An addon domain is a hosting feature that lets one account serve an additional domain name from the same control panel and server resources. In practice, it maps a new hostname to its own document root or site content while still being managed under the same user account. This is common in shared hosting, where customers may run multiple sites without needing separate server accounts.
In cyber security, addon domains matter because they expand the attack surface of a single hosting account. If permissions, path handling, or file validation are weak, a routine domain-management action can influence filesystem locations or configuration files outside the intended boundary. That can lead to arbitrary file creation, website defacement, or even privilege escalation in control-panel bugs. Defenders should treat addon-domain creation as a sensitive administrative function, restrict who can use it, and monitor for unexpected changes to web roots and server-side files.