I Audited 12 Open Source JWT Implementations and Found the Same 6 Mistakes

I spent last month reviewing JWT implementations across 12 open-source Node.js projects on GitHub — ranging from starter templates with 2k stars to production boilerplates used by teams at real companies. I found the same 6 mistakes in almost every one. None of these projects are bad. The developers are skilled. The mistakes are subtle, copy-paste errors from tutorials that nobody questioned. Here they are. Mistake 1 — The Secret Is Literally "secret" I found this in three separate projects: const token = jwt . sign ({ userId : user . id }, " secret " , { expiresIn : " 1h " }); This secret is in every JWT tutorial on the internet. It is in the jwt.io documentation. It is in the jsonwebtoken README. Developers copy it and forget to replace it. A 6-character ASCII secret has approximately 42 bits of entropy. A GPU cluster cracks it from a dictionary in milliseconds. Generate a real secret here — it takes 3 seconds and produces a 256-bit cryptographically random key. Mistake 2 — jwt.decode() Used in Auth Middleware // DANGEROUS — this is in a production auth middleware const decoded = jwt . decode ( req . headers . authorization . split ( " " )[ 1 ]); if ( ! decoded . userId ) return res . status ( 401 ). send ( " Unauthorized " ); jwt.decode() does not verify the signature. It reads the payload regardless of whether the token is valid, expired, or forged. An attacker can craft any payload they want and it will pass this check. The fix is two characters: jwt.verify() . const decoded = jwt . verify ( token , process . env . JWT_SECRET , { algorithms : [ " HS256 " ] }); Mistake 3 — Algorithm Not Specified in verify() // Missing algorithms option jwt . verify ( token , secret ); Without { algorithms: ['HS256'] } , the library trusts whatever algorithm is in the token's header. An attacker can create a token with alg: none and an empty signature — and jwt.verify() will accept it. Always specify the expected algorithm explicitly. Mistake 4 — Secret Committed to Version Cont