http://www.eeye.com/database/advisories/ad06081999/ad06081999.html


Retina vs. IIS4, Round 2

Systems Affected:

Internet Information Server 4.0 (IIS4)
Microsoft Windows NT 4.0 SP3 Option Pack 4
Microsoft Windows NT 4.0 SP4 Option Pack 4
Microsoft Windows NT 4.0 SP5 Option Pack 4

Release Date:

June 8, 1999

Advisory Code:

AD06081999

Description:

We have been debating how to start out this advisory. How do you explain that
90% or so of the Windows NT web servers on the Internet are open to a hole that
lets an attacker execute arbitrary code on the remote web server? So the story
starts...

The Goal:

Find a buffer overflow that will affect 90% of the Windows NT web servers on the
Internet. Exploit this buffer overflow.

The Theory:

There will be overflows in at least one of the default IIS filtered 
extensions (i.e. .ASP, .IDC, .HTR).  The way we think the exploit will take place is
that IIS will pass the full URL to the DLL that handles the extension. Therefore if
the ISAPI DLL does not do proper bounds checking it will overflow a buffer
taking IIS (inetinfo.exe) with it and allow us to execute arbitrary code on the
remote server.

Entrance Retina:

At the same time of working on this advisory we have been working on the AI
mining logic for Retina's HTTP module. What better test scenario than this? We
gave Retina a list of 10 or so extensions common to IIS and instructed it to find
any possible holes relating to these extensions. 

The Grind:

After about an hour Retina found what appeared to be a hole. It displayed that
after sending "GET /[overflow].htr HTTP/1.0" it had crashed the server. We all
crossed our fingers, started up the good ol' debugger and had Retina hit the
server again.

Note: [overflow] is 3k or so characters... but we will not get into the string
lengths and such here. View the debug info and have a look for yourself.

The Registers:
   EAX = 00F7FCC8 EBX = 00F41130
   ECX = 41414141 EDX = 77F9485A
   ESI = 00F7FCC0 EDI = 00F7FCC0
   EIP = 41414141 ESP = 00F4106C
   EBP = 00F4108C EFL = 00000246

  Note: Retina was using "A" (0x41 in hex) for the character to overflow with.
  If you're not familiar with buffer overflows a quick note would be that getting
  our bytes into any of the registers is a good sign, and directly into EIP makes
  it even easier :)

Explain This:

The overflow is in relation to the .HTR extensions. IIS includes the capability to
allow Windows NT users to change their password via the web directory
/iisadmpwd/. This feature is implemented as a set of .HTR files and the ISAPI
extension file ISM.DLL. So somewhere along the line when the URL is passed
through to ISM.DLL, proper bounds checking is not done and our 
overflow takes place. The .HTR/ISM.DLL ISAPI filter is installed by default on IIS4
servers. Looks like we got our 90% of the Windows NT web servers part down.
However can we exploit this?

The Exploit:

Yes. We can definitely exploit this and we have. We will not go into much detail
here about how the buffer is exploited and such. However, one nice thing to note
is that the exploit has been crafted in such a way to work on SP4 and SP5
machines, therefore there is no guessing of offsets and possible accidental
crashing of the remote server. 

Click here for more details about the exploit and the code.
<http://www.eeye.com/database/advisories/ad06081999/ad06081999-exploit.html>

The Fallout:

Almost 90% of the Windows NT web servers on the Internet are affected by this
hole. Everyone from NASDAQ to the U.S. Army to Microsoft themselves. No, we
did not try it on the above mentioned. But it is easy to verify if a web server is
exploitable without using the exploit. Even a server that's locked in a guarded
room behind a Cisco Pix can be broken into with this hole. This is a reminder to all
software vendors that testing for common security holes in your software is a
must. Demand more from your software vendors.

The Request. (Well one anyway.)

Dear Microsoft,

One of the things that we found out is that IIS did not log any trace of our
attempted hack. We recommend that you pass all server requests to the logging
service before passing it to any ISAPI filters etc...The logging service should be, as
named, an actual service running in a separate memory space so that when inetinfo
goes down intrusion signatures are still logged. 

Retina vs. IIS4, Round 2. KO.

Fixes:

    1.Remove the extension .HTR from the ISAPI DLL list. Microsoft has just
       updated their checklist to include this interim fix.
       <http://microsoft.com/security/products/iis/CheckList.asp>
    2.Apply the patch supplied by Microsoft when available.
       <http://www.microsoft.com/security/default.asp>

Vendor Status:

We contacted Microsoft on June 8th 1999, eEye Digital Security Team provided all
information needed to reproduce the exploit. and how to fix it. Microsoft security
team did confirm the exploit and are releasing a patch for IIS.

Related Links

Retina - The Network Security Scanner
http://www.eEye.com/retina/ 

Retina - Brain File used to uncover the hole
http://www.eEye.com/database/advisories/ad06081999/ad06081999-brain.html

Exploit - How we did it and the code.
http://www.eEye.com/database/advisories/ad06081999/ad06081999-exploit.html

NetCat - TCP/IP "Swiss Army knife"
http://www.l0pht.com/~weld/netcat/ 

Greetings go out to:

The former Secure Networks Inc., L0pht, Phrack, ADM, Rhino9, Attrition, HNN
and any other security company or organization that believes in full disclosure.

Copyright (c) 1999 eEye Digital Security Team

Permission is hereby granted for the redistribution of this alert 
electronically. It is not to be edited in any way without express consent of eEye. If
you wish to reprint the whole or any part of this alert in any other medium
excluding electronic medium, please e-mail alert@eEye.com for permission.

Disclaimer:

The information within this paper may change without notice. Use of this
information constitutes acceptance for use in an AS IS condition. There are NO
warranties with regard to this information. 
In no event shall the author be liable for any damages whatsoever arising out of or
in connection with the use or spread of this information. Any use of this
information is at the user's own risk.

Please send suggestions, updates, and comments to: 

eEye Digital Security Team

info@eEye.com 
www.eEye.com 

Retina vs. IIS4, Round 2 - The Brain 
<http://www.eeye.com/database/advisories/ad06081999/ad06081999-brain.html>
Retina vs. IIS4, Round 2 - The Exploit
<http://www.eeye.com/database/advisories/ad06081999/ad06081999-exploit.html>