NSFOCUS Security Advisory(SA2001-06)

Topic:  Microsoft IIS ssinc.dll Buffer Overflow Vulnerability

Release Date£º 2001-08-17

CVE CAN ID : CAN-2001-0506
BUGTRAQ ID : 3190

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

 - Microsoft IIS 4.0
 - Microsoft IIS 5.0

Impact:
=========

NSFOCUS Security Team has found a buffer overflow vulnerability in a dynamic
link library (ssinc.dll) of Microsoft IIS 4.0/5.0 when processing server side
include files. Exploitation of it, an attacker could obtain SYSTEM privilege.

Description£º
============

Microsoft IIS supports SSI (Server Side Include) function. IIS use ssinc.dll as
a SSI interpreter. By default setting, extensions like .stm, .shtm and .shtml
would be mapped to interpreter process (Ssinc.dll).

SSI supports "#include" directive, mostly in this form:

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

When processing "#include" directive, ssinc.dll would check for the name of
the directory under which the .shtml file resides, append it before the
include file and form a new path string.

For example:

Create a file named "test.shtml" with the following content and save it under
"wwwroot/abcd/":

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

The new path string would be "/abcd/ABCD". Ssinc.dll would copy it to a
buffer of 0x804(2052) bytes.

When obtaining Server-side include filename from test.shtml, ssinc.dll would
perform length check for it. In case that it is longer than 0x801 bytes,
ssinc.dll would cut it to 0x801 bytes and append '\0' at the end. Thus, the
include filename (including the trailing '\0') won't be longer than 0x802(2550)
bytes.

But it does not check the length of the new path string appending current
directory name. Thus, if we set the contained filename to be a string
longer than 0x801 bytes and save "test.shtml" under a directory (name of which
is longer than 9 bytes), a buffer overflow would occur and overwrite the EBP
and EIP saved in stack completely (The trailing '\0' would overwrite the first
argument).

As ssinc.dll is running in LOCAL SYSTEM context, in case that an attacker
carefully form the overflow data, he might change the procedure flow and run
arbitrary code with SYSTEM privilege.

To launch an attack, the attacker would need the following two conditions:

1. Privilege to create file or directory under Web directory.
2. Ability to access created file through Web service.


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

1. Create a file "test.shtml" with following file content:

   <!--#include file="AAAA[...]AA"-->

   Number of 'A' should be over 2049.

2. Create a directory "a" under Web directory.
   Copy "test.shtml" to "a" directory.

3. Request "test.shtml" through web browser:
   http://webhost/a/test.shtml

4. IIS would return a blank page which indicates that an overflow has occurred.
   Meanwhile the trailing '\0' has overwritten the last byte of saved EBP.

   On the contrary, in case that the contained file has a shorter name like
   'AA', IIS would return a SSI file '/a/AA' error message when receiving
   the request.



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

 1. Disable the write access to Web directory of untrusted user.
 2. Remove .shtml, .shtm and .stm mappings if SSI service is not needed.

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

2001.6.11  We informed Microsoft of this vulnerability.
2001.6.11  Microsoft replied that the bug has been reproduced.
2001.8.15  Microsoft has released one security bulletin(MS01-044) concerning
           this flaw.

The bulletin is live at :

http://www.microsoft.com/technet/security/bulletin/MS01-044.asp

Patches are available at:

. Microsoft IIS 4.0:
  http://www.microsoft.com/Downloads/Release.asp?ReleaseID=32061

. Microsoft IIS 5.0:
  http://www.microsoft.com/Downloads/Release.asp?ReleaseID=32011

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

The Common Vulnerabilities and Exposures (CVE) project has
assigned the name CAN-2001-0506 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)