Bypassing SSL Pinning in Flutter Apps: Advanced Techniques for Security Testing

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Introduction

SSL pinning is a security mechanism used by mobile applications to prevent man-in-the-middle (MITM) attacks by ensuring the app communicates only with servers holding a specific certificate. However, during penetration testing, bypassing SSL pinning is often necessary to analyze API traffic. Flutter apps, in particular, can pose challenges due to missing native libraries or obfuscation. This article explores verified methods to bypass SSL pinning in Flutter applications, even when traditional tools fail.

Learning Objectives

  • Understand why Flutter apps may lack `libapp.so` and how to work around it.
  • Learn dynamic SSL pinning bypass techniques using Frida and Objection.
  • Explore alternative approaches when static patching (e.g., reflutter) is ineffective.

You Should Know

1. Inspecting APK Architecture with `apktool`

Command:

apktool d -r -s target.apk

Step-by-Step Guide:

1. Decompile the APK to check for `lib/arm64-v8a/libapp.so`.

  1. If missing, extract a universal APK variant or download a compatible `libapp.so` from a similar Flutter app.
  2. Repack the APK using `apktool b` and sign it for testing.

2. Dynamic Bypass with Frida

Command:

frida -U -f com.example.app -l ssl_pinning_bypass.js

Step-by-Step Guide:

  1. Use Frida to hook into the Dart runtime and override `HttpClient` methods:
    Interceptor.attach(Module.findExportByName("libapp.so", "ssl_crypto_x509_session_verify_cert_chain"), { 
onEnter: function(args) { 
console.log("Bypassing SSL pinning..."); 
this.returnValue = 1; // Force validation success 
} 
});
  2. Attach to the running app process if static patching fails.

3. Using Objection for Quick Bypass

Command:

objection explore --startup-command "android sslpinning disable"

Step-by-Step Guide:

  1. Objection automates Frida scripts to disable common pinning methods.
  2. Works for Flutter apps using platform-level pinning (e.g., OkHttp).

4. Manual Certificate Pinning Override

Command (Android):

adb push burp-cert.der /data/local/tmp/cert-der.crt

Step-by-Step Guide:

  1. Add a custom CA certificate to the device.
  2. Use `Xposed` or `Magisk` modules to force the app to trust user-installed certificates.

5. Reverse Engineering with `jadx`

Command:

jadx --deobf target.apk

Step-by-Step Guide:

  1. Analyze decompiled Dart code for custom pinning logic.
  2. Patch the Smali code to bypass checks if Frida fails.

What Undercode Say

  • Key Takeaway 1: Flutter’s lack of `libapp.so` often indicates intentional obfuscation—dynamic analysis (Frida) is more reliable than static patching.
  • Key Takeaway 2: Universal APKs or cross-architecture library extraction can resolve `reflutter` errors.

Analysis:

Flutter’s growing adoption demands adaptable pentesting techniques. While reflutter simplifies patching, its dependency on native libraries makes it brittle. Frida’s runtime manipulation offers a robust alternative, especially for apps stripping debug symbols. Future-proof testing will require combining static analysis (e.g., jadx) with dynamic tools to handle advanced obfuscation.

Prediction

As Flutter evolves, expect more apps to strip native libraries or implement runtime pinning checks. Automated tools like reflutter may become obsolete, shifting focus to Frida-based scripting and AI-assisted reverse engineering (e.g., GPT-4 for Dart code analysis). Penetration testers must prioritize dynamic analysis skills to keep pace.

IT/Security Reporter URL:

Reported By: Akshay Padwalkar – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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