What’s Vulnerable › Vulnerabilities

CVE-2026-9537: Mojo::JWT versions before 1.02 for Perl verify HMAC signatures with a non-constant-time string comparison

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-07-17 · updated 2026-10-24
• Weakness: CWE-208

Description

Mojo::JWT versions before 1.02 for Perl verify HMAC signatures with a non-constant-time string comparison. The decode() method compares the supplied signature to the recomputed HMAC with Perl's eq operator, which stops at the first differing byte, so the comparison time varies with the number of matching leading bytes. A caller that decodes attacker supplied tokens leaks the expected signature through this timing variation, which can be aggregated over many requests to recover the signature and forge a token.

Affected products

• JBERGER Mojo::JWT: before 1.02 — fixed in 1.02

How to fix

Upgrade to a fixed version: JBERGER Mojo::JWT 1.02. Check the vendor advisory for mitigations.

References

• https://github.com/jberger/Mojo-JWT/commit/b8aefb846613e44b5b12bc170898ffd5b05094a2.patch
• http://www.openwall.com/lists/oss-security/2026/07/17/11

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