Mojo::JWT versions prior to 1.02 verify HMAC signatures using Perl's eq operator, which performs a byte-by-byte comparison and returns false at the first differing byte. This results in variable comparison times depending on the number of matching leading bytes, creating a timing side-channel (CWE-208). An attacker who can supply tokens to the decode() method can measure these timing variations to infer the correct signature incrementally, potentially allowing token forgery. The vulnerability affects all versions before 1.02. There is no vendor advisory or patch information available at this time.

An attacker able to send attacker-controlled tokens to the decode() method can exploit timing discrepancies to recover the HMAC signature used for token verification. This can lead to token forgery, allowing unauthorized actions or privilege escalation within systems relying on Mojo::JWT for authentication or integrity checks. The vulnerability does not impact confidentiality or availability directly but compromises integrity.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until an official fix is available, users should consider implementing constant-time comparison functions for HMAC verification or avoid using vulnerable versions of Mojo::JWT. Monitoring vendor channels for updates is recommended.