RE: New Whitepaper - .NET Framework Rootkits: Backdoors inside your Framework

["Erez Metula" <erezmetula () 2bsecure co il>]

Hi Ragan,
Performance seems to be the main cause for the lack of signature check (although the overhead penalty occurs once per 
loaded DLL).
I also believe that Microsoft did some threat modeling and came to the right conclusion that since the checking 
mechanism is in the hands of the attacker anyway, and he is able to disable it regardless of how it works, it would be 
pointless to trust it like you mentioned. 
This is the reason why I believe that even if they do change the signature mechanism (which will probably be broken 
shortly) the idea of modifying .NET Framework DLL will last for a long time.. ☺

And it's also important to mention - I think the main issue is not the mechanism itself but the fact that now he know 
that attackers have another place to hide malicious code – it's not just the BIOS, Kernel, Drivers, etc. but also .NET 
Framework core DLL.

Cheers,
Erez.
________________________________________
From: Ragan, Rob R [mailto:rob.ragan () hp com] 
Sent: Thursday, November 13, 2008 10:18 PM
To: Erez Metula; websecurity () webappsec org; webappsec () lists securityfocus com
Subject: RE: New Whitepaper - .NET Framework Rootkits: Backdoors inside your Framework

Nice presentation and paper. When a signed assembly is installed in the GAC, the system hashes the contents of the file 
containing the manifest and compares the hash with the digital signature embedded in the PE file, after unsigning it 
with the public key. It is interesting that the assembly isn’t part of the hash with framework DLLs. Perhaps this is 
the case for performance reasons. Is there too much overhead to check the signature of these files as they’re used so 
often? Let’s say performance isn’t an issue, would it be a pointless gesture to trust the checking mechanism on a 
machine that has been victimized by a rootkit? Modifying the framework in the way described does require admin 
privileges. 

By the way, Reflexil is a Reflector plug-in designed for easy modification of assemblies based on Mono.Cecil.  
http://www.mono-project.com/Cecil  “With Cecil, you can load existing managed assemblies, browse all the contained 
types, modify them on the fly and save back to the disk the modified assembly.”

I prefer it to a text editor. It might save a couple steps from the modify and recompile sections of your paper. Also 
it has a feature for removing strong naming. 

http://sebastien.lebreton.free.fr/reflexil/ 
http://www.codeproject.com/KB/msil/reflexil.aspx 

Enjoy,

Rob Ragan
HP Application Security Center
770.343.7050 Tel

From: Erez Metula [mailto:erezmetula () 2bsecure co il] 
Sent: Thursday, November 13, 2008 10:39 AM
To: websecurity () webappsec org; webappsec () lists securityfocus com
Subject: [WEB SECURITY] New Whitepaper - .NET Framework Rootkits: Backdoors inside your Framework

Paper Name
=========

.NET Framework Rootkits - Backdoors inside your Framework (Author: Erez Metula)
 

Paper Description
=============

The paper introduces a new method that enables an attacker to change the .NET language, and to hide malicious code 
inside its core.
It covers various ways to develop rootkits for the .NET framework, so that every EXE/DLL that runs on a modified 
Framework will behave differently than what it's supposed to do. Code reviews will not detect backdoors installed 
inside the Framework since the payload is not in the code itself, but rather it is inside the Framework implementation. 
Writing Framework rootkits will enable the attacker to install a reverse shell inside the framework, to steal valuable 
information, to fixate encryption keys, disable security checks and to perform other nasty things as described in this 
paper. 



Paper Summary
============
 
Framework modification can be achieved by tampering with a Framework DLL and "pushing" it back into the Framework.
The process is composed of several steps, described thoroughly at the corresponding whitepaper.
It also exposes a flaw in the manner in which a .NET Framework DLL is loaded, and how it is possible to bypass its 
signature mechanism.
Instead of re-signing tampered DLL's with a spoofed Microsoft signature key - surprisingly, it was found during this 
research that the modified DLL can be directly copied to the correct location at the file system, because the SN 
mechanism does not check the actual signature of a loaded DLL but blindly loads the DLL based on the directory name 
with the corresponding signature name!
It is important to mention that this technique does not requires "full trust" permissions, which further proves the 
fact that the GAC / CAS protection mechanisms are broken.

This paper also introduces ".Net-Sploit" - a new tool for building MSIL rootkits that will enable the user to inject 
preloaded/custom payload to the Framework core DLL.

You can find the detailed whitepaper, .NET-Sploit tool, source code, and the OWASP presentation at:
http://www.applicationsecurity.co.il/.NET-Framework-Rootkits.aspx