The Ultimate Guide to JWT Vulnerabilities and Attacks (with Exploitation Examples)

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JWT vulnerabilities pose serious security risks in modern web applications. Common attacks include signature bypass, algorithm confusion (switching from RS256 to HS256), weak secret brute-forcing, and injection attacks through header parameters like 'kid' and 'jku'. The guide covers exploitation techniques for each

#security

#authentication

#cryptography

#web-security

#jwt

Jun 15, 2025•13m read time•From pentesterlab.com

Table of contents

🧪 JSON Web Algorithms (JWA)🔄 One Website, Many JWT Implementations 🔓 1. Signature Not Verified ❌ 2. None Algorithm Attack 🧂 3. Trivial Secret (Weak HMAC Keys)🔀 4. Algorithm Confusion (RSA to HMAC)🔀 4b. Algorithm Confusion (ECDSA to HMAC)🪤 5. kid Injection (Key ID Manipulation)🧬 6. Embedded JWK (CVE-2018-0114)🌐 7. JKU / X5U Header Abuse 🧙 8. CVE-2022-21449 (Psychic Signature)📚 Final Thoughts: Mastering JWT Security