Blacklisting

Blacklisting is the practice of compiling and using lists of entities that are denied access to a service, resource, or opportunity because they are deemed untrustworthy or harmful. Lists may target individuals, organizations, domains, IP addresses, or other identifiers. The term has historical roots in governance and law, but in technology it is often implemented as denylists or blocklists to describe access restrictions.

Blacklists are created through manual flagging, automated reputation systems, third-party feeds, or regulatory sanctions. Access is restricted by matching identifiers against the list and applying a policy such as blocking email delivery, denying network access, or refusing service.

Common applications include email security (spam blocks and DNS-based blocklists), network security (IP or domain blocks), content moderation (blocking sites or accounts), and finance or sanctions compliance (watchlists and fraud or anti-terrorism lists). In employment or media contexts, historical blacklists have restricted individuals from opportunities, illustrating potential harms from improper use.

Criticism centers on false positives, due process concerns, privacy implications, and the risk of discrimination or collateral harm. Best practices emphasize accuracy, timely updates, avenues to appeal, transparency about criteria, and the use of allowlists or risk-based scoring to reduce harm. Some regions also regulate how lists are compiled and used to protect rights.

Alternatives and related concepts include denylists or allowlists, reputation systems, and dynamic threat intelligence. Blacklisting can be an effective risk-management tool when designed with governance, oversight, and accountability.