JWT Security Best Practices & Common Vulnerabilities

JSON Web Tokens (JWTs) are the standard mechanism for stateless authentication in modern microservice architectures. However, improper JWT handling can compromise your entire backend.

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Top 3 JWT Security Vulnerabilities

1. Algorithm Confusion Attack (RS256 to HS256)

If your server accepts both RS256 (asymmetric) and HS256 (symmetric) algorithms, an attacker can obtain your public key (which is public), re-sign a forged token using HS256 with the public key as the secret, and bypass authentication.

2. Token Storage in LocalStorage (XSS Risk)

Storing JWTs in localStorage or sessionStorage exposes them to Cross-Site Scripting (XSS) attacks. If an attacker injects malicious JS into your page, they can extract the JWT token instantly.

3. Missing Expiration (& Revocation Void)

JWTs are stateless; once issued, they remain valid until expiration. Without an active token revocation list (or Redis blacklist), revoked user accounts retain access.

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Recommended Architecture

• Store tokens inside HTTP-Only, SameSite=Strict, Secure cookies.
• Use asymmetric RS256/ES256 algorithms with regular key rotation.
• Keep JWT payload minimal (never store PII or passwords inside JWT claims).