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The Alerter service is a Microsoft Windows service which is used to send administrative alerts. It alerts users about security, access, and user session problems. Alert messages are sent from a server to a client computer, and the Windows Messenger service must be running on the client computer for the user to receive alert messages.
The service differs from the Messenger service in that it is mainly used for server application, such as performance monitor, to send alert messages; where the Messenger service is used for client-to-client communications.
An explanation is that this service can be abused by an attacker who can trick valid users into doing some actions that may harm their accounts or your network (social engineering attack).