How to Use Reflexil for Runtime Method PatchingRuntime method patching is a powerful technique used by developers, reverse engineers, and security researchers to change the behavior of a .NET program without recompiling its source code. Reflexil is a well-known plugin for .NET disassemblers (notably Reflector and ILSpy through various forks or integrations) that allows interactive editing of an assembly’s Intermediate Language (IL). This article explains what Reflexil is, when and why you might patch methods at runtime, safety and legal considerations, and gives a detailed, step-by-step walkthrough showing how to patch methods using Reflexil, with practical examples and troubleshooting tips.
What is Reflexil?
Reflexil is an interactive IL editor that integrates with .NET decompilers/disassemblers and lets you view, modify, and save IL code inside assemblies. Instead of editing high-level C# source and recompiling, Reflexil works directly at the IL level, enabling changes to method bodies, injecting new instructions, modifying metadata, adding or removing method references, and more.
Why choose IL editing with Reflexil?
- It operates on compiled assemblies when source code is unavailable.
- It can perform precise modifications that might be difficult or impossible at the source level.
- It enables quick experimentation and testing of small runtime behavior changes.
Legal and safety considerations
- Only patch assemblies you have the right to modify. Unauthorized modification of proprietary software may violate terms of service, licenses, or laws.
- Back up originals. Always keep an unmodified copy of the assembly to restore if something goes wrong.
- Be careful with signed assemblies: patches will break strong-name signatures unless you re-sign the assembly. Patched assemblies can be detected and rejected by signature checks.
- Runtime patching can introduce crashes, memory corruption, or undefined behavior if IL is malformed.
Tooling prerequisites
- A .NET disassembler that supports Reflexil. Historically Reflexil integrated with Red Gate .NET Reflector; forks and plugins exist for tools like ILSpy or dnSpy. Ensure you have a compatible combination.
- Reflexil plugin installed and enabled in the disassembler.
- Target assembly (DLL or EXE) you intend to patch.
- .NET runtime and (optionally) a decompiler view (C#) to help understand logic.
- A testing environment (sandbox or isolated VM) to run the patched assembly safely.
Basic Reflexil workflow overview
- Open the target assembly in the disassembler.
- Navigate to the method you want to patch.
- Inspect the IL and (optionally) the decompiled C# to understand behavior.
- Use Reflexil to edit the method body: insert, remove, or replace IL instructions; change operands; modify metadata.
- Save the modified assembly.
- Test the patched assembly under controlled conditions and iterate if needed.
Step-by-step example: Patching a method to skip a permission check
Scenario: You have a small .NET assembly with a class App.Security that contains a method bool HasAccess(string user). You want to patch the method at runtime so it always returns true, effectively bypassing the permission check for testing.
- Open the assembly
- Launch your disassembler (Reflector, ILSpy with Reflexil, or dnSpy variant with IL editor).
- Load the EXE or DLL into the tool.
- Locate the method
- Use the tree view to expand namespaces and types until you find App.Security.HasAccess(string).
- View the decompiled C# to understand what conditions need bypassing.
- View IL code
- Switch to the IL/method body view. A typical HasAccess IL might load parameters, call other methods, compare values, and return a bool.
- Replace method body with simple return
- In Reflexil, select the method and choose to edit the method body.
- The easiest way to force the method to always return true is to clear existing instructions and insert IL that loads the constant 1 (true) and returns. The IL sequence to return true typically is:
- ldc.i4.1
- ret
- Insert instructions
- In Reflexil’s instruction editor, remove all existing instructions (or mark them).
- Use the Add/Insert instruction function to add:
- ldc.i4.1 (pushes integer 1 onto the evaluation stack)
- ret (returns with the value on the stack)
- If the method is declared to return System.Boolean, this will correctly return true. If it’s void or returns another type, adjust accordingly (for reference types, push a null with ldnull; for integers use ldc.i4.0 or ldc.i4.1 as appropriate).
- Save the assembly
- Use Reflexil to save changes. If the assembly is strong-named, the save will likely break the signature; you’ll need to re-sign or disable signature checks in your test environment.
- Optionally, export or save as a new filename to preserve the original.
- Test the patched assembly
- Run the application or unit tests that exercise HasAccess to confirm it now returns true for all inputs.
- If it crashes, revisit IL for mismatched stack behavior or incorrect return types.
More advanced patches
- Replacing a call: Swap out a call to a method (call or callvirt) with a different method reference. Use Reflexil to change the operand of the call instruction to point to another MethodDef/MethodRef.
- Injecting logging: Insert IL to call Console.WriteLine or a custom logger. You’ll need to import references to the target type and its method, and ensure proper argument push instructions are present.
- Conditional branch modification: Change branch targets to alter code flow. For example, replace brfalse.s with br.s or vice versa, or adjust offsets by modifying the operand to jump to a different instruction index.
- Adding new helper methods: You can add new methods to a type (or new types) using the disassembler’s API and then call them from patched code. Reflexil can create method definitions and add IL into them.
Handling method signatures and the evaluation stack
IL has strict rules: the stack must be balanced and the types must match the method signature. Common mistakes:
- Returning a value of the wrong type.
- Not providing required arguments before calling a method.
- Leaving extra values on the evaluation stack before a ret.
Fixes:
- Use ldnull for reference returns, ldc.i4.0/1 for integers/booleans, ldc.r8/ldc.r4 for floats, and appropriate conversion instructions when needed.
- When modifying call instructions, ensure parameters expected by the called method are pushed in correct order.
Strong-name and signing issues
- Patching breaks strong-name (snk) signatures. To run patched assemblies that were originally strong-named:
- Re-sign the assembly with your own key (only possible if the runtime doesn’t validate original signature integrity for your use case).
- Use a test environment that disables strong-name verification using sn.exe’s -Vr option (Windows) or equivalent. This should only be used for local testing.
Troubleshooting common errors
- BadImageFormatException: often caused by corrupt or malformed IL or incorrect PE format after save. Reopen the modified assembly in the disassembler to inspect the method body for malformed IL.
- MissingMethodException / TypeLoadException: caused by incorrect method references or removed dependencies. Ensure imported references exist and assembly versions are correct.
- InvalidProgramException: IL verification failed at runtime due to stack imbalance or illegal IL. Double-check stack behavior and operand types.
- Application crashes: use a debugger (Visual Studio, WinDbg) to inspect the call stack and exception. Compare original and patched IL to pinpoint differences.
Example: Insert a logging call before return
Suppose a method returns an integer, and you want to log the return value before returning.
-
Identify the method and find the ret(s) that return the integer.
-
Before each ret, insert:
- dup (duplicate the integer on the stack so you can call Console.WriteLine and keep the value for return)
- call System.Void [mscorlib]System.Console::WriteLine(System.Int32)
- ret
-
Ensure using the correct method reference for Console.WriteLine(int). Reflexil can import that method reference into the assembly if not already present.
Best practices
- Work on copies and use version control (store original and patched assemblies).
- Make small incremental edits and test each change.
- Keep IL edits minimal and well-documented as comments or external notes.
- Prefer adding new methods and redirecting calls when possible rather than surgical edits of complex IL.
- Use a dedicated, isolated test environment.
Alternatives to Reflexil
- dnSpy: built-in IL editor and debugging support; very popular for live debugging and patching.
- Mono.Cecil: programmatic assembly rewriting for automated or repeatable patches.
- Harmony (for runtime hooking): a dynamic method-patching library that patches methods at runtime without changing the assembly file.
- Manual recompilation: when source is available, change source and recompile — usually safer and clearer.
Conclusion
Reflexil is a powerful tool for direct IL-level editing of .NET assemblies, enabling runtime method patching when source code isn’t available or quick experiments are needed. Follow safe practices: work on copies, understand IL stack rules, watch for signature issues, and test thoroughly. Mastering IL instructions and using careful, minimal changes will reduce errors and make debugging far easier.
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