reFlutter

Read more on the blog: https://swarm.ptsecurity.com/fork-bomb-for-flutter/

This framework helps with Flutter apps reverse engineering using the patched version of the Flutter library which is already compiled and ready for app repacking. This library has snapshot deserialization process modified to allow you perform dynamic analysis in a convenient way.

Key features:

• socket.cc is patched for traffic monitoring and interception;
• dart.cc is modified to print classes, functions and some fields;
• display absolute code offset for functions;
• contains minor changes for successful compilation;
• if you would like to implement your own patches, manual Flutter code changes are supported using a specially crafted Dockerfile.

Supported engines

• Android: arm64, arm32;
• iOS: arm64;
• Release: Stable, Beta

Install

# Linux, Windows, MacOS
pip3 install reflutter==0.8.6

Usage

impact@f:~$ reflutter main.apk

Please enter your Burp Suite IP: <input_ip>

SnapshotHash: 8ee4ef7a67df9845fba331734198a953
The resulting apk file: ./release.RE.apk
Please sign the apk file

impact@f:~$ reflutter main.ipa

Options:

• -p, --patch-dump — dump mode: patch the engine to emit dump.dart (classes/methods/offsets) on start, and print a frida.js hint instead of the proxy instructions.
• -n, --no-interact — never prompt for a Burp IP (implies 127.0.0.1); useful for old engines in CI.
• -b <Snapshot_Hash>, --build-engine — engine build mode: print the engine commit for a snapshot hash and (when run inside a flutter/flutter checkout) apply the reFlutter source patches. See scripts/local-release.

Traffic interception

You need to specify the IP of your Burp Suite Proxy Server located in the same network where the device with the Flutter application is. Then configure the Proxy in BurpSuite -> Listener Proxy -> Options tab:

• Add port: 8083
• Bind to address: All interfaces
• Request handling: Support invisible proxying = True

No certificate installation or root access is required for Android. reFlutter also allows bypassing some of the Flutter certificate pinning implementations.

⚠️ Note: Engines up to and including Flutter 3.24.x (snapshot hash 80a49c7111088100a233b2ae788e1f48) still carry the hardcoded proxy IP and get patched in place. Starting with 3.27.x the hardcoded IP is gone — configure the proxy directly on the device instead.

On Android

Use ADB to configure the device’s proxy:

adb -s <device> shell "settings put global http_proxy <proxy_ip:port>"

Sign, align, and install the APK. Optionally configure TunProxy to route traffic through Burp Suite.

On iOS

Sign and install the IPA. Configure Potatso to use your Burp Suite proxy server.

Usage on Android

The resulting apk must be aligned and signed. A recommended tool is uber-apk-signer:

java -jar uber-apk-signer.jar --allowResign -a release.RE.apk

Run the app on a device. Determine _kDartIsolateSnapshotInstructions via binary search. reFlutter writes the dump file to the app's root folder and sets 777 permissions. Retrieve it using:

adb -d shell "cat /data/data/<PACKAGE_NAME>/dump.dart" > dump.dart

Library:'package:anyapp/navigation/DeepLinkImpl.dart' Class: Navigation extends Object {
String* DeepUrl = anyapp://evil.com/ ;
...

Usage on iOS

After running reflutter main.ipa, execute the app on device. The dump file path is printed to Xcode console logs:

Current working dir: /private/var/mobile/Containers/Data/Application/<UUID>/dump.dart

Retrieve the file from the device.

Frida

frida-tools==13.7.1
frida==16.7.19

Use dump offsets in the Frida script:

frida -U -f <package> -l frida.js

To find _kDartIsolateSnapshotInstructions:

readelf -Ws libapp.so

Look for the Value field.

To Do

• Display absolute code offset for functions;
• Extract more strings and fields;
• Add socket patch;
• Extend engine support to Debug using Fork and Github Actions;
• Improve detection of App.framework and libapp.so inside zip archive

Build Engine

Engines are built with scripts/local-release (macOS; builds v2 + v3 for iOS and Android arm64/arm/x64, verifies every patch landed, and uploads the release assets) based on data in enginehash.csv. The engine commit for a snapshot hash is resolved via reflutter -b <Snapshot_Hash>. Snapshot hash is retrieved from:

https://storage.googleapis.com/flutter_infra_release/flutter/<hash>/android-arm64-release/linux-x64.zip

Custom Build

Manual Flutter code patching is supported using Docker:

git clone https://github.com/Impact-I/reFlutter && cd reFlutter
docker build -t reflutter -f Dockerfile .

Run with:

docker run -it -v "$(pwd):/t" -e HASH_PATCH=<Snapshot_Hash> -e COMMIT=<Engine_commit> reflutter

Example:

docker run -it -v "$(pwd):/t" -e HASH_PATCH=aa64af18e7d086041ac127cc4bc50c5e -e COMMIT=d44b5a94c976fbb65815374f61ab5392a220b084 reflutter

Example: Build Android ARM64 (Linux/Windows)

docker run -e WAIT=300 -e x64=0 -e arm=0 -e HASH_PATCH=<Snapshot_Hash> -e COMMIT=<Engine_commit> --rm -iv${PWD}:/t reflutter

Flags:

• -e x64=0: disables x64 build
• -e arm64=0: disables arm64 build
• -e arm=0: disables arm32 build
• -e WAIT=300: time in seconds to modify source before build
• -e HASH_PATCH: snapshot hash from enginehash.csv
• -e COMMIT: engine commit hash