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Flutter is Google’s cross-platform UI toolkit that lets developers write a single Dart code-base which the Engine (native C/C++) turns into platform-specific machine code for Android & iOS. The Engine bundles a Dart VM, BoringSSL, Skia, etc., and ships as the shared library libflutter.so (Android) or Flutter.framework (iOS). All actual networking (DNS, sockets, TLS) happens inside this library, nie in the usual Java/Kotlin Swift/Obj-C layers. That siloed design is why the usual Java-level Frida hooks fail on Flutter apps.

Intercepting HTTPS traffic in Flutter

This is a summary of this blog post.

Why HTTPS interception is tricky in Flutter

  • SSL/TLS verification lives two layers down in BoringSSL, so Java SSL‐pinning bypasses don’t touch it.
  • BoringSSL uses its own CA store inside libflutter.so; importing your Burp/ZAP CA into Android’s system store changes nothing.
  • Symbols in libflutter.so are stripped & mangled, hiding the certificate-verification function from dynamic tools.

Fingerprint the exact Flutter stack

Knowing the version lets you re-build or pattern-match the right binaries.

StepCommand / FileOutcome
Get snapshot hashpython3 get_snapshot_hash.py libapp.soadb4292f3ec25…
Map hash → Engineenginehash list in reFlutterFlutter 3 · 7 · 12 + engine commit 1a65d409…
Pull dependent commitsDEPS file in that engine commit• dart_revision → Dart v2 · 19 · 6
• dart_boringssl_rev → BoringSSL 87f316d7…

Find get_snapshot_hash.py here.

Target: ssl_crypto_x509_session_verify_cert_chain()

  • Located in ssl_x509.cc inside BoringSSL.
  • Returns bool – a single true is enough to bypass the whole certificate chain check.
  • Same function exists on every CPU arch; only the opcodes differ.

Option A – Binary patching with reFlutter

  1. Clone the exact Engine & Dart sources for the app’s Flutter version.
  2. Regex-patch two hotspots:
  • In ssl_x509.cc, force return 1;
  • (Optional) In socket_android.cc, hard-code a proxy ("10.0.2.2:8080").
  1. Re-compile libflutter.so, drop it back into the APK/IPA, sign, install.
  2. Pre-patched builds for common versions are shipped in the reFlutter GitHub releases to save hours of build time.

### Option B – Live hooking with Frida (the “hard-core” path) Because the symbol is stripped, you pattern-scan the loaded module for its first bytes, then change the return value on the fly.

// attach & locate libflutter.so
var flutter = Process.getModuleByName("libflutter.so");

// x86-64 pattern of the first 16 bytes of ssl_crypto_x509_session_verify_cert_chain
var sig = "55 41 57 41 56 41 55 41 54 53 48 83 EC 38 C6 02";

Memory.scan(flutter.base, flutter.size, sig, {
onMatch: function (addr) {
console.log("[+] found verifier at " + addr);
Interceptor.attach(addr, {
onLeave: function (retval) { retval.replace(0x1); }  // always 'true'
});
},
onComplete: function () { console.log("scan done"); }
});

Proszę wklej zawartość pliku src/mobile-pentesting/android-app-pentesting/flutter.md, a przełożę odpowiedni tekst na polski zachowując oryginalny markdown i html.

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

Wskazówki dotyczące portowania

  • For arm64-v8a or armv7, pobierz pierwsze ~32 bajty funkcji z Ghidra, skonwertuj na ciąg hex oddzielony spacjami i zamień sig.
  • Zachowaj po jednym wzorcu na wydanie Flutter, przechowuj je w ściągawce do szybkiego użycia.

Wymuszanie ruchu przez proxy

Flutter sam ignoruje ustawienia proxy urządzenia. Najprostsze opcje:

  • Android Studio emulator: Ustawienia ▶ Proxy → ręcznie.
  • Physical device: evil Wi-Fi AP + DNS spoofing, lub Magisk module edytujący /etc/hosts.

Quick Flutter TLS bypass workflow (Frida Codeshare + system CA)

When you only need to observe a pinned Flutter API, połączenie rooted/writable AVD, system-trusted proxy CA i drop-in skryptu Frida jest często szybsze niż reverse-engineering libflutter.so:

  1. Install your proxy CA in the system store. Follow Install Burp Certificate to hash/rename Burp’s DER certificate and push it into /system/etc/security/cacerts/ (writable /system required).

  2. Drop a matching frida-server binary and run it as root so it can attach to the Flutter process:

adb push frida-server-17.0.5-android-x86_64 /data/local/tmp/frida-server
adb shell "su -c 'chmod 755 /data/local/tmp/frida-server && /data/local/tmp/frida-server &'"
  1. Zainstaluj narzędzia po stronie hosta i wyenumeruj docelowy pakiet.
pip3 install frida-tools --break-system-packages
adb shell pm list packages -f | grep target
  1. Uruchom aplikację Flutter z hookiem Codeshare, który neutralizuje kontrole pinów BoringSSL.
frida -U -f com.example.target --codeshare TheDauntless/disable-flutter-tls-v1 --no-pause

The Codeshare script overrides the Flutter TLS verifier so every certificate (including Burp’s dynamically generated ones) is accepted, side-stepping public-key pin comparisons.

  1. Route traffic through your proxy. Skonfiguruj GUI proxy Wi‑Fi emulatora lub wymuś je przez adb shell settings put global http_proxy 10.0.2.2:8080; jeśli bezpośrednie routowanie zawiedzie, użyj adb reverse tcp:8080 tcp:8080 lub host-only VPN.

Once the CA is trusted at the OS layer and Frida quashes Flutter’s pinning logic, Burp/mitmproxy regains full visibility for API fuzzing (BOLA, token tampering, etc.) without repacking the APK.

Offset-based hook of BoringSSL verification (no signature scan)

When pattern-based scripts fail across architectures (e.g., x86_64 vs ARM), directly hook the BoringSSL chain verifier by absolute address within libflutter.so. Workflow:

  • Extract the right-ABI library from the APK: unzip -j app.apk "lib/*/libflutter.so" -d libs/ and pick the one matching the device (e.g., lib/x86_64/libflutter.so).
  • Analyze in Ghidra/IDA and locate the verifier:
  • Source: BoringSSL ssl_x509.cc function ssl_crypto_x509_session_verify_cert_chain (3 args, returns bool).
  • In stripped builds, search for the string "ssl_client" and inspect XREFs; identify the function taking three pointer-like args and returning a boolean.
  • Compute the runtime offset: take the function address shown by Ghidra and subtract the image base used during analysis to get the relative offset (RVA). Example: 0x02184644 - 0x00100000 = 0x02084644.
  • Hook at runtime by base + offset and force success:
// frida -U -f com.target.app -l bypass.js --no-pause
const base = Module.findBaseAddress('libflutter.so');
// Example offset from analysis. Recompute per build/arch.
const off  = ptr('0x02084644');
const addr = base.add(off);

// ssl_crypto_x509_session_verify_cert_chain: 3 args, bool return
Interceptor.replace(addr, new NativeCallback(function (a, b, c) {
return 1; // true
}, 'int', ['pointer', 'pointer', 'pointer']));

console.log('[+] Hooked BoringSSL verify_cert_chain at', addr);

Uwagi

  • Przelicz przesunięcie dla każdej docelowej kompilacji i architektury CPU; compiler/codegen differences break hardcoded signatures.
  • Ten bypass powoduje, że BoringSSL akceptuje dowolny chain, umożliwiając HTTPS MITM niezależnie od pins/CA trust wewnątrz Flutter.
  • Jeśli podczas debugowania wymusisz przekierowanie ruchu, aby potwierdzić blokowanie TLS, np.:
iptables -t nat -A OUTPUT -p tcp -j DNAT --to-destination <Burp_IP>:<Burp_Port>

… nadal będziesz potrzebować powyższego hooka, ponieważ weryfikacja odbywa się wewnątrz libflutter.so, a nie w systemowym trust store Androida.

Referencje

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