What are allowlists and denylists?

An allowlist lists what is permitted, and everything else is blocked. A denylist lists what is blocked, and everything else is permitted. Firewalls, email filters and application controls use both. Allowlists are stricter and safer, for example letting a production machine talk only to the three systems it needs, but they take work to maintain. Denylists are easier but only stop what is already known to be bad. These terms replace the older whitelist and blacklist. In detection, an allowlist also means marking known harmless activity, such as a backup server, so it stops causing alerts.

← All 69 questions on What is…? Network and security termsAll topics