Definition
Blocklisting, also known as blacklisting, is a security practice where specific items, such as IP addresses, email senders, or software applications, are explicitly denied access or execution. This approach maintains a definitive list of known threats or unwanted entities, preventing them from entering or operating within a system. In practice, security systems automatically check incoming data or requests against the blocklist, rejecting any matches. This is crucial for preventing malware, spam, and unauthorized access, thereby protecting sensitive information and system integrity. For instance, an email server might blocklist known spam domains to keep inboxes clean. Blocklisting differs from allowlisting (whitelisting), which permits only explicitly approved items. This concept is fundamental to cybersecurity and appears on exams like CompTIA Security+ and CISSP.