CVE-2026-75509: joserfc claim-validation bypass via array-typed single-string claims (iss/sub/jti)
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 Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
joserfcto a version that resolves this vulnerability.Fixed in 1.7.3
Event History
Frequently Asked Questions
Which deployments are affected?
Deployments using the PyPI joserfc package prior to version 1.7.3 are affected when JWTClaimsRegistry validates issuer or subject claims as intended single-string equality checks.
What does an attacker need to exploit this issue?
An attacker needs to provide a JWT with an array-valued iss or sub claim that includes the expected value. The affected validation applies membership matching to list-valued claims rather than enforcing the intended equality check.
Is a default configuration known to be affected?
The available information does not identify a default configuration. Exposure depends on use of JWTClaimsRegistry for issuer or subject claim validation.
What is the remediation?
Upgrade joserfc to version 1.7.3, which fixes the claim-validation behavior.