JOSE RFC Implementation Issuer Validation Bypass in joserfc

Executive Summary

joserfc is a Python library for JOSE standards. Prior to version 1.7.3, it incorrectly validated JWT claims by allowing array values for single-string claims like issuer (iss) or subject (sub). This let attackers bypass validation by including the expected issuer in an array, tricking the system into accepting tokens with malicious issuers. The issue was fixed in version 1.7.3 by restricting array checks to multi-valued claims like audience (aud).

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Detection Guidance

Check if your system uses joserfc versions prior to 1.7.3 by running: pip show joserfc. If installed, verify the version matches or exceeds 1.7.3. Inspect JWT validation logic for iss, sub, jti, or nonce claims to ensure they are not processed as arrays.

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Impact Analysis

This vulnerability allows attackers to bypass issuer validation in JWT tokens, potentially enabling signature forgery and impersonation. Applications using joserfc versions before 1.7.3 for JWT validation could accept tokens with incorrect or malicious issuers, leading to unauthorized access or data modification. The impact is high for integrity as unauthorized users could alter data.

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Compliance Impact

This vulnerability could violate compliance requirements that mandate strict JWT validation, such as ensuring proper issuer verification for authentication tokens. Non-compliance may result from unauthorized access or data modification risks, potentially breaching GDPR's integrity principles or HIPAA's access controls. Organizations must ensure patched versions are used to maintain compliance.

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Mitigation Strategies

Upgrade joserfc to version 1.7.3 or later using pip install --upgrade joserfc. Review JWT validation code to ensure iss and sub claims are validated as single strings, not arrays. Apply patches if custom JWT validation logic exists.

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