OAuth2 Scope Validation Bypass in Apache CXF

Executive Summary

This vulnerability exists in Apache CXF's OAuth2 Dynamic Client Registration endpoint. The authorization server accepts and stores the scope value from client registration requests without validating it against a predefined allowlist. This allows clients to self-assign privileged scopes during registration.

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Detection Guidance

To detect this vulnerability, check the Apache CXF version in use. If it is below 4.2.3, 4.1.8, or 3.6.12, the system is vulnerable. Use commands like 'cxf --version' or inspect the application's dependency files (e.g., pom.xml for Maven or build.gradle for Gradle) to verify the version.

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Impact Analysis

An attacker could register a client with elevated privileges by specifying privileged scopes. This could lead to unauthorized access to protected resources or data if the client is granted excessive permissions.

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Compliance Impact

This vulnerability could violate compliance requirements by allowing unauthorized access to sensitive data, potentially breaching GDPR's data protection principles or HIPAA's access control mandates. Proper scope validation is critical for maintaining compliance.

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Mitigation Strategies

Immediately upgrade Apache CXF to version 4.2.3, 4.1.8, or 3.6.12 or later. After upgrading, review client registrations to ensure no privileged scopes were self-assigned. Monitor logs for suspicious registration attempts.

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