🛡️ Part 13: SSL Pinning Bypass in Flutter — How Attackers Spoof Certificates to Intercept…

This article was translated specially for the channel FlutterPulseYou'll find lots of interesting things related to Flutter on this channel. Don't hesitate to subscribe!🚀

Your Flutter app uses HTTPS, right?

🔙 Check Previous Part [Firebase Anonymous Auth Hijacking]

But did you know:

Even HTTPS isn't enough if SSL pinning is not properly implemented or is easily bypassed?

Attackers can intercept encrypted traffic between your app and server using man-in-the-middle (MITM) techniques, spoof TLS certificates, and steal or modify sensitive data — especially if your Flutter app:

• Doesn't use SSL pinning
• Implements it incorrectly or only at the plugin level
• Is vulnerable to Frida or certificate forging-based bypasses

This article uncovers how SSL/TLS pinning works in Flutter, how attackers break it, and how to defend against advanced bypass strategies.

🔍 What's the Problem?

Most developers rely on HTTPS and assume it's secure by default.

But tools like:

• Charles Proxy
• Burp Suite
• mitmproxy
• Frida with certificate override
• Custom CA injection on rooted devices

...can intercept all app traffic, even if it's over HTTPS, unless the app validates server identity via SSL pinning.

🕵️‍♂️ Real-World Exploit Scenarios

💣 Exploit 1: MITM on a Device Without Pinning

On a real or emulator device:

adb install myapp.apk
adb reverse tcp:8080 tcp:8080 # redirect traffic to proxy

In Charles/Burp, attacker installs their CA certificate → Flutter app accepts it as trusted root.
Result: Traffic decrypted.

💣 Exploit 2: Frida Script to Bypass SSL Pinning

If the app implements pinning (even native), this Frida script can override certificate checks:

Java.perform(function () {
  var TrustManagerImpl = Java.use('com.android.org.conscrypt.TrustManagerImpl');
  TrustManagerImpl.checkTrustedRecursive.implementation = function () {
    console.log("SSL Pinning bypassed");
    return;
  };
});

Result: Certificate is blindly accepted, app sends/receives decrypted traffic via attacker proxy.

💣 Exploit 3: Weak or No SSL Pinning in Flutter Plugins

Most HTTP calls in Flutter use packages like:

• http
• dio
• http_client_helper

Without explicit pinning, they rely on system CA list → attacker just needs to add a custom CA to the device.

📉 Real-World Impact

Risk Level: CRITICAL

🛡️ How to Fix It

✅ Fix 1: Implement SSL Pinning in Flutter Using dio or http

Dio Example:

final dio = Dio();
(dio.httpClientAdapter as DefaultHttpClientAdapter).onHttpClientCreate = (client) {
  SecurityContext sc = SecurityContext(withTrustedRoots: false);
  sc.setTrustedCertificates('assets/cert.pem'); // Your pinned cert

  return HttpClient(context: sc);
};

http Example:

final client = HttpClient(context: SecurityContext()
  ..setTrustedCertificates('assets/cert.pem'));

final request = await client.getUrl(Uri.parse("https://api.example.com"));

⚠️ You must include only your certificate, not the system trust store.

✅ Fix 2: Use Public Key Pinning Instead of Certificate Pinning

Certificates expire; public keys don't (as often).

Use SHA-256 fingerprint of your server's public key:

openssl x509 -in cert.pem -pubkey -noout | openssl pkey -pubin -outform DER | openssl dgst -sha256 -binary | base64

Then compare this hash manually in your code before trusting the connection.

✅ Fix 3: Use Third-Party Libraries for Pinning

Plugins like ssl_pinning_plugin offer easier ways to manage cert pinning.

But beware: these can still be bypassed on rooted devices without native-side enforcement or runtime hardening.

✅ Fix 4: Detect Frida and Debugger Attaching

Use native code to detect Frida presence:

public static boolean isFridaPresent() {
  return Build.FINGERPRINT.contains("generic") || 
         checkFridaProcesses();
}

Or monitor /proc/<pid>/maps for frida-agent.so.

✅ Fix 5: Don't Ignore SSL Errors or Use badCertificateCallback

This is a critical mistake:

HttpClient()
  ..badCertificateCallback = (cert, host, port) => true; // ❌ NEVER DO THIS

This disables all validation and makes MITM trivial.

❌ Anti-Patterns to Avoid

✅ Developer Checklist

• Pin SSL certificates or public keys in all network calls
• Disable system CA trust when pinning
• Never use badCertificateCallback = true
• Rotate certificates safely with overlapping periods
• Detect Frida or debugger presence
• Validate fingerprint or cert expiration in app builds
• Log/report SSL pinning failures in production builds

👀 Up Next

🔐 Part 14: Deep Link & URI Hijacking in Flutter — How Malicious Apps Spoof or Intercept Navigation Intents

Thank you for reading this article

If I missed something or made an error, please let me know in the comments. I'm always eager to learn and improve.

Give a clap 👏 if you found this article helpful.