Medium severity · published Aug 24, 2026
CVE-2026-75509
joserfc claim-validation bypass via array-typed single-string claims (iss/sub/jti)
6.5
CVSS
0.1%
EPSS
No
On KEV
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.
CVSS vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:H/A:N
Attack vector
Network
Complexity
High
Privileges
None
User interaction
None
Confidentiality
Low
Integrity
High
Availability
None
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.1% · higher than 2% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
JO
joserfc · authlib
< 1.7.3 · source: vendor
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-75509
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-75509
github.com
https://github.com/authlib/joserfc/commit/76ee6a59bf5773c0af00b99076c5e199031f97f1
github.com
https://github.com/authlib/joserfc/security/advisories/GHSA-r74j-q665-7rpj
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