JSON Web Tokens are everywhere. They're the de facto standard for stateless API authentication and they're also one of the most misunderstood and misimplemented security mechanisms in web development. This article covers JWT from first principles, including common mistakes that create security vulnerabilities, and walks through a complete, production-ready implementation in Python with FastAPI. What Is a JWT? A JSON Web Token (JWT) is a compact, URL-safe token that encodes a JSON payload and optionally signs or encrypts it. It's used to transmit verified claims between parties most commonly, "this user is authenticated and has these permissions." A JWT looks like this: eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJ1c2VyXzEyMyIsImV4cCI6MTcxNjY1MDAwMH0.3rW7TE8iCCiLBOBs6nLBpWxXuV3zqkL9mYpC2HBkd4s Enter fullscreen mode Exit fullscreen mode It's three Base64URL-encoded segments joined by dots: HEADER.PAYLOAD.SIGNATURE Enter fullscreen mode Exit fullscreen mode Header { "alg" : "HS256" , "typ" : "JWT" } Enter…