Issuer impersonation via array-valued JWT claims
Published Aug 24, 2026 · Updated Aug 25, 2026
Authentication bypass in Authlib joserfc before 1.7.3 allows remote attackers to impersonate trusted issuers with crafted JWTs. JWTClaimsRegistry applies list membership to iss, sub, jti, and nonce instead of rejecting arrays where single strings are required. Exploitation requires an application to rely on the affected claim validation and accept a signed token whose array contains the configured trusted issuer.
Summary
What happened
Authentication bypass in Authlib joserfc before 1.7.3 allows remote attackers to impersonate trusted issuers with crafted JWTs. JWTClaimsRegistry applies list membership to iss, sub, jti, and nonce instead of rejecting arrays where single strings are required. Exploitation requires an application to rely on the affected claim validation and accept a signed token whose array contains the configured trusted issuer.
The record
- CVE
- CVE-2026-75509
- Published
- Aug 24, 2026
- Updated
- Aug 25, 2026
- Vendor
- Authlib
- Product
- joserfc
- Classifications
- CWE-345, CWE-290, T1606
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Aug 24, 2026CVE publishedPublication date reported by the CVE source.
- Aug 25, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=< 1.7.3
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- Upstream array-valued issuer proof of conceptproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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