What is a domain controller?

A domain controller is the server that checks sign-ins and holds the central list of users, computers and permissions in many company networks, typically running Microsoft's Active Directory. When someone logs in to a company computer, the domain controller confirms who they are and what they may access. That makes it the most valuable target for an attacker: whoever controls it controls the company's accounts. Protect it with separate administrator accounts, multi-factor authentication where possible and tight network access. A workstation that suddenly talks to the domain controller in unusual ways is worth checking.

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