Authentication Bypass in python-jose via Empty HMAC Secrets

Executive Summary

python-jose through 3.5.0 has an authentication bypass flaw where unauthenticated attackers can forge JWTs. This happens because HMACKey and CryptographyHMACKey accept zero-length HMAC secrets. If an application's verification key is empty (e.g., an oct JWK with empty k), attackers can create tokens with arbitrary claims that jwt.decode will accept.

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Detection Guidance

To detect this vulnerability, inspect applications using python-jose versions before 3.5.0 for JWT verification keys that may resolve to empty. Check for JWKs with empty 'k' values in HS256/HS384/HS512 algorithms. Review logs for unauthorized JWT tokens or unexpected authentication bypass events.

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Impact Analysis

This vulnerability allows attackers to bypass authentication and impersonate users by forging JWTs. If exploited, it could lead to unauthorized access to sensitive data, privilege escalation, or other malicious actions depending on the application's use of JWTs.

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Compliance Impact

This vulnerability could violate compliance requirements by enabling unauthorized access to personal or sensitive data, undermining data integrity and confidentiality. GDPR and HIPAA require strict access controls and data protection, which this flaw could compromise.

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Mitigation Strategies

Immediately upgrade python-jose to version 3.5.0 or later. Review and update JWT verification keys to ensure they are non-empty. Implement strict validation for JWKs to reject empty HMAC secrets. Monitor for suspicious authentication attempts or forged tokens.

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