JWT Security Best Practices: Signing, Validation, and Storage Rules
The critical JWT security rules: always verify signatures, never trust alg:none, avoid localStorage, and set short expiry.
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Tags: security, jwt, best-practices
JWT Security Best Practices: What Can Go Wrong JWTs are widely used and widely misimplemented. Several well-documented attacks have affected major systems. Understanding these vulnerabilities and their mitigations is the difference between using JWTs correctly and creating a critical authentication bypass. Attack 1: The "alg:none" Vulnerability This is arguably the most notorious JWT vulnerability. The JWT spec allows , which means no signature. An attacker can: Take a valid JWT Decode the payload Modify the payload (e.g., change to ) Re-encode with in the header Remove the signature (or submit an empty third section) If the server naively reads the algorithm from the token's header and skips verification for , the attack succeeds. Mitigation: Always specify the allowed algorithm…
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