B(l)utter

Flutter Mobile Application Reverse Engineering Tool by Compiling Dart AOT Runtime

Currently the application supports only Android libapp.so (arm64 only). Also the application is currently work only against recent Dart versions.

For high priority missing features, see TODO

Termux

• Same as debian but needs ndk . if you dont want ndk then remove android library dependencies related files in dartsdk
• I actually liked fmt library thats the main reason replaced standard format
• Install fmt: pkg install fmt
• It should work for both dartsdk stable/beta builds didnt checked for dev builds
• If any error related to capstone first check if is present in include dir
  pkg-config --cflags capstone

OR You can copy paste below command to install all requirements:

pip install requests pyelftools && pkg install -y git cmake ninja build-essential pkg-config libicu capstone fmt

Note

In case you face errors related to no member named 'format', you need to replace all occurance of std::format with fmt::format using below shell command:

find -type f -exec sed -i 's/std::format/fmt::format/g' {} +

Blutter.mp4

Environment Setup

This application uses C++20 Formatting library. It requires very recent C++ compiler such as g++>=13, Clang>=16.

I recommend using Linux OS (only tested on Deiban sid/trixie) because it is easy to setup.

Debian Unstable (gcc 13)

• Install build tools and depenencies

apt install python3-pyelftools python3-requests git cmake ninja-build \
    build-essential pkg-config libicu-dev libcapstone-dev

Windows

• Install git and python 3
• Install latest Visual Studio with "Desktop development with C++" and "C++ CMake tools"
• Install required libraries (libcapstone and libicu4c)

python scripts\init_env_win.py

• Start "x64 Native Tools Command Prompt"

macOS Ventura and Sonoma (clang 16)

• Install XCode
• Install clang 16 and required tools

brew install llvm@16 cmake ninja pkg-config icu4c capstone
pip3 install pyelftools requests

Usage

Blutter can analyze Flutter applications in several ways.

APK File

If you have an .apk file. Simply provide the path to the APK file and the output directory as arguments:

python3 blutter.py path/to/app.apk out_dir

.so File(s)

Blutter can also analyze .so files directly. This can be done in two ways:

1. Analyzing .so files extracted from an APK:

   If you have extracted the lib directory from an APK file, you can analyze it using Blutter. Provide the path to the lib directory and the output directory as arguments:

   python3 blutter.py path/to/app/lib/arm64-v8a out_dir

   The blutter.py will automatically detect the Dart version from the Flutter engine and use the appropriate executable to extract information from libapp.so.

2. Analyzing libapp.so with a known Dart version:

   If you only have libapp.so and know its Dart version, you can specify it to Blutter. Provide the Dart version with --dart-version option, the path to libapp.so, and the output directory as arguments:

   python3 blutter.py --dart-version X.X.X_android_arm64 libapp.so out_dir

   Replace X.X.X with your lib dart version such as "3.4.2_android_arm64".

If the Blutter executable for the required Dart version does not exist, the script will automatically checkout the Dart source code and compile it.

Update

You can use git pull to update and run blutter.py with --rebuild option to force rebuild the executable

python3 blutter.py path/to/app/lib/arm64-v8a out_dir --rebuild

Output files

• asm/* libapp assemblies with symbols
• blutter_frida.js the frida script template for the target application
• objs.txt complete (nested) dump of Object from Object Pool
• pp.txt all Dart objects in Object Pool

Directories

• bin contains blutter executables for each Dart version in "blutter_dartvm<ver>_<os>_<arch>" format
• blutter contains source code. need building against Dart VM library
• build contains building projects which can be deleted after finishing the build process
• dartsdk contains checkout of Dart Runtime which can be deleted after finishing the build process
• external contains 3rd party libraries for Windows only
• packages contains the static libraries of Dart Runtime
• scripts contains python scripts for getting/building Dart

Generating Visual Studio Solution for Development

I use Visual Studio to delevlop Blutter on Windows. --vs-sln options can be used to generate a Visual Studio solution.

python blutter.py path\to\lib\arm64-v8a build\vs --vs-sln

TODO

• More code analysis
  • Function arguments and return type
  • Some psuedo code for code pattern
• Generate better Frida script
  • More internal classes
  • Object modification
• Obfuscated app (still missing many functions)
• Reading iOS binary
• Input as ipa