A blacklist, in IT, is a collection of entities that are blocked from communicating with or logging into a computer, site or network. Blocked entities are typically identified as IP addresses, user IDs, domains, email addresses, MAC addresses or programs. Blacklisting is a common feature in antivirus programs, intrusion prevention/detection systems and spam filters. Blacklists save effort by limiting interactions with known sources of problems and preventing potential attacks.
Network administrators and users alike employ blacklists to block entities who would be likely to cause problems. The problem entities could be malware networks, spammers, hackers, DoS (denial of service) attackers or abusive site or forum users, among a plethora of other possibilities. Application blacklisting prevents the execution of undesirable programs, which include applications that are known to contain security threats or vulnerabilities and those that are deemed inappropriate for a given business environment. Hackers and IT security researchers sometimes use blacklists differently, seeking interaction with blacklisted entities to provide information.
Assembled blacklists are available for particular sources of problems, such as known malware networks or spammers. Spamhaus, for example, provides recommended block lists for IP addresses known to be associated with spam and those known to be hijacked by third-party exploits. Due to differences in online communication needs and the variety of online threats, however, blacklists work best when customized.
Whitelisting, the opposite approach, validates permitted entities and blocks communications from all other sources.