Both penetration tests and automated vulnerability scans are useful tools for managing vulnerabilities. While these are different testing methods, they are complementary and both should be performed.
A vulnerability scan is an automated, low-cost method for testing common network and server vulnerabilities. This is sometimes referred to as an automated pen test. Many automated tools are available and most are easily configured by the end user to scan for published vulnerabilities on a scheduled basis. While an automated vulnerability scan is very efficient and cost-effective in identifying common vulnerabilities such as missing patches, service misconfigurations, and other known weaknesses, they are not as accurate in validating the accuracy of vulnerabilities nor do they fully determine the impact through exploitation. Automated scanners are more prone to reporting false positives (incorrectly reporting weaknesses) and false negatives (failing to identify vulnerabilities, especially those impacting web applications). Automated Vulnerability Scanning is mandated by the Payment Card Industry Data Security Standard (PCI DSS) as noted in requirement 11.2.
A penetration test focuses on the environment as a whole. In many ways, it picks up where the scanners leave off to provide a comprehensive analysis of the overall security posture. While scripts and tools are leveraged by a penetration tester, their use is largely limited to reconnaissance activities. The bulk of a penetration test is manual by nature. A penetration test identifies vulnerabilities scanners cannot, such as wireless flaws, web application vulnerabilities, and vulnerabilities not yet published. Further, pen testing includes attempts to safely exploit vulnerabilities, escalate privileges, and ultimately demonstrate how an attacker could gain access to sensitive information assets. Penetration testing frequently applies “test scenarios” specific to an organization as well. For example, a university may grant access to student workers, a hospital may leverage third party service providers, or a consultancy may have unique access rights for their engineers. Each of these scenarios would require different positioning of the penetration tester within the environment and requires adjustments to the methodology. Penetration testing is also mandated by the PCI DSS as noted in requirement 11.3.
Penetration testing and automated vulnerability scans both serve a purpose and both types of testing belong in a comprehensive vulnerability assessment program. Automated vulnerability scanning should be scheduled to run on a frequent basis, ideally at least weekly, with network penetration tests scheduled quarterly or when significant changes are planned to an environment.