# CVE-2026-75509: authlib joserfc issuer validation bypass in JWT claims

Source: https://junglewise.ai/threats/cve-2026-75509-authlib-joserfc-issuer-validation-bypass-in-jwt-claims  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-24. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-75509 · Severity: medium · CVSS 6.5 · Published 2026-08-24

joserfc is a Python library used to handle JSON Web Tokens (JWTs) and cryptographic signing operations. The library incorrectly validates the issuer claim when it is formatted as an array instead of a string, allowing attackers to forge tokens with a manipulated issuer claim that passes validation. This could allow an attacker to forge authentication tokens and impersonate trusted issuers.

Technologies: [Authlib Joserfc](https://junglewise.ai/threats/technologies/joserfc). Vendors: [Authlib](https://junglewise.ai/threats/vendors/authlib).

## Quick answers

**Is CVE-2026-75509 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-75509 being exploited in the wild.

**How severe is CVE-2026-75509?** Medium severity, with a CVSS score of 6.5, published 24 August 2026.

**Which products and versions are affected?** Authlib Joserfc before 1.7.3.

**Is there a fix for CVE-2026-75509?** A patch was published on 2026-07-03: Fix committed; version 1.7.3 released Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-75509 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-75509?** Check whether you run Authlib Joserfc at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The vulnerability is a logic error in JWTClaimsRegistry's claim validation. The registry applies membership matching (array containment check) to list-valued claims including iss, sub, and jti, which RFC 7519 defines as single StringOrURI values. An attacker can craft a JWT with an array-typed iss claim containing both a malicious issuer and the expected legitimate issuer; the flawed membership check accepts this as a match when an equality check was intended. The vulnerability requires the application to rely on iss validation for authentication/authorization decisions, but no special privileges or user interaction are needed. The fix (version 1.7.3) adds type validation to reject array-typed iss, sub, jti, and nonce claims.

## Affected products

- authlib joserfc before 1.7.3

## Timeline

- 2026-08-24: disclosed
- 2026-07-03: patched: Fix committed; version 1.7.3 released

## References

- [https://github.com/authlib/joserfc/commit/76ee6a59bf5773c0af00b99076c5e199031f97f1](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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