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ASP.DLL Include File Buffer Overflow
From: "Brett Moore" <brett.moore () security-assessment com>
Date: Wed, 19 Jul 2006 10:09:03 +1200

========================================================================
= ASP.DLL Include File Buffer Overflow
=
= MS Bulletin posted: 
= http://www.microsoft.com/technet/security/Bulletin/MS06-034.mspx
=
= Affected Software:
=    IIS 5.0
=    IIS 5.1
=    IIS 6.0
=
= Public disclosure on July 19, 2006
========================================================================

== Overview ==

A buffer overflow exists in ASP.DLL that can be exploited by creating
a .asp file containing a parameter for the include SSI command.

  <!-- #include file="<long buffer>" -->OVERFLOWDATA

The include function in ASP.DLL, checks if the parameter is longer than 
260 bytes. If it is then an error is caused, but before causing the
error
a miscalculated copy is done.

  mov     edi, [ebp+var_228]   ; load length of parameter
  cmp     edi, 104h            ; check if larger than 260 bytes
  jbe     short loc_
  mov     esi, [ebp+var_22C]   ; load address of parameter
  lea     eax, [edi+esi-104h]  ; load eax with the address of the last 
                               ; 260 bytes of the parameter
                               ; (length of string+source of string)-
104h
  lea     edx, [ebp+var_211]   ; load edx with address on stack
  sub     edx, eax             ; 
  mov     cl, [eax]            ;  \
  mov     [edx+eax], cl        ;   do the copy
  inc     eax                  ;   and overflow the stack
  test    cl, cl               ;  /
  jnz     short loc_7096D1F3   ;  

Funnily enough, the solution was to remove this copy as the resulting 
data was never actually used.

== Exploitation ==

Exploitation requires the ability to upload or somehow create a file
with
a .asp extension in a folder that will allow .asp processing. 

Since ASP.DLL usually runs under the IWAM_ account, there is no
privilege
escalation through this vulnerability. It is however possible to bypass
any security restrictions enforced by ASP. It also allows for the 
execution of APIS that have no ASP equivalent.

== Solutions ==

- Install the vendor supplied patch.

== Credit ==

Discovered and advised to Microsoft February, 2006 by Brett Moore of
Security-Assessment.com

Same Bug Different App
http://www.security-assessment.com/Presentations/SBDA_Ruxcon_2005.ppt

In memory of;
  http://www.nsfocus.com/english/homepage/research/0305.htm
and
  http://www.eeye.com/html/research/advisories/AD20001003.html

== About Security-Assessment.com ==

Security-Assessment.com is a leader in intrusion testing and security
code review, and leads the world with SA-ISO, online ISO17799 compliance
management solution. Security-Assessment.com is committed to security
research and development, and its team have previously identified a
number of vulnerabilities in public and private software vendors
products.

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