Mojo::JWT versions earlier than 1.02 verify the HMAC of JSON Web Tokens by comparing the supplied signature to the recomputed one with Perl's direct equality operator, which stops as soon as a differing byte is found. This non‑constant‑time comparison leaks measurable timing information to a caller that decodes attacker‑supplied tokens. By sending a large number of crafted tokens and measuring the response times, an adversary can recover the exact signature and then forge a valid JWT, enabling them to bypass authentication or authorization checks.

All installations of the JBERGER Mojo::JWT Perl library older than version 1.02 are affected. The issue arises wherever the library’s decode() function is invoked to validate tokens supplied by external users.

The CVSS score of 5.3 indicates moderate severity; the EPSS score is less than 1 %, signifying currently low but non‑zero exploitation probability. Nevertheless, the attack requires only repeated token decoding and timing measurement, a capability readily available to anyone with network access to the service. The vulnerability is not present in the CISA KEV catalog.