JWT Security: Best Practices and Common Pitfalls
JSON Web Tokens (JWTs) are a powerful and popular standard for stateless authentication in modern applications. However, if used incorrectly, they can lead to serious security vulnerabilities. In this article, we'll cover how to securely issue, store, rotate, and validate JWTs to protect your application from common pitfalls.
Claims and Algorithms
The most critical step in JWT security is to verify the token's signature on the server. Never trust a token's payload without first validating its signature. Pin the expected algorithm and be wary of algorithm downgrade attacks. Always validate standard claims like **`exp`** (expiration), **`nbf`** (not before), and **`iat`** (issued at). Also, check the **`aud`** (audience) and **`iss`** (issuer) claims to ensure the token is intended for your application and came from a trusted source. Keep your tokens short-lived and avoid over-packing them with user data.
Storage and Transport
Where a token is stored directly impacts its security. Never store long-lived session tokens in browser storage like `localStorage` or `sessionStorage`. Instead, prefer **secure, HTTP-only, and `SameSite=Strict` cookies**. When transmitting tokens, always use **TLS/SSL** to ensure the data is encrypted. Avoid exposing sensitive information in URL parameters. For debugging, use the URL Encoder/Decoder to properly encode parameters and avoid common pitfalls.
Revocation and Rotation
One of the biggest challenges with JWTs is their stateless nature, which makes server-side revocation difficult. For high-security applications, you can pair JWTs with a server-side session list to enable immediate revocation. A more common solution is to use a **short-lived access token** combined with a **long-lived refresh token**. Design your refresh tokens to be single-use and implement replay protection to mitigate against replay attacks. Regular rotation of keys and tokens is an essential part of a robust security strategy.
Debugging Tools
During development and testing, use a tool like the JWT Decoder to inspect a token's headers and claims. Confirm that the algorithm, expiration, and custom claims are correct before deploying your application. This helps you catch potential security flaws before they make it to production.