-1090

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

 - Microsoft IIS 4.0 for Far East editions ( < SP6 )
 - Microsoft IIS 5.0 for Far East editions (Chinese (Traditional and 
                     Simplified), Japanese, and Korean (Hangeul))
Non-affected system:
==================

 - Microsoft IIS 4.0 for Far East editions( SP6/SP6a )
 - Microsoft IIS 5.0/4.0 other editions

Impact: 
=========


NSFOCUS security team has found a security flaw in Microsoft IIS 4.0/5.0 
(Far East editions) when responding to a HTTP request containing incomplete
double-byte characters(DBCS). It could lead to exposure of files under Web
directory to a remote attacker.

Description£º
============

Microsoft IIS for Far East editions include Chinese (Traditional and 
Simplified), Japanese, and Korean (Hangeul), all of which use
double-byte character set(DBCS). When IIS receives a HTTP request
with non-ASCII character in the file name, it will check if  it is a
lead-byte(Lead-byte ranges: 0x81 - 0x9F, 0xE0 - 0xFC). If it is, then 
IIS go on checking for a trail-byte. And if a trail-byte is not available, 
IIS will simply drop the lead-byte. All this will result in an opening of 
a different file.

Submitting a malformed URL, attacker can entrap IIS to call some certain
ISAPI DLL, open some kinds of file that it can not interpret. attacker 
may obtain the content of file like plain text file(.asp, .ini, .asa 
etc.)or binary system file(.exe) under Web or virtual directory.

This problem has been solved in IIS 4.0 with SP6 before it comes forth
again in IIS 5.0.

Other IIS 4.0/5.0 including English versions are unaffected.


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

[Proof of concept code will be available soon]

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

1¡¢Remove unnecessary ISAPI mapping like HTR, HTW,IDQ etc.
    
2¡¢Turn on "Check that file exists" option in every necessary 
   ISAPI mapping.
   
3¡¢If you are using vulnerable IIS 4.0 with prior SP6, update to SP6.

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

Microsoft has been informed on Oct 8, 2000. 

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

The Common Vulnerabilities and Exposures (CVE) project has 
assigned the name CAN-2000-1090 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)