In the context of cybersecurity, a security whitelist, also known as an allow list, is a method that only permits predefined actions while blocking everything else, operating on a "deny by default" policy. This approach contrasts with blocklists, which allow everything by default except specified exceptions, thereby offering greater potential security but with operational challenges of ensuring all legitimate processes are allowed. Whitelisting is often used in email, IP, and application security to prevent unauthorized access and reduce attack surfaces, though it can be cumbersome in dynamic environments where flexibility is required. Blocklists, on the other hand, provide ease of access but demand constant updating to guard against emerging threats. The trade-off between security and accessibility is a common challenge within the C-I-A triad of information security, necessitating a balanced implementation of either approach based on specific use cases and operational needs.