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PROXYING FLUTTER TRAFFIC ON ANDROID WITH CLAUDE

Proxying Flutter Traffic on Android with Claude: A Comprehensive Technical Guide

Understanding the Flutter Networking Stack on Android

We operate within a complex ecosystem where mobile applications, particularly those built with cross-platform frameworks like Flutter, present unique challenges for traffic analysis and manipulation. To effectively proxy Flutter traffic on an Android device, we must first dissect the underlying networking architecture. Flutter applications do not rely on the native Android HttpURLConnection or the deprecated OkHttp clients directly. Instead, they utilize the Dart dart:io library, specifically the HttpClient class, which implements its own HTTP protocol stack in pure Dart. This architectural decision isolates Flutter’s networking layer from the traditional Java/Kotlin based proxies that operate at the system level.

When a Flutter application makes a network request, the Dart runtime handles socket creation, TLS handshakes, and HTTP framing entirely in user space. This means that standard system-wide proxy configurations, often set via Wi-Fi settings or ADB commands, may be ignored by the Flutter engine unless the application explicitly opts into respecting them. Furthermore, Flutter’s default behavior often involves certificate pinning or strict certificate validation, which can cause SSL/TLS handshake failures when intercepting traffic with a man-in-the-middle (MITM) proxy like Charles Proxy or Burp Suite.

We must understand that the Dart HttpClient bypasses the Android framework’s default network stack. Consequently, simply configuring a proxy in the Android system settings is insufficient for capturing traffic generated by a Flutter application. We need to intervene at a lower level or modify the application’s behavior to acknowledge our proxy settings. This requires a deep understanding of both the Android network stack and the Dart runtime environment. The objective is to intercept, decrypt, and analyze HTTPS traffic without modifying the Flutter application source code, a task that becomes significantly more complex when dealing with compiled binaries.

The Role of Dart in Network Requests

Dart uses a non-blocking, event-driven architecture. When an HTTP request is initiated, the Dart HttpClient opens a socket connection directly to the target server. If the application uses HTTPS, the Dart runtime performs the TLS handshake. This handshake is critical. If the proxy presents a self-signed certificate (as most interception proxies do), the Dart runtime will typically reject the connection unless the application developer has disabled certificate validation (which is common only in debug builds) or the device’s trust store is modified to accept the proxy’s Certificate Authority (CA).

We observe that standard Android VPNs or proxy tools often fail here because they rely on capturing traffic from the Java VM or the Linux kernel’s network stack. Since Dart utilizes its own user-space networking, these tools see the encrypted traffic before it is handed off to the socket, rendering decryption impossible without cooperation from the application layer. This necessitates the use of tools that can inject into the process or modify the environment in which the Dart code executes.

Introducing Claude: The AI-Assisted Traffic Manipulation Workflow

While the concept of using an AI like Claude for traffic proxying might seem abstract, we utilize it as a sophisticated assistant to generate the necessary code snippets, configuration files, and dynamic instrumentation strategies required to bypass Flutter’s networking constraints. In this context, “Claude” acts as a force multiplier, helping us construct complex Frida scripts, modify APKs, and automate the analysis of network protocols. We do not rely on Claude to perform the proxying itself, but rather to accelerate the engineering tasks required to make the proxy work.

We leverage Claude’s ability to generate context-aware code to solve specific hurdles encountered during the proxying process. For example, when faced with a Flutter application employing SSL pinning, we can query Claude to generate a Frida script that hooks into the Dart VM’s TrustManager or the underlying BoringSSL implementation used by the Dart runtime. This collaborative approach allows us to bypass security mechanisms dynamically, without needing to recompile the Flutter application.

Setting up the Development Environment

Before we can proxy traffic, we must prepare the Android device and our analysis workstation. This environment is the foundation of our operation.

  1. Root Access: We require a rooted Android device. Root access is non-negotiable because we need to install custom certificates into the system trust store and potentially modify application data or inject code into running processes. Tools like Magisk are essential here. We often utilize Magisk modules to facilitate systemless modifications, ensuring we can revert changes if necessary.
  2. Proxy Tool: We need a robust proxy server. Burp Suite (Professional or Community) and Charles Proxy are industry standards. They allow us to intercept requests, view headers, modify payloads, and inspect responses. We configure these tools to listen on all network interfaces, typically on port 8080 or 8888.
  3. Frida: Frida is a dynamic instrumentation toolkit that lets us inject snippets of JavaScript into native apps. It is the Swiss Army knife for reverse engineering on mobile. We install the Frida server on the Android device and the Frida client tools on our workstation.
  4. ADB (Android Debug Bridge): Essential for communicating with the device, pushing files, and executing commands.
  5. Claude (AI Assistant): We keep Claude accessible to generate scripts and troubleshoot errors in real-time.

Method 1: System-Level Proxying with Modified Trust Stores

The most straightforward approach to proxying Flutter traffic involves configuring the application to trust our proxy’s certificate. Since Flutter’s HttpClient validates certificates against the device’s trust store, we must install our proxy’s CA certificate into the Android system’s root certificate authority store.

Extracting the Proxy CA Certificate

First, we export the certificate from our proxy tool. In Burp Suite, this is done by navigating to the Proxy tab, selecting Import/Export CA Certificate, and choosing the Certificate in DER format. We save this file as burp.cer on our workstation.

Installing the Certificate on a Rooted Device

On a non-rooted device, user-installed certificates are stored in the user’s credential store, but Flutter (and many other apps) often ignore this for various security reasons, especially on Android 7 (Nougat) and above which enables Network Security Configuration by default. On a rooted device, we can place the certificate in the system store.

We execute the following steps via ADB:

  1. Convert the certificate to PEM format (if necessary) and calculate the hash.
  2. Push the certificate to /system/etc/security/cacerts/.
  3. Set correct permissions (644) and ownership (root:root).
  4. Reboot the device.

We often rely on Magisk modules like “AlwaysTrustUserCerts” to automate this process. This module automatically symlinks user-installed certificates into the system store, bypassing the need for manual file manipulation. However, even with the certificate in the system store, Flutter might still refuse the connection if it employs Certificate Pinning.

Method 2: Dynamic Instrumentation with Frida and Claude

When system-level proxying fails due to SSL pinning, we turn to dynamic instrumentation. We use Frida to hook into the application at runtime and disable the certificate validation logic. This is where we heavily utilize Claude to craft precise hooks for the specific Flutter binary.

Understanding Flutter’s Binary Structure

A Flutter application consists of a thin Java/Kotlin wrapper (the “platform side”) and a large native shared library (libflutter.so) containing the Dart VM and the compiled application code. The networking logic resides within the Dart VM, which interfaces with the underlying OS via BoringSSL. To bypass pinning, we must hook into the BoringSSL functions or the Dart VM’s internal methods.

Generating Frida Scripts with Claude

We can ask Claude to generate a Frida script tailored to the specific application. A generic script might look like this, but we refine it based on the target app’s behavior:

Prompt to Claude: “Write a Frida script to bypass SSL pinning in a Flutter application on Android by hooking into BoringSSL’s SSL_CTX_set_verify function to disable certificate verification.”

Resulting Strategy: We intercept the SSL_CTX_set_verify function, which is responsible for setting the verification callback for a SSL context. By replacing the callback with a function that always returns success (true), we effectively disable certificate validation for that context.

// Example Frida Script generated via AI assistance
Java.perform(function() {
    var SSL_CTX_set_verify = Module.findExportByName("libssl.so", "SSL_CTX_set_verify");
    
    if (SSL_CTX_set_verify) {
        Interceptor.replace(SSL_CTX_set_verify, new NativeCallback(function(ctx, mode, callback) {
            // Replace the verification callback with a function that always returns 1 (success)
            var newCallback = new NativeCallback(function(preverify_ok, ctx) { return 1; }, 'int', ['int', 'pointer']);
            // Call the original function but with our bypass callback
            var nativeFunction = new NativeFunction(SSL_CTX_set_verify, 'void', ['pointer', 'int', 'pointer']);
            nativeFunction(ctx, mode, newCallback);
        }, 'void', ['pointer', 'int', 'pointer']));
        console.log("[+] SSL_CTX_set_verify hooked");
    } else {
        console.log("[-] SSL_CTX_set_verify not found");
    }
});

We run this script using the Frida client: frida -U -f com.example.app -l bypass_ssl.js

This injects the script into the app process before the code fully executes, allowing us to bypass pinning checks.

Advanced Hooking: Dart VM Internal Methods

Sometimes, BoringSSL hooks are insufficient because the Dart VM handles validation internally. In these cases, we look for exports in libflutter.so. We can ask Claude to analyze the symbols in libflutter.so related to certificate handling. We look for mangled C++ symbols that might correspond to Dart_NativeEntry or specific TLS handlers.

We use tools like nm or objdump on the libflutter.so file to list symbols. Once we identify potential targets, we can hook them with Frida. For instance, if we find a function that acts as a certificate validator, we can NOP (No Operation) the instructions that perform the check or force the function to return a valid status.

Method 3: VPN-Based Proxying with Packet Tunneling

If we cannot root the device or modify the app via Frida, we can use a VPN-based approach. This method creates a local VPN on the Android device that captures all network traffic and routes it through our proxy. ProxyDroid or Postern are classic tools, but modern Android versions restrict VPN usage for interception.

Using Clash or V2Ray for Traffic Redirection

We can configure a VPN app like Clash for Android to act as a transparent proxy. We set up a local SOCKS5 or HTTP proxy on our workstation (e.g., Burp Suite listening on port 8080). We then configure Clash to forward all traffic to this upstream proxy.

However, the challenge remains: Flutter ignores system proxy settings. To solve this, we use TProxy (Transparent Proxy) within the Linux kernel, accessible via root. We use a tool like Drony to force specific apps to use a proxy, but Drony works by creating a local VPN and injecting packets.

The Workflow:

  1. Install Drony on the Android device.
  2. Configure Drony to intercept traffic from the target Flutter app.
  3. Set the upstream proxy in Drony to our Burp Suite instance.
  4. Enable Drony.

Drony captures the packets, applies the proxy settings, and forces the traffic through Burp. This works because Drony operates at the network layer, manipulating the socket connections before they leave the device. It effectively tricks the app into connecting through the proxy, even if the app ignores system settings.

Method 4: Reverse Engineering and Patching the APK

For a permanent solution or when dynamic instrumentation is too unstable, we can reverse engineer the APK, patch the network configuration, and repackage the application.

Decompiling the APK

We use APKTool to decompile the application: apktool d app-release.apk

This disassembles the APK into Smali code and resources. Since Flutter apps are largely native, the Smali code primarily handles the bridge between the Android OS and the Flutter engine.

Modifying Network Security Configuration

Android 7.0+ allows developers to define a custom Network Security Configuration file (network_security_config.xml). If the app defines one, it might restrict cleartext traffic or enforce certificate pinning. We look for this file in res/xml/. If it exists, we can modify it to allow user-added certificates or disable cleartext traffic restrictions.

<!-- Modified network_security_config.xml -->
<network-security-config>
    <base-config cleartextTrafficPermitted="true">
        <trust-anchors>
            <certificates src="system" />
            <certificates src="user" />
        </trust-anchors>
    </base-config>
</network-security-config>

By adding <certificates src="user" />, we allow certificates installed by the user (our proxy CA) to be trusted.

Patching the AndroidManifest.xml

We also check the AndroidManifest.xml for android:usesCleartextTraffic="false". If present, we change it to true to allow HTTP traffic (useful for debugging non-encrypted endpoints, though HTTPS is preferred). We ensure the android:networkSecurityConfig attribute points to our modified configuration file.

Rebuilding and Signing the APK

After modifications, we rebuild the APK: apktool b app-release -o modified.apk

We must sign the APK before installing it. We generate a keystore if we don’t have one and sign the modified APK: jarsigner -verbose -sigalg SHA256withRSA -digestalg SHA-256 -keystore my-release-key.keystore modified.apk alias_name

Finally, we install the modified APK onto the device. This version of the app will now trust our proxy’s certificate, allowing us to intercept traffic easily via Burp Suite or Charles Proxy without needing root or Frida hooks.

Method 5: MitM Proxying with Proxyman and SSL Unpinning

Proxyman is a modern alternative to Charles Proxy, offering a cleaner interface and easier certificate installation. When dealing with Flutter traffic, Proxyman’s “Certificate Pinning Bypass” guide is extensive.

We follow the Proxyman methodology for Android:

  1. Install Proxyman Certificate: Use Proxyman’s “Tools” -> “Install Certificate on Android” feature. This automates the process of pushing the certificate to the device.
  2. Android 7+ Bypass: For Android 7 and above, Proxyman recommends using a script to bypass user certificate restrictions. We use a Python script provided by Proxyman to automate the pushing of certificates to the system partition (requires root).
  3. VPN Option: Proxyman also offers a “Proxyman VPN” tool, which acts as a local VPN to capture traffic without root, similar to Drony.

However, for Flutter, the VPN approach is often the only viable non-root method. We configure the Proxyman VPN to forward traffic to the Proxyman desktop app via a SOCKS5 connection. This setup creates a tunnel that captures the traffic. The Flutter app sees a local VPN connection, and the traffic is routed to Proxyman, where it is decrypted (provided the certificate is trusted).

Troubleshooting Common Issues

Even with the best tools, proxying Flutter traffic can be fraught with errors. We address the most common issues below.

“Certificate Unknown” or SSL Handshake Failures

If we see “Certificate Unknown” in the proxy logs, it means the client (Flutter app) is rejecting the proxy’s certificate. This usually happens because:

Solution: We revert to Method 2 (Frida). We must find the specific function responsible for the validation. We can use Frida’s Interceptor.attach to trace calls to SSL_read and SSL_write to see if data is being exchanged before the handshake fails. If the handshake fails immediately, we know the pinning logic is triggered early.

Traffic Not Appearing in Proxy

If traffic does not appear in the proxy logs, the app might be bypassing the proxy settings entirely.

Performance Latency

Proxied traffic is slower due to the overhead of interception and decryption.

Leveraging Magisk Modules for Proxying

As a repository for Magisk modules (Magisk Module Repository), we can develop or utilize existing modules to streamline this process. A custom Magisk module can automate the installation of the proxy certificate into the system store and even enable TPROXY support in the Linux kernel.

For example, a “Universal Proxy Injector” module could:

  1. Detect the active proxy settings.
  2. Patch the network SecurityConfig of installed apps dynamically (risky and complex).
  3. Enable IP forwarding and iptables rules to redirect traffic to a local proxy port.

We can use the Magisk environment to load persistent Frida servers or set up environment variables that force the Dart VM to respect proxy settings (though this is difficult as Dart doesn’t read standard HTTP_PROXY env vars on Android).

Comparative Analysis of Techniques

We have presented five distinct methods. Each has trade-offs regarding complexity, root requirement, and success rate.

| Method | Root Required? | Difficulty | Success Rate (Pinned) | Best For | | : — | : — | : — | : — | : — | | System Trust Store | Yes | Low | Low | Simple apps

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