Weak JWT secrets leave HS256 tokens open to offline cracking and forgery

A developer explains that JWT signing secrets are often far too short to be safe. The RFC 7518 specification requires HMAC-SHA2 keys at least as long as the hash output, which is 256 bits or 32 bytes for HS256. Because tokens are only signed rather than encrypted, attackers can attempt to guess weak secrets offline using tools like hashcat, then forge tokens such as admin credentials. The author stresses that length alone is not enough, recommending secrets drawn from a cryptographically secure random generator such as openssl rand or Python's secrets module. The post also advises storing secrets in environment variables, pinning the verification algorithm, and noting that decoding a token does not verify its authenticity.