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 successfull 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==0.8.0

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

Configure Burp Suite proxy server to listen on *:8083
Proxy Tab -> Options -> Proxy Listeners -> Edit -> Binding Tab

Then enable invisible proxying in Request Handling Tab
Support Invisible Proxying -> true

impact@f:~$ reflutter main.ipa

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. Next, you should configure the Proxy in BurpSuite -> Listener Proxy -> Options tab

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

You don't need to install any certificates. On an Android device, you don't need root access as well. reFlutter also allows to bypass some of the flutter certificate pinning implementations.

Usage on Android

The resulting apk must be aligned and signed. I use uber-apk-signer java -jar uber-apk-signer.jar --allowResign -a release.RE.apk. To see which code is loaded through DartVM, you need to run the application on the device. Note that you must manually find what _kDartIsolateSnapshotInstructions (ex. 0xB000 ) equals to using a binary search. reFlutter writes the dump to the root folder of the application and sets 777 permissions to the file and folder. You can pull the file with adb command

impact@f:~$ adb -d shell "cat /data/data/<PACKAGE_NAME>/dump.dart" > dump.dart

file contents

Library:'package:anyapp/navigation/DeepLinkImpl.dart' Class: Navigation extends Object {
String* DeepUrl = anyapp://evil.com/ ;
 Function 'Navigation.': constructor. (dynamic, dynamic, dynamic, dynamic) => NavigationInteractor {

              Code Offset: _kDartIsolateSnapshotInstructions + 0x0000000000009270

                   }

 Function 'initDeepLinkHandle':. (dynamic) => Future<void>* {

              Code Offset: _kDartIsolateSnapshotInstructions + 0x0000000000412fe8

                   }

 Function '_navigateDeepLink@547106886':. (dynamic, dynamic, {dynamic navigator}) => void {

              Code Offset: _kDartIsolateSnapshotInstructions + 0x0000000000002638

                   }

       }

Library:'package:anyapp/auth/navigation/AuthAccount.dart' Class: AuthAccount extends Account {
PlainNotificationToken* _instance = sentinel;

 Function 'getAuthToken':. (dynamic, dynamic, dynamic, dynamic) => Future<AccessToken*>* {

               Code Offset: _kDartIsolateSnapshotInstructions + 0x00000000003ee548

                   }

 Function 'checkEmail':. (dynamic, dynamic) => Future<bool*>* {

               Code Offset: _kDartIsolateSnapshotInstructions + 0x0000000000448a08

                   }
 Function 'validateRestoreCode':. (dynamic, dynamic, dynamic) => Future<bool*>* {

               Code Offset: _kDartIsolateSnapshotInstructions + 0x0000000000412c34

                   }
 Function 'sendSmsRestorePassword':. (dynamic, dynamic) => Future<bool*>* {

               Code Offset: _kDartIsolateSnapshotInstructions + 0x00000000003efb88

                   }
       }

Usage on iOS

Use the IPA file created after the execution of reflutter main.ipa command. To see which code is loaded through DartVM, you need to run the application on the device. reFlutter will print the dump file path to the Xcode console logs with the reFlutter tag Current working dir: /private/var/mobile/Containers/Data/Application/<UUID>/dump.dart Next, you will need to pull the file from the device

Frida

The resulting offset from the dump can be used in the frida script

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

To get value for _kDartIsolateSnapshotInstructions you can use readelf -Ws libapp.so Where is the value you need in 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

The engines are built using reFlutter in Github Actions to build the desired version, commits and snapshot hashes are used from this table. The hash of the snapshot is extracted from storage.googleapis.com/flutter_infra_release/flutter/<hash>/android-arm64-release/linux-x64.zip

release

Custom Build

If you would like to implement your own patches, manual Flutter code change is supported using specially crafted Docker

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

Build command:

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

Linux, Windows

EXAMPLE BUILD ANDROID ARM64:

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 building for x64 architecture, use to reduce building time>
-e arm64=0 <disables building for arm64 architecture, use to reduce building time>
-e arm=0 <disables building for arm32 architecture, use to reduce building time>
-e WAIT=300
-e HASH_PATCH=[Snapshot_Hash]
-e COMMIT=[Engine_commit] <here you specify commit for your engine version, take it from enginehash.csv table or from flutter/engine repo>