What security measure protects web servers by ensuring the validity of digital certificates?

Certificate pinning is a security measure that directly relates to the validation of digital certificates used in secure communications. It involves associating a host with their expected X.509 certificate (the format of digital certificates) or public key. This means that when a web browser or application connects to a web server, it checks the server's certificate against pre-configured certificates or keys. If there is a discrepancy, the connection can be blocked, thus helping to prevent man-in-the-middle attacks and other types of spoofing that exploit vulnerabilities in certificate validation.

This method is particularly effective in ensuring that the server clients are communicating with is indeed trusted and has not been compromised. It adds an additional layer of security by hardcoding the expected certificates or keys, which eliminates the risk of accepting fraudulent certificates that could be issued by compromised certificate authorities.

In contrast, other options like access control lists, encryption protocols, and multi-factor authentication serve different purposes within security frameworks. Access control lists manage permissions for different users or systems, encryption protocols secure data in transit and at rest, and multi-factor authentication provides an additional layer of user verification, but none focus specifically on certificate validation for web servers like certificate pinning does.