Reverser Android Malware

Android APK Malware Analysis

Full analysis workflow for suspicious Android APKs — from static manifest review through dynamic instrumentation and C2 extraction.

Quick Reference

# Decode APK resources and smali
apktool d <TARGET>.apk -o ./decoded/

# Decompile to Java source
jadx -d ./java_src/ <TARGET>.apk

# Quick permission audit
aapt dump permissions <TARGET>.apk

# Hash and signature check
sha256sum <TARGET>.apk
apksigner verify --print-certs <TARGET>.apk

# Automated scan
docker run -p 8000:8000 opensecurity/mobile-security-framework-mobsf
# Upload APK to http://localhost:8000

# List Frida processes on device
frida-ps -U

MITRE ATT&CK Mapping

1. Static Analysis — APK Structure

Unpack and examine the APK structure and manifest.

# Decode APK (keeps smali + resources)
apktool d <TARGET>.apk -o ./apk_decoded/

# Examine AndroidManifest.xml for suspicious permissions
cat ./apk_decoded/AndroidManifest.xml | grep -iE 'permission|receiver|service|provider'

# High-risk permissions to flag:
# SEND_SMS, RECEIVE_SMS, READ_SMS — SMS interception (banking trojans)
# READ_CONTACTS, READ_CALL_LOG — data exfiltration
# RECORD_AUDIO, CAMERA — spyware
# ACCESSIBILITY_SERVICE — overlay attacks, keylogging
# BIND_DEVICE_ADMIN — device admin abuse (ransomware)
# INSTALL_PACKAGES — dropper / sideloader
# REQUEST_IGNORE_BATTERY_OPTIMIZATIONS — persistent background service

# Check for accessibility service (major red flag for credential stealers)
grep -r "android.accessibilityservice" ./apk_decoded/AndroidManifest.xml

# List receivers (broadcast receivers for SMS, boot, etc.)
grep -A5 "receiver" ./apk_decoded/AndroidManifest.xml

# Check for native libraries
ls -la ./apk_decoded/lib/*/

# Certificate analysis
apksigner verify --print-certs -v <TARGET>.apk
# Self-signed with debug key? → highly suspicious
# Check: keytool -printcert -jarfile <TARGET>.apk

2. Decompile to Java

Recover readable source from DEX bytecode.

# jadx — best-in-class Android decompiler
jadx -d ./java_src/ --show-bad-code <TARGET>.apk
# --show-bad-code includes methods that failed to decompile cleanly

# Search for key behaviors
grep -rn "SmsManager\|sendTextMessage\|getDefault()" ./java_src/
grep -rn "HttpURLConnection\|OkHttpClient\|Retrofit\|Volley" ./java_src/
grep -rn "getDeviceId\|getSubscriberId\|getLine1Number\|IMEI" ./java_src/
grep -rn "getInstalledPackages\|getInstalledApplications" ./java_src/
grep -rn "TelephonyManager\|PackageManager" ./java_src/
grep -rn "Runtime\.exec\|ProcessBuilder\|\"su\"" ./java_src/
grep -rn "DexClassLoader\|PathClassLoader\|InMemoryDexClassLoader" ./java_src/
grep -rn "crypto\|Cipher\|SecretKey\|AES\|DES" ./java_src/
grep -rn "AccessibilityService\|onAccessibilityEvent" ./java_src/

# Extract hardcoded URLs / IPs / domains
grep -rnE 'https?://[^\s"]+|[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}' ./java_src/

# Alternative: dex2jar + JD-GUI
d2j-dex2jar <TARGET>.apk -o <TARGET>.jar
# Open <TARGET>.jar in JD-GUI

3. Smali Analysis

For obfuscated samples where jadx fails, work at smali level.

# Smali is already extracted by apktool in step 1
# Navigate to entry points
find ./apk_decoded/smali* -name "*.smali" | head -20

# Find the main activity
grep -r "android.intent.action.MAIN" ./apk_decoded/AndroidManifest.xml
# Then read the corresponding smali file

# Search for reflection (used to hide API calls)
grep -rn "Ljava/lang/reflect" ./apk_decoded/smali*/
grep -rn "invoke-virtual.*Method;->invoke" ./apk_decoded/smali*/

# Search for dynamic class loading (dropper behavior)
grep -rn "DexClassLoader\|InMemoryDexClassLoader" ./apk_decoded/smali*/

# Search for crypto operations
grep -rn "Ljavax/crypto" ./apk_decoded/smali*/

# Search for command execution (root exploits)
grep -rn "Runtime;->exec\|\"su\"" ./apk_decoded/smali*/

# Decode obfuscated strings — common pattern: XOR or Base64 in static init
grep -rn "const-string\|sget-object" ./apk_decoded/smali*/ | grep -i "Base64\|decode"

4. Dynamic Analysis with Frida

Instrument the running APK to intercept behavior.

# Start Frida server on rooted device/emulator
adb push frida-server-<version>-android-<arch> /data/local/tmp/frida-server
adb shell "chmod 755 /data/local/tmp/frida-server && /data/local/tmp/frida-server &"

# List running apps
frida-ps -Ua

# Hook encryption calls to capture plaintext
frida -U -l hook_crypto.js -f <PACKAGE_NAME> --no-pause

// hook_crypto.js — intercept javax.crypto.Cipher
Java.perform(function() {
    var Cipher = Java.use("javax.crypto.Cipher");

    Cipher.doFinal.overload("[B").implementation = function(input) {
        console.log("[*] Cipher.doFinal called");
        console.log("    Algorithm: " + this.getAlgorithm());
        console.log("    Input: " + bytesToHex(input));
        var result = this.doFinal(input);
        console.log("    Output: " + bytesToHex(result));
        return result;
    };

    // Hook URL connections for C2 traffic
    var URL = Java.use("java.net.URL");
    URL.$init.overload("java.lang.String").implementation = function(url) {
        console.log("[*] URL: " + url);
        return this.$init(url);
    };

    // Hook SMS sending
    var SmsManager = Java.use("android.telephony.SmsManager");
    SmsManager.sendTextMessage.implementation = function(dest, sc, text, sentPI, delPI) {
        console.log("[!] SMS to: " + dest + " body: " + text);
        // Don't actually send
        // return this.sendTextMessage(dest, sc, text, sentPI, delPI);
    };
});

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('');
}

# SSL pinning bypass for traffic interception
frida -U -l ssl_bypass.js -f <PACKAGE_NAME> --no-pause
# Then proxy through Burp/mitmproxy on port 8080

# Trace all JNI calls (native ↔ Java bridge)
frida-trace -U -i "JNI_*" -f <PACKAGE_NAME>

5. Network Traffic Analysis

# Set up transparent proxy on emulator
adb shell settings put global http_proxy <HOST_IP>:8080

# mitmproxy with Android CA installed
mitmproxy --mode transparent --listen-port 8080

# For certificate-pinned apps, use Frida SSL bypass first
# Then capture traffic in mitmproxy/Burp

# tcpdump on device (root required)
adb shell tcpdump -i any -w /sdcard/capture.pcap
adb pull /sdcard/capture.pcap
wireshark capture.pcap

# Analyze DNS queries
tshark -r capture.pcap -Y "dns.qr == 0" -T fields -e dns.qry.name | sort -u

6. Native Library Analysis

# List native libraries in APK
unzip -l <TARGET>.apk | grep "\.so$"

# Extract and analyze
unzip <TARGET>.apk lib/ -d ./native_libs/

# Identify architecture
file ./native_libs/lib/*/*.so

# Strings from native libs
strings -n 8 ./native_libs/lib/arm64-v8a/*.so | grep -iE 'http|socket|exec|system|decrypt|key|aes'

# Ghidra/IDA for full native RE
# Focus on: JNI_OnLoad, Java_<package>_<class>_<method> exports
readelf -s ./native_libs/lib/arm64-v8a/*.so | grep -i "java_\|jni_onload"

# Check for packer/protector in native code
# Bangcle, Qihoo 360, Baidu, Tencent packers wrap the real DEX
rabin2 -I ./native_libs/lib/arm64-v8a/*.so

7. Automated Analysis with MobSF

# Run MobSF
docker run -it --rm -p 8000:8000 opensecurity/mobile-security-framework-mobsf

# Upload via API
curl -F "file=@<TARGET>.apk" http://localhost:8000/api/v1/upload \
  -H "Authorization: <API_KEY>"

# Get scan results
curl "http://localhost:8000/api/v1/scan" \
  -X POST -d "scan_type=apk&file_name=<TARGET>.apk&hash=<FILE_HASH>" \
  -H "Authorization: <API_KEY>"

# MobSF checks:
# - Manifest analysis (permissions, exported components)
# - Code analysis (hardcoded secrets, insecure APIs)
# - Binary analysis (native libs, packer detection)
# - CVSS scoring and compliance mapping

Tools & Resources

Detection Signatures

Error Handling & Edge Cases

Decision Gate

IF APK has suspicious permissions (SMS + Accessibility + Internet):
  → Full static analysis (manifest + jadx + strings)
  → Extract C2 URLs and hardcoded secrets
  → Dynamic analysis with Frida hooks on crypto/network/SMS
ELSE IF APK has native libraries with obfuscated exports:
  → Likely packed — dump real DEX with frida-dexdump
  → Analyze unpacked DEX with jadx
  → RE native lib with Ghidra for JNI logic
ELSE IF APK loads code dynamically (DexClassLoader):
  → Dropper — intercept payload download with Frida
  → Analyze second-stage DEX separately
ELSE:
  → Standard jadx decompilation
  → Behavioral analysis in MobSF
  → Network traffic capture for C2 identification