C#

dnEditor – Opensource .NET decompiler using dnlib

Thought I’d share another neat project with you today created by ViRb3 which you can find at: dnEditor on GitHub.

It’s a .NET decompiler in it’s early stages of development which uses dnlib by 0xd4d as the engine to load assemblies. This means it’ll load pretty much every .NET assembly, obfuscated or not, without any issues. It has support for IL->C# decompilation using ILSpy as a base. It has a simple but user-friendly interface similar to ILSpy and Reflector:

dnEditor user interface

If you have used Reflexil before you’ll find a familiar interface in the instruction editor in dnEditor:

Instruction editor in dnEditor

Another cool feature is the color-coded IL blocks and instructions similar to SimpleAssemblyExplorer:

IL blocks

You should keep your eyes on this project as it’s being actively developed and I might even take a shot at contributing some to it myself in the future.

Make sure you check out ViRb3’s profile on GitHub: ViRb3 GitHub profile 

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RTN-Team forum

If you’re interested in the work I do on this blog, and have a general interest in programming and/or reverse engineering you should check out a forum I’m a part of over at RTN-Team forum. It focuses on questions/help/releases related to reverse engineering but there a lot of experienced coders available to help out too.

It’s a small community right now and we want to keep it rather small, but with high quality content and members. Quality over quantity, you know. So please consider going over there and signup and hopefully join the community.

Once again the link is: https://www.rtn-team.cc/board/

There is some more content coming to the blog as well soon, so stay tuned for that. 🙂

How does SJITHook work?

In my last post I released a project called SJITHook, which is basically just a small class to easily create a hook to the .NET just-in-time compiler in order to be able to modify or analyze methods at runtime. I’m sure a lot of you guys know how it works, but for those who don’t I’ll try to explain it right now. But before anything, you should read about the .NET just-in-time compiler to keep up.

There are 2 DLL’s used in every standard .NET application, either Mscorjit.dll or Clrjit.dll depending on what .NET version the assembly is targetting. Mscorjit targets 2.0 and below while Clrjit.dll target 4.0 and above. They have two things in common though, which makes it a lot easier for us to write hooks for them. They both have an export called “getJit” which returns the address of the V(irtual function)Table in a class called CILJit. If you’re unaware of what a VTable is then read: Virtual method table. They also both have the same signature for the function we’re gonna hook, called “compileMethod”. This method takes your raw IL code along with a LOT of metadata crap and other stuff and compiles it to native assembly code. So each time a function is ran for the first time, it’s ran through compileMethod, turned in to assembly code, and then if it’s called again it jumps straight to the assembly code. A lot of obfuscators and protections take advantage of this process. They do this by encrypting the IL method bodies in an assembly, so they are unreadable to a static decompiler and then decrypts the code in a compileMethod hook and then passes it on to the original compileMethod in clrjit/mscorjit.

I created a quick diagram to hopefully help visualize what this means:

L

Let’s take an example of the code in SJITHook where I retrieve the pointer to the VTable:

ss (2014-05-12 at 07.37.51)

 

The returned pointer is the one referred to as “pVTable” in the diagram above too. So in order to get a pointer to the VTable (or more exactly, the first entry in the VTable) we dereference pVTable:

ss (2014-05-12 at 07.47.06)

It’s a quite straight forward process from here on. It basically goes:

  1. Save original address of compileMethod. (See this)
  2. Overwrite the first entry in the VTable that points to original compileMethod, with our own function. It needs to have the exact same signature and calling convention to not mess up stack. (See this)
  3. In our hooked compileMethod function we can do whatever we want, but we have to call original compileMethod at the end. (See this)
  4. When we’re finished, we can unhook by rewriting address of original compileMethod into first entry in VTable. (See this)

If we take a look at the hook from a physical memory point of view, this is what the VTable looks like (first line is compileMethod pointer):

before hook: ss (2014-05-12 at 08.15.04)

 

after hook:   ss (2014-05-12 at 08.18.40)

There are however a few things we have to keep in mind when doing this. Most importantly, we have to recognize that our own functions will trigger the JIT and in turn our hook which creates an infinite loop and crashes the application. This is why we have to ‘prepare’ our own defined methods before creating the hook. Using RuntimeHelpers.PrepareMethod/Delegate forces the method to be ran through the JIT even though it’s not executed:

ss (2014-05-12 at 08.01.33)

 

Something to note is that there are numerous ways of hooking a function, this is just one of them. The reason I use this method is because it’s easy (automatic even) to make it support both x86 and x64 bit application with the usage of .NET’s IntPtr type. Read more about hooks here: http://en.wikipedia.org/wiki/Hooking#Runtime_modification.

I hope this gave you some insight on what SJITHook does and how it works internally. And if there’s still things that are unclear, feel free to leave a comment below and I’ll do my best to answer it. 🙂

SJITHook (Simple JIT hook) up on GitHub

Just a small little project I decided to release. It allows you to easily create a JIT hook in a .NET application. It can be used to do stuff like dynamic method decryption at runtime (commonly used in obfuscation). You can find the project here: SJITHook.

If you’re unsure how to use it, please read the ReadMe.

Example of what it can do:

ss (2014-05-11 at 08.15.09)

I’ll soon release a blog entry here explaining exactly what this project does, and how it works more in depth. 🙂

Updating method at runtime (native)

I thought I’d show you something fun to do in C#; changing method bodies at runtime, AFTER they’ve been JIT’d. This is a fairly simple process actually. The first thing we have to do is load our assembly using reflection:

 Assembly asm = Assembly.LoadFile(Path.GetFullPath("test.exe"));
 Type mainType = asm.EntryPoint.DeclaringType;
 MethodInfo method = mainType.GetMethod("Foo",
 BindingFlags.InvokeMethod | BindingFlags.NonPublic | BindingFlags.Static);

The method we’re gonna use looks like this:

Let’s invoke it with 1000 as parameter. Expected output would be 1000*1000:

Console.WriteLine("Original value: " + method.Invoke(null, new object[] { 1000 }));

And indeed it shows 1000*1000:

Now let’s change this method to show something else! First of all we would need to force the method to be JIT’d in order for the assembly code to even be present in memory. But since we already invoked the method, it has already been JIT’d, but if we didn’t we could use:

RuntimeHelpers.PrepareMethod(method.MethodHandle);

Okay now to the interesting part. Before I start with this you should read .NET Framework Internals: MethodDesc to understand why this works. First thing we need to do is retrieve a pointer to the native method:

IntPtr pBody = method.MethodHandle.GetFunctionPointer();

Since we know our target int is 0x3e8 (1000), we iterate the body until we find it, we then write our new 0x539 (1337) in place of it:

unsafe
{
   var ptr = (byte*) pBody.ToPointer();
   for (var i = 0; i < 100; i++)
   {
      // Assuming our 0xE8 byte is the first one
      if ((*(ptr+i)) == 0xe8)
           (*(int*)(ptr+i)) = 0x539;
   }
}

That’s it! Let’s call the method the same way we did in the beginning:

Console.WriteLine("New value: " + method.Invoke(null, new object[] { 1000 }));

It should just return 1000*1000 as before, right? Of course not, silly. We replaced the 1000 with 1337:

I’m not sure what this could be used for in practice. Perhaps some sort of obfuscation? But then you’d probably be better off using MethodRental.SwapMethodBody to work with the IL code rather than assembly. Hope you found this little example interesting. 🙂

Download files used: http://www.multiupload.nl/UR4EBGQ3BT