CVE-2026-75509: joserfc claim-validation bypass via array-typed single-string claims (iss/sub/jti)

Severity: medium (CVSS 6.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-08-24 · updated 2026-10-12
Weakness: CWE-290

Description

joserfc is a Python library that provides an implementation of several JSON Object Signing and Encryption (JOSE) standards. Prior to version 1.7.3, JWTClaimsRegistry applies membership matching to list-valued iss and sub claims, allowing an array-valued iss that contains the expected issuer to pass an intended equality check and enabling issuer-validation bypass. This issue is fixed in version 1.7.3.

Affected products

authlib joserfc: before 1.7.3

References

https://github.com/authlib/joserfc/commit/76ee6a59bf5773c0af00b99076c5e199031f97f1
https://github.com/authlib/joserfc/security/advisories/GHSA-r74j-q665-7rpj
https://github.com/authlib/joserfc/security/advisories/GHSA-r74j-q665-7rpj

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