CVE-2026-75509: CWE-290: Authentication Bypass by Spoofing in authlib joserfc
Description
A vulnerability in the authlib joserfc Python library prior to version 1.7.3 allows issuer-validation bypass due to improper handling of array-valued 'iss' claims in JWT token validation. This can lead to authentication bypass by spoofing the issuer claim. The issue is fixed in version 1.7.3.
CVSS v3.1
Score 6.5medium
Affected software
authlib
joserfc
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AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The authlib joserfc library, which implements JSON Object Signing and Encryption (JOSE) standards, had a flaw in JWTClaimsRegistry prior to version 1.7.3. The library applied membership matching to list-valued 'iss' and 'sub' claims, allowing an array-valued 'iss' claim containing the expected issuer to pass an equality check incorrectly. This enabled an attacker to bypass issuer validation, effectively causing an authentication bypass by spoofing the issuer claim. The vulnerability is identified as CWE-290 (Authentication Bypass by Spoofing) and CWE-345 (Insufficient Verification of Data Authenticity). The issue is resolved in version 1.7.3.
Potential Impact
An attacker can bypass issuer validation in JWT tokens by exploiting the improper handling of array-valued 'iss' claims, potentially allowing unauthorized access or impersonation. The CVSS 3.1 score is 6.5 (medium severity) with low confidentiality impact, high integrity impact, and no availability impact. No known exploits are reported in the wild.
Mitigation Recommendations
Upgrade to authlib joserfc version 1.7.3 or later where this vulnerability is fixed. No other mitigations are indicated by the vendor advisory.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- GitHub_M
- Date Reserved
- 2026-08-17T20:49:21.598Z
- Cvss Version
- 3.1
- State
- PUBLISHED
Threat ID: 6a8ca814acd9273b49150594
Added to database: 08/24/2026, 20:22:44 UTC
Last enriched: 09/10/2026, 21:03:43 UTC
Last updated: 10/07/2026, 18:48:23 UTC
Views: 103
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