7 JWT Security Mistakes Developers Still Make in 2026

JSON Web Tokens power authentication for millions of applications. Yet developers keep making the same security mistakes. Here are seven we see constantly.

1. Storing JWTs in localStorage

localStorage is accessible to any JavaScript on the page. A single XSS vulnerability exposes every token. Use httpOnly cookies instead — they are invisible to JavaScript.

2. Not Validating the Algorithm

The alg: "none" attack is old but still works against libraries that do not explicitly reject it. Always specify the expected algorithm when verifying: jwt.verify(token, secret, { algorithms: ["HS256"] }).

3. Using Weak Secrets

HMAC-SHA256 with a short secret is trivially brute-forceable. Use at least 256 bits of entropy. Generate with: openssl rand -hex 32.

4. Not Checking Expiration

Tokens without exp claims live forever. Always set expiration. Short-lived access tokens (15 min) with refresh tokens is the standard pattern.

5. Putting Sensitive Data in the Payload

JWT payloads are Base64-encoded, not encrypted. Anyone can decode them. Never put passwords, credit card numbers, or secrets in the payload.

6. Not Handling Token Revocation

JWTs are stateless — you cannot "invalidate" a token once issued. Implement a token blocklist or use short expiration times with refresh token rotation.

7. Decoding Without Verifying

Decoding a JWT (reading the payload) is not the same as verifying it (checking the signature). Client-side decoding for display is fine. Server-side decisions must always verify the signature first.

Use our JWT Decoder to inspect tokens safely in your browser, and our JWT Generator to create properly signed tokens for testing.