Assembly Manipulation and C# / VB.NET Code Injection

2022-04-28 04:19:20

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Introduction

Reflector and JustDecompile are great tools for doing an in-depth examination of various types of assemblies and also for disassembling IL code towards a supported .NET language. However, they are unable to modify either the structure or the IL code of assemblies. Reflexil allows such modifications by using the powerful Mono.Cecil library written by Jb EVAIN. Reflexil runs as a plug-in and is directed especially towards IL code handling. It accomplishes this by proposing a complete instruction editor and by allowing C#/VB.NET code injection, as we will see in the following two examples.

Demo Application

Let‘s use a very simple application that is able to add two numbers:

Assembly Manipulation and C# / VB.NET Code Injection

Collapse | Copy Code

using System;
using System.Windows.Forms;

namespace ReflexilDemo
{
    public partial class DemoForm : Form
    {
        public DemoForm()
        {
            InitializeComponent();
        }

        private void ComputeAndDisplay(decimal x, decimal y)
        {
            MessageBox.Show(String.Format("{0}+{1}={2}", x, y, x + y));
        }

        private void DisplayResultButton_Click(object sender, EventArgs e)
        {
            ComputeAndDisplay(LeftOperand.Value, RightOperand.Value);
        }
    }
}

Using Instruction Editor

Using the instruction editor, let‘s update the ComputeAndDisplay method body by calling the overloaded methodMessageBox.Show, which takes a title as a second parameter for the modal window displaying the result. In order to do so, we have to initially put a string parameter on the stack with ldstr opcode:

Then we have to update the "call" instruction to use the overloaded method MessageBox.Show, which uses the previously created parameter:

It is time to save our work and to test our patched assembly:

Our assembly is now using the overloaded method and the suitable parameter:

Instruction Editor Features

The instruction editor supports all opcodes defined in Mono.Cecil. The following operands are supported:

Primitive types: byte, sbyte, int32, int64, single, double
String
Instruction reference
Multiple instructions references (switch)
Parameter or variable reference
Internal generic type reference
Type, field or method reference using a browser for selecting the suitable element. This browser is like the reflector‘s one (lazy loading, icons, etc.)

In a quite comprehensible way, the Reflector and Reflexil object models are not synchronous: updates made on the IL code will not impact the disassemble window of Reflector. Reflexil and Mono.Cecil do not perform any checks of the emitted code. The only constraint is about coherence between the operand type used for a given opcode. For those who find IL manipulation difficult, the following example shows how to update a method body with C# or VB.NET.

Using C#/VB.NET Code Injection

You can choose your preferred injection language and input/display bases (binary, octal, decimal, and hexadecimal) with the configuration form:

Let‘s use the "Replace all with code" feature on the body of the ComputeAndDisplay method:

A compile window enables us to view the resulting IL code. A basic support for intellisense/insight is provided:

We can do it again with VB.NET language. Note that in this simple case, we obtain an identical IL code (it is not always the case):

Let‘s save and test our patched assembly:

C#/VB.NET Code Injection Features

The code is compiled using System.CodeDom in a separate AppDomain for a correct resource release. Once compiled, instructions are extracted and then reinserted in the original method body. Parameters, variables, methods, fields and types references are adjusted to match the original assembly. Code injection is limited: it is not possible to refer either to the fields or to the methods defined in ancestral types of the owner type of the original method. You can choose the target framework used by the compilation process.

Method Attributes Editor

You can easily update a method signature or change his visibility scope. You are also able to change the return type:

Method parameters (and variables) are also editable. Reflexil can load symbols (MDB and PDB files are supported) to display original variable names:

Exception Handlers

Reflexil allows to add/update/delete exception handlers associated with a method body. The following types are supported:

Catch
Filter (the VB.NET Where clause in Try/Catch blocks)
Finally
Fault

Type Attributes Editor

Like methods, you can change any type visibility scope. So you can expose a previously private type to the world:

Member manipulation

Reflexil is able to rename, delete or inject class, interface, struct, enum, event, field, method, constructor, property or assembly reference.

Smart injection : injecting a new property will generate a field, getter/setter methods and IL code.

Resources Editor

You can inject and alter embedded resources, linked resources and assembly linked resources. A complete hex editor is provided to update, export or import files.

Custom attributes Editor

Custom attributes are fully supported.

Assembly and Assembly Reference Editor

With the assembly editor, you can use a different entry point:

You can also update all information regarding identification: version, public key, name and culture. Note that you can also alter any referenced assembly so you can use a different version:

Module Editor

With the module editor, you can simply change the kind of your application (transform an executable assembly to a DLL library for instance):

Signed Assemblies Support

When saving a signed assembly, the resulting assembly is placed in the "delay signed" state. Reflexil can use SDK tools to fix it.

Reflexil is able to remove assembly strong name and update referencing assemblies. You can also do it by yourself with the assembly editor: remove the public key and set the HasPublicKey flag to false.

Deobfuscation Support

Reflexil is able to remove obfuscated code thanks to de4dot.

Babel NET, CliSecure, CodeFort, CodeVeil, CodeWall, CryptoObfuscator, DeepSea, Dotfuscator, dotNET Reactor, Eazfuscator NET, Goliath NET, ILProtector, MaxtoCode, MPRESS, Rummage, Skater NET, SmartAssembly, Spices Net and Xenocode are supported.

Assembly Verifier

With the assembly verifier (using peverify.exe from .NET SDK), you can determine whether the IL code and associated metadata meet type safety requirements.

Conclusion

Reflexil is completely based on Mono.Cecil. The interesting thing is that Mono.Cecil can load assemblies without the help of the runtime, so there is no resource release constraint and AppDomain isolation for example. There is no relation between System.Type and Mono.Cecil.TypeDefinition, whereas they both materialize the .NET type concept. If we want to programmatically reproduce our first example (Show overload), we can write it very easily, thanks to Mono.Cecil.

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