Fix JWT Vulnerabilities (Weak Signing, None Algo) in Symfony
JWT implementation in Symfony, typically via LexikJWTAuthenticationBundle, is a prime target for 'alg: none' and secret-brute-forcing attacks. If you're using default configurations or weak symmetric keys, you're essentially handing out root shells. This guide covers hardening the bundle configuration to enforce cryptographic integrity.
The Vulnerable Pattern
# config/packages/lexik_jwt_authentication.yaml lexik_jwt_authentication: secret_key: '12345' # Vulnerable: Hardcoded/Weak secret # Missing signature_algorithm: defaults or weak implementations might allow 'none' via header injection token_ttl: 3600Or custom implementation allowing dynamic ‘alg’ header
$token = (new Builder())->withHeader(‘alg’, $header->alg)->getToken();
The Secure Implementation
To kill the 'none' algorithm attack, you must explicitly define the 'signature_algorithm' in your bundle config. Switching to RS256 (Asymmetric) is the gold standard; even if an attacker steals your public key, they cannot forge tokens. The Lcobucci encoder in Symfony is preferred as it strictly validates the 'alg' header against your configuration, rejecting any token that attempts to downgrade to 'none' or 'HS256' when 'RS256' is expected. Always use environment variables for passphrases and store keys outside the web root.
# config/packages/lexik_jwt_authentication.yaml lexik_jwt_authentication: # Use Asymmetric Encryption (RS256) instead of Symmetric (HS256) secret_key: '%kernel.project_dir%/%env(JWT_SECRET_KEY)%' public_key: '%kernel.project_dir%/%env(JWT_PUBLIC_KEY)%' pass_phrase: '%env(JWT_PASSPHRASE)%' token_ttl: 3600 encoder: # Force the Lcobucci encoder which enforces strict algorithm checks service: lexik_jwt_authentication.encoder.lcobucci signature_algorithm: RS256.env
Generate keys: openssl genpkey -algorithm RSA -out config/jwt/private.pem -pkeyopt rsa_keygen_bits:4096
JWT_SECRET_KEY=config/jwt/private.pem JWT_PUBLIC_KEY=config/jwt/public.pem JWT_PASSPHRASE=use-a-strong-random-string
Your Symfony API
might be exposed to JWT Vulnerabilities (Weak Signing, None Algo)
74% of Symfony apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.
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