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NSFOCUS Security Advisory(SA2003-05)

Topic: Microsoft IIS ssinc.dll Over-long Filename Buffer Overflow Vulnerability

Release Date: 2003-5-30

CVE CAN ID: CAN-2003-0224

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

- - Microsoft IIS 5.0

Unaffected system:
======================
- - Microsoft IIS 4.0
- - Microsoft IIS 5.1

Summary:
=========

NSFOCUS Security Team has found a buffer overflow vulnerability in the way
a dynamic linking library (ssinc.dll)that is shipped with Microsoft IIS 4.0/5.0
handles the files it contains. Exploiting the vulnerability local attackers
could gain local system privilege.

Description:
============

Microsoft IIS supports SSI(Server Side Include) functionality. ssinc.dll is
a SSI interpreter. By default extended name .stm,.shtm and .shtml will be
mapped to interpreter(ssinc.dll).

SSI supports "#include" command. Typically it is used as the following:

<!--#include file="filename"-->

When handling "#include" command ssinc.dll will first attempt to gain the 
physical path of the shtml file, so it will copy the URI request to a buffer
the size of which is fixed. For example, when requesting the following 
shtml file:
http://iishost/abc/test.shtml

It will copy "/abc/test.shtml" to a buffer the size of which is fixed. Because
the length of the shtml filename has not been checked during the copying,
it causes a stack buffer overflow.

NSFocus Security Team has found a similar problem. See
http://www.nsfocus.net/index.php?act=advisory&do=view&adv_id=17
The corresponding Microsoft Security Bulletin is:
http://www.microsoft.com/technet/security/bulletin/ms01-044.asp

This vulnerability occurred before that one.

IIS has checked the over-long shtml filename to avoid buffer overflow,
while its method is to shorten the filename to legal length, then attempt
to open it. After that ssinc.dll will still use the original URI to finish
the copy. Therefore, by creating a WEB file with special length local attackers
could bypass the check, then request a shtml file with an over-long filename
to cause a buffer overflow, which would cause IIS cease to respond. By carefully
crafting the overflow data attackers could run arbitrary code with Local System
privilege.

Two conditions are required to carry out the attack:

1. Attackers need to have the privilege to create files on web directory.
2. Attackers need to be able to access the created files via web service.

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

1. Disable untrusted users' writing privilege to web directory.
2. If SSI functionality is not required, remove .shtml, .shtm, .stm mapping.
3. Install IIS Lockdown tools provided by Microsoft:
   http://www.microsoft.com/downloads/details.aspx?familyid=dde9efc0-bb30-
   47eb-9a61-fd755d23cdec&displaylang=en

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

2002.11.05  Inform vendor about the issue
2003.05.28  Microsoft has issued a Security Bulletin(MS03-018) and the related patch.

Detailed Microsoft Security Bulletin is available at:

http://www.microsoft.com/technet/security/bulletin/ms03-018.asp

Patches are available at:

http://microsoft.com/downloads/details.aspx?FamilyId=2F5D9852-4ADD-44F8-8715-AC3D7D7D94BF

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

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CAN-2003-0224 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.

Credit:
=========

This vulnerability was discovered by Ren Guang Yuan of NSFOCUS Security Team.

DISCLAIMS:
==========
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DISTRIBUTION OR REPRODUCTION OF THE INFORMATION IS PROVIDED THAT THE
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Copyright 1999-2003 NSFOCUS. All Rights Reserved. Terms of use.


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

PGP Key: http://www.nsfocus.com/homepage/research/pgpkey.asc
Key fingerprint = F8F2 F5D1 EF74 E08C 02FE  1B90 D7BF 7877 C6A6 F6DA
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