JWT (JSON Web Token) authentication has become the standard for modern US web applications. From fintech platforms in New York to SaaS startups in San Francisco, JWTs provide a stateless, scalable authentication mechanism that works across web, mobile and API contexts. In this guide, we cover everything American developers need to know about JWT authentication — from token structure to security best practices.
What Is JWT Authentication?
JWT authentication is a stateless authentication mechanism. Instead of storing session data on the server (as in traditional session-based auth), the server issues a signed token to the client after login. The client sends this token with each subsequent request, and the server verifies the signature to authenticate the user.
The "stateless" part is key: the server does not need to look up session data in a database or cache to verify the token. The signature alone proves the token was issued by the server and has not been tampered with. This makes JWTs ideal for distributed systems, microservices and APIs where multiple servers need to verify tokens independently.
JWT Token Structure
A JWT consists of three parts separated by periods: header.payload.signature. Each part is Base64url-encoded.
Header: Specifies the token type (JWT) and the signing algorithm (e.g., HS256, RS256). Example: {"alg":"HS256","typ":"JWT"}
Payload: Contains claims — statements about the user and the token. Common claims include sub (subject/user ID),iat (issued at time),exp (expiration time) and custom claims like role or email.
Signature: Created by signing the encoded header and payload with a secret key (HS256) or private key (RS256). The signature ensures the token has not been tampered with.
You can inspect any JWT by pasting it into the JWT Decoder, which decodes the header and payload for inspection.
Signing Algorithms: HS256 vs RS256
HS256 (HMAC with SHA-256): Uses a shared secret for both signing and verification. The same secret must be present on the server that signs the token and any server that verifies it. Simpler to implement but requires secure secret distribution.
RS256 (RSA Signature with SHA-256): Uses a private key to sign and a public key to verify. The private key stays on the authentication server; the public key can be shared with any service that needs to verify tokens. More secure for distributed systems because the signing key never leaves the auth server.
For US production applications, RS256 is generally recommended because it separates signing and verification keys. If an attacker compromises a verification key (public key), they still cannot forge tokens. With HS256, compromising the shared secret allows token forgery.
Access Tokens and Refresh Tokens
A robust JWT authentication system uses two types of tokens:
Access token: Short-lived (15 minutes to 1 hour). Sent with each API request to authenticate the user. If compromised, the attacker has access only for a short window.
Refresh token: Long-lived (days to weeks). Used to obtain new access tokens without requiring the user to log in again. Stored securely (httpOnly cookie) and sent only to the token refresh endpoint, not with every request.
When the access token expires, the client sends the refresh token to a dedicated endpoint, which validates it and issues a new access token. If the refresh token is revoked or expired, the user must log in again. This approach balances security (short-lived access tokens) with user experience (long-lived sessions via refresh tokens).
Where to Store JWTs
Token storage is a critical security decision. The three common options are:
localStorage: Accessible via JavaScript, which makes it vulnerable to XSS attacks. If an attacker injects a script into your page, they can read the token and steal the user's session. Not recommended for sensitive applications.
httpOnly cookies: Not accessible via JavaScript, which protects against XSS. However, cookies are automatically sent with every request, which can lead to CSRF attacks if not properly mitigated. Use SameSite=Strict or SameSite=Lax to prevent CSRF.
In-memory (JavaScript variable): The most secure option for access tokens. The token exists only in the running application's memory and is lost when the page is closed or refreshed. Combine with a refresh token in an httpOnly cookie for seamless re-authentication.
For US applications handling sensitive data (finance, healthcare, personal information), in-memory access tokens with httpOnly refresh token cookies is the recommended approach.
JWT Security Best Practices
Keep tokens short-lived. Access tokens: 15 minutes to 1 hour. Refresh tokens: 7 to 30 days. Short lifetimes limit the damage if a token is compromised.
Never put sensitive data in the payload. JWT payloads are Base64url-encoded, not encrypted. Anyone who intercepts the token can read its contents. Do not include passwords, credit card numbers or other sensitive data in the payload.
Always verify the signature. Never accept a JWT without verifying its signature. An unsigned or improperly signed token may be a forgery.
Validate the expiration. Always check the exp claim and reject expired tokens. Do not rely on the client to send only valid tokens.
Implement token revocation. JWTs are stateless, which means you cannot revoke a token without a server-side check. If you need revocation (e.g., when a user logs out or changes their password), maintain a blacklist of revoked tokens or use short access token lifetimes with revocable refresh tokens.
Use HTTPS. JWTs sent over HTTP can be intercepted. Always use HTTPS for any endpoint that sends or receives tokens.
Using a Free JWT Decoder
The JWT Decoder from Automarkly decodes JWT headers and payloads in your browser. Paste a JWT, and the tool shows the decoded header and payload as formatted JSON. The tool also identifies the signing algorithm and expiration time.
The decoder is useful for:
Debugging: Inspect token contents to verify claims, expiration and algorithm.
Learning: See how JWTs are structured by decoding real tokens.
Security review: Verify that no sensitive data is included in the payload.
For US developers, JWT authentication is a fundamental skill for building modern, secure web applications. Use the JWT Decoder to inspect tokens, implement the security best practices covered in this guide and build authentication systems that protect your users' data.