Apereo CAS Client Certificate Validation Bypass

Executive Summary

This vulnerability in Apereo CAS Client allows it to accept any CA-trusted certificate for a hostname if the URL matches a configured allowlist or regex pattern. An attacker with a man-in-the-middle position can intercept CAS exchanges, capture the Ticket-Granting Ticket (TGT), and obtain Service Tickets on behalf of the victim.

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Detection Guidance

To detect this vulnerability, inspect CAS client configurations for improper certificate validation settings. Check if the client accepts any CA-trusted certificate for hostnames matching allowlists or regex patterns. Review network traffic for unexpected certificate chains or MITM attempts during CAS exchanges.

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Impact Analysis

An attacker could intercept your authentication exchanges, steal your TGT, and impersonate you to access services. This could lead to unauthorized access to your accounts and sensitive data.

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Compliance Impact

This vulnerability could lead to unauthorized access to personal or sensitive data, violating GDPR and HIPAA requirements for data protection and access control. Organizations may face compliance penalties and reputational damage.

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Mitigation Strategies

Update to the latest patched versions of Apereo CAS Client (4.1.0+) and Jasig CAS Client (3.6.4+). If no patches are available, disable certificate validation allowlists or regex patterns that accept any CA-trusted certificate. Review network traffic for unusual CAS exchange patterns or unauthorized Service Ticket requests.

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