What Is a JWT and How Does It Keep You Logged In Securely?

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JSON Web Tokens (JWTs) are the standard mechanism modern web applications use to verify a user's identity after login, replacing traditional server-side sessions. A JWT consists of three Base64Url-encoded parts — header, payload, and signature — joined by dots, where the signature cryptographically proves the token was issued by the server and has not been tampered with. Unlike session-based authentication, JWTs are stateless: the server validates the token's signature on each request without storing any session data, making them highly scalable across multiple servers. Importantly, JWTs are signed but not encrypted, meaning anyone holding the token can read its contents, so sensitive data like passwords should never be stored in the payload. Best practices include setting short expiry times, using refresh tokens for longer sessions, and storing tokens in HttpOnly cookies rather than localStorage to reduce exposure to cross-site scripting attacks.

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