11.3.1 Perform external penetration testing at least annually and after any significant infrastructure or application upgrade or modification (such as an operating system upgrade, a sub-network added to the environment, or a web server added to the environment).
11.3.1.a Examine the scope of work and results from the most recent external penetration test to verify that penetration testing is performed as follows:
• Per the defined methodology
• At least annually
• After any significant changes to the environment.
11.3.1.b Verify that the test was performed by a qualified internal resource or qualified external third party and, if applicable, organizational independence of the tester exists (not required to be a QSA or ASV).
Penetration testing conducted on a regular basis and after significant changes to the environment is a proactive security measure that helps minimize potential access to the CDE by malicious individuals.
The determination of what constitutes a significant upgrade or modification is highly dependent on the configuration of a given environment. If an upgrade or modification could allow access to cardholder data or affect the security of the cardholder data environment, then it could be considered significant. Performing penetration tests after network upgrades and modifications provides assurance that the controls assumed to be in place are still working effectively after the upgrade or modification.