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Introduction:
A newly released European Union age verification prototype app promised robust identity checks, but security researchers quickly discovered that its “protection” melts away with basic file manipulation. Storing sensitive security controls—PIN lockout counters and biometric toggles—in plaintext files accessible to any user with root privileges turns the app into a digital house of cards. While the EU clarified this is a proof-of-concept not intended for production, the design flaws highlight a dangerous trend: developers trusting local file integrity without cryptographic binding or hardware-backed security.
Learning Objectives:
- Understand how insecure local storage (plaintext files) undermines mobile app authentication
- Learn to audit Android app data directories for misconfigured security controls
- Implement proper credential binding using Android Keystore and encrypted shared preferences
You Should Know:
- Anatomy of the Hack: Why a Text File Should Never Guard Your Identity
The core vulnerability stems from treating client-side files as authoritative sources of truth. The EU app stored three critical artifacts in /data/data/com.eu.ageverification/shared_prefs/:
– `pin_config.xml` – contained the user’s PIN hash (weakly salted) and a flag `biometric_enabled`
– `attempt_counter.txt` – plain integer tracking failed PIN entries
– `identity_claim.dat` – base64-encoded verified credentials (name, age, government ID hash)
Because Android’s app sandbox protects these files from unprivileged apps but not from root, an attacker with physical access and a rooted device can modify them. The step-by-step exploit:
Step 1 – Gain root access (requires unlocked bootloader + Magisk or similar).
Step 2 – Locate app data directory using `adb shell` or terminal emulator:
adb shell su cd /data/data/com.eu.ageverification/shared_prefs/ ls -la
Step 3 – Reset lockout counter (bypass “too many attempts”):
echo "0" > attempt_counter.txt
Step 4 – Disable biometric check by changing `biometric_enabled` from `true` to `false` in pin_config.xml:
<!-- Before --> <boolean name="biometric_enabled" value="true" /> <!-- After --> <boolean name="biometric_enabled" value="false" />
Step 5 – Delete PIN hash file (or corrupt it). Restart the app – it will prompt for a new PIN setup, then blithely re-encrypt the existing identity credentials under the attacker’s chosen PIN.
Why it works: The app never cryptographically bound the PIN to the identity data. No authentication tag tied the stored credential to the original enrollment. A proper design would derive a key from the PIN using a KDF (e.g., PBKDF2) and use that key to wrap the identity data – making PIN changes impossible without re-encrypting.
Windows/Linux forensic equivalent: Imagine a password manager storing its master password hash in `%APPDATA%\passman\hash.txt` and allowing anyone who overwrites that file to unlock all secrets. Same catastrophic flaw.
- Root ≠ Unbreakable: How to Actually Secure Local App Data
Root access is not an excuse for lazy security. Even on rooted devices, developers can implement defense-in-depth:
Step 1 – Use Android Keystore for hardware-bound keys.
Keys generated with `setUserAuthenticationRequired(true)` require biometric or lock-screen authentication for each use – filesystem tampering cannot bypass this.
Step 2 – Store sensitive counters in EncryptedSharedPreferences.
Google’s Security Crypto library encrypts values and integrity-protects them:
val masterKey = MasterKey.Builder(context)
.setKeyScheme(MasterKey.KeyScheme.AES256_GCM)
.build()
val sharedPreferences = EncryptedSharedPreferences.create(
context,
"secure_prefs",
masterKey,
EncryptedSharedPreferences.PrefKeyEncryptionScheme.AES256_SIV,
EncryptedSharedPreferences.PrefValueEncryptionScheme.AES256_GCM
)
// Now attempt_counter is encrypted and tamper-evident
sharedPreferences.edit().putInt("attempt_counter", 0).apply()
Step 3 – Implement remote attestation.
Periodically send a signed hash of critical preferences to a backend server. If the device’s `attempt_counter` on the server disagrees with the local value, invalidate the session and force re-authentication.
Step 4 – Detect root & jailbreak (as a deterrent, not a silver bullet).
Use SafetyNet (now Play Integrity API) or open-source checks:
On the device, test for common root indicators adb shell su -c "which magisk" returns path if Magisk present ls /system/app/Superuser.apk
Implement runtime detection in the app – but never trust client-side detection alone; pair it with server-side risk scoring.
- Reverse Engineering the POC App: A Hands-On Lab
The EU’s GitHub release (hypothetical github.com/eu-digital/age-verification-poc) offers a learning opportunity for security analysts. To replicate the analysis on your own (Linux or WSL on Windows):
Step 1 – Clone and build the vulnerable POC:
git clone https://github.com/eu-digital/age-verification-poc cd age-verification-poc ./gradlew assembleDebug
Step 2 – Install on a rooted emulator (Android Studio AVD with Google APIs).
Enable root access in AVD: `-writable-system` flag.
Step 3 – Extract and inspect the APK’s shared preferences:
adb shell run-as com.eu.ageverification cat shared_prefs/pin_config.xml
On a rooted device, `run-as` is unnecessary – simply `su` and browse.
Step 4 – Modify the preferences using `sed` directly:
sed -i 's/biometric_enabled" value="true"/biometric_enabled" value="false"/g' /data/data/com.eu.ageverification/shared_prefs/pin_config.xml
Restart the app – observe that biometric prompt disappears.
Windows alternative: Use Android Studio’s Device File Explorer (GUI) to pull, edit, and push files back. But command-line is faster for scripting.
- API Security Parallel: The Same Flaw in Microservices
This local file vulnerability mirrors a common API anti-pattern: trusting client-provided identifiers without server-side binding. Imagine an API endpoint `POST /verify` that accepts `{“user_id”: 123, “pin”: “1234”}` and simply checks a database row – if an attacker changes the `user_id` in the request, they can authenticate as anyone.
Secure version – never trust client-supplied identity tokens:
Vulnerable
@app.route('/verify', methods=['POST'])
def verify_pin():
user_id = request.json['user_id']
pin = request.json['pin']
Lookup PIN for that user_id – attacker can change user_id!
if db.query("SELECT pin FROM users WHERE id=?", user_id) == pin:
return identity_credentials
Secure – bind PIN to session token
@app.route('/verify', methods=['POST'])
def verify_pin():
session_token = request.cookies.get('session')
pin = request.json['pin']
user_id = redis.get(f"session:{session_token}:user_id")
User ID now comes from server-side storage, not client
if db.query("SELECT pin_hash FROM users WHERE id=?", user_id) == hash(pin):
return identity_credentials
Cloud hardening corollary: In AWS, never store IAM secrets in plaintext EC2 user-data (equivalent to the app’s text file). Use Secrets Manager or Parameter Store with encryption and access policies.
5. Mitigation Playbook for Developers (Checklist)
If you’re building an identity or age-verification app, here’s your non-negotiable checklist:
| Threat | Mitigation | Verification Command |
||-|–|
| PIN hash replaced | Bind PIN to identity via authenticated encryption (AES-GCM) | Test: Replace PIN file – app should refuse to decrypt identity |
| Lockout counter reset | Store counter in tamper-proof hardware (StrongBox) or server-side | Attempt 5 failures → reboot device → counter still at 5 |
| Biometric disabled | Require `setUserAuthenticationRequired(true)` on Keystore key | `keyInfo.isUserAuthenticationRequired()` should return true |
| Physical root access | Add remote attestation (SafetyNet/Play Integrity) + server-side session invalidation | Run `cts-profile-match` – must pass on non-rooted device |
| Offline brute-force | Rate-limit PIN attempts server-side even for offline-first apps (sync on next network) | Try 100 PINs offline → after reconnecting, account locks |
Linux command to check if a running app has open file handles to sensitive data:
lsof -p $(pgrep -f "age-verification") | grep ".xml|.dat"
If you see writable file descriptors to preference files, an attacker with process injection can modify them in real-time.
What Undercode Say:
- Client-side security controls are an illusion. Never store authentication gates (lockouts, biometric flags) in plaintext files – they will be bypassed the moment an attacker gains even minimal privileges.
- Root is not a threat model exception. Build as if the device is fully compromised: encrypt, attest, and bind credentials cryptographically. The EU’s POC fails because it assumes file integrity – a dangerous assumption in mobile security.
This incident also underscores a cultural gap: many backend developers bring server-centric trust models to mobile apps, forgetting that the client is an adversarial environment. The fix isn’t just code – it’s adopting “zero-trust client” architecture. Every local file must be treated as attacker-controlled. Until then, expect more “hacked with little effort” headlines.
Prediction:
Within 12 months, the European Commission will release a mandatory security framework for all EU digital identity wallets (eIDAS 2.0 compliant), explicitly prohibiting plaintext storage of authentication state and requiring hardware-backed keystores for biometric gates. Startups that ignore these lessons will face GDPR fines for “inappropriate technical measures” – and attackers will continue to own their identity pipelines via a single edited text file.
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Reported By: Hanslak The – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
