seikaikyo/performing-dynamic-analysis-of-android-app

Performing Dynamic Analysis of Android App

When to Use

Use this skill when:

• Static analysis results need runtime validation on an actual Android device
• The target app uses obfuscation (DexGuard, custom packers) that prevents effective static analysis
• Testing requires observing actual API calls, decrypted data, or runtime-generated values
• Assessing root detection, tamper detection, or anti-debugging implementations

Do not use this skill on production environments without authorization -- dynamic instrumentation can alter app behavior and trigger security alerts.

Prerequisites

• Rooted Android device or emulator (Genymotion, Android Studio AVD with writable system)
• Frida server installed on device matching the architecture (arm64, x86_64)
• Python 3.10+ with frida-tools and objection packages
• ADB configured and device connected
• Target APK installed on device

Workflow

Step 1: Setup Frida Server on Android Device

# Check device architecture
adb shell getprop ro.product.cpu.abi
# Output: arm64-v8a

# Download matching Frida server from GitHub releases
# https://github.com/frida/frida/releases
# Push to device
adb push frida-server-16.x.x-android-arm64 /data/local/tmp/frida-server
adb shell chmod 755 /data/local/tmp/frida-server
adb shell /data/local/tmp/frida-server &

# Verify Frida connection
frida-ps -U

Step 2: Enumerate Application Attack Surface

# List all packages
frida-ps -U -a

# Attach Objection for high-level exploration
objection --gadget com.target.app explore

# List activities, services, receivers
android hooking list activities
android hooking list services
android hooking list receivers

# List loaded classes
android hooking list classes
android hooking search classes com.target.app

Step 3: Hook Sensitive Methods

# Hook all methods of a class
android hooking watch class com.target.app.auth.LoginManager

# Hook specific method with argument dumping
android hooking watch class_method com.target.app.auth.LoginManager.authenticate --dump-args --dump-return

# Hook crypto operations
android hooking watch class javax.crypto.Cipher --dump-args
android hooking watch class java.security.MessageDigest --dump-args

# Hook network calls
android hooking watch class okhttp3.OkHttpClient --dump-args
android hooking watch class java.net.URL --dump-args

Step 4: Write Custom Frida Scripts for Deep Analysis

// hook_crypto.js - Intercept encryption/decryption operations
Java.perform(function() {
    var Cipher = Java.use("javax.crypto.Cipher");

    Cipher.doFinal.overload("[B").implementation = function(input) {
        var mode = this.getAlgorithm();
        console.log("[Cipher] Algorithm: " + mode);
        console.log("[Cipher] Input: " + bytesToHex(input));

        var result = this.doFinal(input);
        console.log("[Cipher] Output: " + bytesToHex(result));
        return result;
    };

    function bytesToHex(bytes) {
        var hex = [];
        for (var i = 0; i < bytes.length; i++) {
            hex.push(("0" + (bytes[i] & 0xFF).toString(16)).slice(-2));
        }
        return hex.join("");
    }
});

# Execute custom Frida script
frida -U -f com.target.app -l hook_crypto.js --no-pause

Step 5: Bypass Root Detection

// root_bypass.js - Common root detection bypass
Java.perform(function() {
    // Bypass RootBeer library
    var RootBeer = Java.use("com.scottyab.rootbeer.RootBeer");
    RootBeer.isRooted.implementation = function() {
        console.log("[RootBeer] isRooted() bypassed");
        return false;
    };

    // Bypass generic file-based root checks
    var File = Java.use("java.io.File");
    var originalExists = File.exists;
    File.exists.implementation = function() {
        var path = this.getAbsolutePath();
        var rootPaths = ["/system/app/Superuser.apk", "/system/xbin/su",
                         "/sbin/su", "/system/bin/su", "/data/local/bin/su"];
        if (rootPaths.indexOf(path) >= 0) {
            console.log("[Root] Blocked check for: " + path);
            return false;
        }
        return originalExists.call(this);
    };

    // Bypass SafetyNet/Play Integrity
    try {
        var SafetyNet = Java.use("com.google.android.gms.safetynet.SafetyNetApi");
        console.log("[SafetyNet] Class found - may need additional bypass");
    } catch(e) {}
});

Step 6: Analyze Network Communication at Runtime

// network_monitor.js - Monitor all HTTP requests
Java.perform(function() {
    // Hook OkHttp3
    try {
        var OkHttpClient = Java.use("okhttp3.OkHttpClient");
        var Interceptor = Java.use("okhttp3.Interceptor");
        var Chain = Java.use("okhttp3.Interceptor$Chain");

        console.log("[OkHttp] Monitoring network requests...");

        var Request = Java.use("okhttp3.Request");
        Request.url.implementation = function() {
            var url = this.url();
            console.log("[OkHttp] URL: " + url.toString());
            return url;
        };
    } catch(e) {
        console.log("[OkHttp] Not found, trying HttpURLConnection");
    }

    // Hook HttpURLConnection
    var URL = Java.use("java.net.URL");
    URL.openConnection.overload().implementation = function() {
        console.log("[URL] Opening: " + this.toString());
        return this.openConnection();
    };
});

Step 7: Extract Decrypted Data and Secrets

# Using Objection for quick extraction
objection --gadget com.target.app explore

# Dump Android Keystore entries
android keystore list
android keystore dump

# Search heap for sensitive objects
android heap search instances com.target.app.model.User
android heap evaluate <handle> "JSON.stringify(clazz)"

# Memory string search
memory search "password" --string
memory search "api_key" --string

Key Concepts

| Term | Definition | |------|-----------| | Dynamic Instrumentation | Modifying application behavior at runtime by injecting code into the running process | | Method Hooking | Replacing or wrapping function implementations to intercept arguments and return values | | Frida Server | Daemon running on the target device that receives instrumentation commands from the host | | Dalvik/ART Runtime | Android runtime environments; Frida hooks at the ART level for Java/Kotlin methods | | Heap Inspection | Examining live objects in the application's memory heap to extract runtime data |

Tools & Systems

• Frida: Dynamic instrumentation toolkit for injecting JavaScript into native Android processes
• Objection: Higher-level Frida wrapper with pre-built Android and iOS security testing commands
• frida-trace: Automated method tracing utility for quick reconnaissance of app behavior
• Drozer: Android security assessment framework for testing IPC and exported components
• Android Studio Profiler: Runtime monitoring for CPU, memory, and network activity

Common Pitfalls

• Frida version mismatch: The Frida server on the device must match the frida-tools version on the host. Version mismatches cause connection failures.
• Anti-Frida detection: Some apps detect Frida by checking for the Frida server process, scanning memory for Frida signatures, or monitoring /proc/self/maps. Use Frida Gadget injection or custom server builds.
• Obfuscated class names: When ProGuard/R8 is applied, class and method names are shortened (e.g., a.b.c.d()). Use android hooking search classes to discover actual runtime names.
• Multi-DEX apps: Large apps split across multiple DEX files may not have all classes loaded at startup. Hook class loaders or use Java.enumerateLoadedClasses() after app is fully initialized.