CVE-2026-75509: CWE-290: Authentication Bypass by Spoofing in authlib joserfc
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 tokens. This authentication bypass flaw enables an attacker to spoof the issuer claim, potentially leading to unauthorized actions. The issue is fixed in version 1.7.3.
AI Analysis
Technical Summary
The authlib joserfc library, which implements JSON Object Signing and Encryption (JOSE) standards, had a flaw in JWTClaimsRegistry before version 1.7.3. It 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 allowed an attacker to bypass issuer validation, effectively bypassing authentication checks based on the issuer claim. The vulnerability is tracked as CVE-2026-75509 and is classified under CWE-290 (Authentication Bypass) and CWE-345 (Insufficient Verification of Data Authenticity).
Potential Impact
An attacker can bypass issuer validation in JWT tokens, potentially allowing unauthorized access or actions that rely on correct issuer authentication. The CVSS score of 6.5 (medium severity) reflects a network attack vector with high impact on integrity but low impact on confidentiality and availability. No known exploits are reported in the wild.
Mitigation Recommendations
Upgrade to authlib joserfc version 1.7.3 or later, where the issuer-validation bypass issue is fixed. No other mitigation is indicated by the vendor advisory or CVE data.
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 tokens. This authentication bypass flaw enables an attacker to spoof the issuer claim, potentially leading to unauthorized actions. The issue is fixed in version 1.7.3.
CVSS v3.1
Score 6.5medium
Affected software
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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 before version 1.7.3. It 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 allowed an attacker to bypass issuer validation, effectively bypassing authentication checks based on the issuer claim. The vulnerability is tracked as CVE-2026-75509 and is classified under CWE-290 (Authentication Bypass) and CWE-345 (Insufficient Verification of Data Authenticity).
Potential Impact
An attacker can bypass issuer validation in JWT tokens, potentially allowing unauthorized access or actions that rely on correct issuer authentication. The CVSS score of 6.5 (medium severity) reflects a network attack vector with high impact on integrity but low impact on confidentiality and availability. No known exploits are reported in the wild.
Mitigation Recommendations
Upgrade to authlib joserfc version 1.7.3 or later, where the issuer-validation bypass issue is fixed. No other mitigation is indicated by the vendor advisory or CVE data.
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
- Remediation Level
- null
Threat ID: 6a8ca814acd9273b49150594
Added to database: 08/24/2026, 20:22:44 UTC
Last enriched: 08/24/2026, 20:37:18 UTC
Last updated: 08/24/2026, 20:42:36 UTC
Views: 4
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