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

```bash
# 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

```bash
# 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

```bash
# 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

```javascript
// 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("");
    }
});
```

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

### Step 5: Bypass Root Detection

```javascript
// 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

```javascript
// 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

```bash
# 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.