Reverse Engineering Team Board - advise on .net executable

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advise on .net executable

I have some experience in win32/native RCE but .net is completely new to me. I have a target .net executable that is obfuscated with Smart Assembly. I have tried DeSmart (1.0.0.9) and it appears to de-obfuscate (the result is readable in reflector.net). However the executable crashes on startup.

The target seems to be produced in vb.net.

Any advice or tips on how to proceed is welcome, I could provide the target exe if needed (by PM please).

Can you give us a link to the target?

I put the file on rapidshare and SendSpace (pwd in pm), thanks!

The file won't run because you only sent me the dlls and not the installer, but if you open the app in cff explorer, go to the methods section under Tables. Go to method 9290, and that is the entrypoint method. I'm not sure if there's some kind of login form or something, because again I can't load the program, but if there is you could probably patch this method to load the mainform instead.

This is the method:

Code:

L_00000000:   call 0x06000005

L_00000005:   call 0x0A000E34

L_0000000A:   call 0x0A0011AB

L_0000000F:   leave.s 0x00000012

L_00000011:   endfinally

L_00000012:   call 0x06002468

L_00000017:   ldarg.0

L_00000018:   callvirt 0x0A000E2F

L_0000001D:   ret

If there's some sort of registration form or a login form, you can probably just patch it to load the mainform, as I said above. You can try patching it in Reflector with the ReflexIL plugin, and if that doesn't work (it should), then open the app in a hex editor and go to 0x108FB0 (method start), and edit the method that way.

I think the problem lies here:

Code:

unit  Namespace_01;

interface

type

strict private Class_01 = class sealed

    // Methods

    function Class_01.Method_00(num1: Integer): string;

    var

        buffer: Byte[];

    begin

        lock (typeof(Class_01))

        beginif (Class_01.Field_00 = nil) then

            begin

                executingAssembly := Assembly.GetExecutingAssembly;

                Class_01.Field_00 := executingAssembly.GetManifestResourceStream(executingAssembly.ManifestModule.ModuleVersionId.ToString('B'));

                publicKeyToken := executingAssembly.GetName.GetPublicKeyToken;

                if (publicKeyToken <> nil) then

                begin

                    index := 0;

                    repeat

                        Class_01.Field_01 := (Class_01.Field_01 xor ((publicKeyToken[index] shl 8) + publicKeyToken[(index + 1)]));

                        inc(index, 2)

                    until (index >= (publicKeyToken.Length - 1))

                end;

                num2 := (((MethodBase.GetCurrentMethod.MetadataToken and $ffffff) - 1) mod $ffff);

                Class_01.Field_01 := (Class_01.Field_01 xor num2)

            end;

            Class_01.Field_00.Position := (num1 - Class_01.Field_01);

            num3 := Class_01.Field_00.ReadByte;

            count := 0;

            if ((num3 and $80) = 0) then

                count := num3;

            if ((num3 and $40) = 0) then

                count := (((num3 and $3f) shl 8) + Class_01.Field_00.ReadByte)

            else

                count := (((((num3 and $1f) shl $18) + (Class_01.Field_00.ReadByte shl $10)) + (Class_01.Field_00.ReadByte shl 8)) + Class_01.Field_00.ReadByte);

            buffer := New(array[count] of Byte);

            Class_01.Field_00.Read(buffer, 0, count)

        end;

        if (buffer.Length = 0) then

            begin

                Result := string.Empty;

                exit

            end;

        bytes := Convert.FromBase64String(Encoding.UTF8.GetString(buffer, 0, buffer.Length));

        begin

            Result := string.Intern(Encoding.UTF8.GetString(bytes, 0, bytes.Length));

            exit

        end

    end;





    // Fields

    strict private class var Field_00: Stream;

    strict private class var Field_01: Integer = 0;



end;

and it called in the code like this:

Code:

    function LandBase.ToString: string;

    begin

        Result := string.Format(Class_01.Method_00($7588), self.Naam)

    end;

It seems to return a string from an encrypted resource and this fails because the public key has changed??

Quote:

Originally Posted by bball0002 (Post 17813)

...
If there's some sort of registration form or a login form, you can probably just patch it to load the mainform, as I said above. You can try patching it in Reflector with the ReflexIL plugin, and if that doesn't work (it should), then open the app in a hex editor and go to 0x108FB0 (method start), and edit the method that way.

I tried but any change made to the file will make it crash.

You are right, using public key token to decode parts of code/resources/strings is very popular method these days.. .NET Reactor does the same.
Easiest solution is to modify your function Class_01.Method_00() to use hardcoded decryption key instead of public key token.

Quote:

Originally Posted by kao (Post 17827)

Easiest solution is to modify your function Class_01.Method_00() to use hardcoded decryption key instead of public key token.

Sounds like a good idea, what confused me however, I tried to get the Public Key Token (with Powershell) like this:

Code:

[System.Reflection.Assembly]::LoadFile('target.exe').GetName().GetPublicKeyToken()

but the result is null, please advise how to get the public key token.

OK, I read the code again and the Public Key Token can be null (which it is). I also checked GetManifestResourceStream(executingAssembly.Manife stModule.ModuleVersionId.ToString('B')) and the result is

Code:

{2602d3bb-5287-4d6f-9a35-61240bcf8c57}

which is a resource (probably encrypted or encoded strings.

How to get the value of this: MethodBase.GetCurrentMethod.MetadataToken ?

PublicKeyToken should be null only if assembly is not signed by strong name key. Are you sure about that? Check the original file, not the one produced by DeSmart.

GetCurrentMethod.MetadataToken is an identifier of the current method, in your case identifier of Class_01.Method_00().

I would use ILDASM and other Microsoft tools for all those tasks. It takes some time to get used to it's output but it's really powerful. It's possible that other tools (for example, Reflector) show method tokens as well, but I'm not sure about that..

Showing strong name token of an assembly (sn.exe is part of Microsoft.NET SDK):

Code:

C:\>sn -T test.exe



Microsoft (R) .NET Framework Strong Name Utility  Version 2.0.50727.42

Copyright (c) Microsoft Corporation.  All rights reserved.



Public key token is 67d0e8149152d3d2

Example ILDASM output from my test assembly:

Code:

.method /*060049B8*/ privatescope hidebysig static void '3wW1VNpxI$PST060049B8'() cil managed noinlining

So, the token of method '3wW1VNpxI$PST060049B8' is 0x060049B8.

I tried sn already it reports: target.exe does not represent a strongly named assembly

ILDasm doesn't work on the original exe, it reports Protected module -- cannot disassemble.

Maybe I can set a breakpoint and inspect the value? Can you recommend a debugger for this?

Using debugger I watched the result and it is 0x00000005 so I inserted opcode ldc.i4.5.
Using reflector the code is now:

Code:

public static string Method_00(int num1)

{

    byte[] buffer;

    lock (typeof(Class_01))

    {

        if (Field_00 == null)

        {

            Assembly executingAssembly = Assembly.GetExecutingAssembly();

            Field_00 = executingAssembly.GetManifestResourceStream(executingAssembly.ManifestModule.ModuleVersionId.ToString("B"));

            byte[] publicKeyToken = executingAssembly.GetName().GetPublicKeyToken();

            if (publicKeyToken != null)

            {

                int num = 0;

                do

                {

                    Field_01 = 7;

                    num += 2;

                }

                while (num < (publicKeyToken.Length - 1));

            }

            Field_01 = 5;

        }

But still crashes

For SmartAss'd targets, I usually use a loader. You can use a loader to modify the method while it's in memory. Just use CFF Explorer to get the Virtual Offset of the opcode that you want to patch, and patch it in memory. If you don't want to make your own loader you can use DUP to create one for you.

Modifying the method in memory bypasses all of those checks that SmartAssembly makes to the methods/exe.