This comprehensive guide explores the assessment of JSON Web Tokens (JWT) in web application security, moving from a high-level cheat sheet to deep-dive exploitation techniques. It details the fundamental structure of JWTs—comprising the header, payload, and signature—and explains how applications utilize them for stateless session management. The article highlights the trade-offs between performance and security, particularly concerning token revocation and insecure client-side storage.
The core of the article focuses on practical attack vectors, including signature validation bypasses, brute-forcing weak HMAC secrets, and algorithm exclusion attacks. It also covers advanced techniques such as key confusion (RS256 vs. HS256), JWK/JKU header injection, and path traversal via the 'kid' parameter. By using tools like Burp Suite's JWT Editor and hashcat, security analysts can identify implementation flaws that allow for unauthorized privilege escalation and data access.
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