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:

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. 

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.
2. Apply the patch supplied by Microsoft when available.

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 
Retina vs. IIS4, Round 2 - The Exploit 
  
 

Retina - The Network Security Scanner for all platforms of windows is available for beta testing.   

eCompany Office™ 
eEye - Digital Security Team is involved in testing and developing security components in eCompany Office. An eCommerce and web site management suite from our parent company.  

PadLock-it - eEye has been consulting for WinWare Inc. to develop PadLock-it the password keeper for all platforms of windows. 
http://winwareinc.com