Penetration testing simulates real-world attacks to identify any remaining exploitable vulnerabilities after controls are implemented. It validates that mitigation has been successful from an attacker's perspective.
"Penetration testing is essential to verify the effectiveness of remediation efforts and ensure vulnerabilities can no longer be exploited."
- CISM Review Manual 15th Edition, Chapter 2: Risk Management, Section: Security Testing and Evaluation* Vulnerability assessments and audits are valuable, but pen testing provides practical assurance.