retina.vs.iis4-round2.txt

`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 [email protected] 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  
  
[email protected]   
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>  
`