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Obfuscation Demo

Learning how to solve the most common issue with obfuscation, using the #1 obfuscator tool in the market.

Agenda:

  1. Run a simple desktop application (works fine :)),
  2. Obfuscate the application,
  3. Run the obfuscated application (throws an exception :(),
  4. Reverse-engineer the application (helps understanding the real cause of the runtime exception),
  5. Fix the issue (re-obfuscate the .exe and verify everything is working as expected).

demo

Pre-requisites

Dotfuscator Community.

Could be installed within Visual Studio as well,

  • VS2022 -> Extensions -> Manage Extensions -> Online -> search for "Dotfuscator Community" -> install (restart Visual Studio).

Obfuscation

  1. Build/Run the application in release mode (or via CLI),

    1. Using Visual Studio

    vs-release-run

    1. Using CLI
    dotnet build -c release

  2. Obfuscation

    1. Using Visual Studio

      1. Open Dotfuscator

        • VS2022 -> Tools -> PreEmptive Protection - Dotfuscator Community.

      2. File -> Open -> browse to obfuscation-room\dotfuscator-settings.xml -> Click Open.

      3. Click Build

      4. You should get something like this

      dotfuscator-build.png

    2. Using CLI

      1. Find dotfuscator.exe, (should be at [VS Install Dir]\Common7\IDE\Extensions\PreEmptiveSolutions\DotfuscatorCE folder),

        1. Or, open dotfuscator GUI, open Task Manager, search for dotfuscatorUI.exe, right click, open file location.

      2. Add the path of the folder to PATH environmental variables,

      3. Restart the CLI (ex. powershell 7) and run the following command

      dotfuscator .\obfuscation-room\dotfuscator-settings.xml
      1. You should get something like this

      dotfuscator-build.png

  3. The obfuscated application will be generated at this location obfuscation-room\obfuscated\obfuscation-demo.exe.

  4. Run the application, click on any button, exception occurs :(

runtime-exception

Reverse Engineering

Let's decompile the obfuscated application using popular ILSpy.

I've shipped ILSpy binaries with this project (Locate it at .\obfuscation-room\ILSpy_binaries_8.2.0.7535-x64.zip).

  1. Extract the content of the .zip file then run ILSpy.exe,

  2. File -> Open -> navigate to \obfuscation-demo\obfuscation-room\obfuscated\obfuscation-demo.exe -> Open,

  3. You should get something like this

ilspy

See, mainwindow.baml, it is the obfuscated version of MainWindow.xaml. Notice how the names of <RepeatButton/>s are changed

  • LeftButton changed to a,
  • RightButton changed to c,
  • RepeatButton_OnClick event handler is renamed to a

Classes a and b are the obfuscated versions of App and MainWindow classes.

Only problem?

Look at the obfuscated version of RepeatButton_OnClick

obfuscated-event-handler

Got it?

Strings in C# are not renamed (LeftButton and RightButton), whereas changed in XAML (a and c). Therefore -at runtime- the code flows to throw new ArgumentOutOfRangeException(); block, hence the exception occurs..

Solution

  1. Open the dotfuscator-settings.xml in dotfuscator GUI -> Renaming -> Exclusions

Exclude the buttons from being renamed.

dotfuscator-solution

  1. Click Build

P.S. Compare the stats between the old build and the new one (after the fix),

  • Before (Fields Renamed: 8 of 8 (100.00%))
[Build Output] Types Renamed:  7 of 7  (100.00%)
[Build Output] Methods Renamed:  5 of 13  (38.46%)
[Build Output] Fields Renamed:  8 of 8  (100.00%)
  • After (Fields Renamed: 6 of 8 (75.00%))
Types Renamed:  7 of 7  (100.00%)
Methods Renamed:  5 of 13  (38.46%)
Fields Renamed:  6 of 8  (75.00%)

You can also compare the old mainwindow.baml with the new one (see how buttons' names left untouched, LeftButton and RightButton).

ilspy-after-the-fix

  1. Run the obfuscated application, click on any button, all work.

DONE.