Mass Assignment with Jwt Tokens

Mass assignment in JWT tokens occurs when attackers manipulate token claims to inject unauthorized properties or override existing ones during token parsing and validation. This vulnerability is particularly dangerous in JWT because tokens are often treated as trusted data once verified, leading to privilege escalation or data exposure.

The most common JWT mass assignment pattern involves the exp (expiration) claim. Consider a scenario where a service validates a token's expiration but then uses the exp value directly in a database query without proper sanitization: