User data is aggregated across a plurality of electronic communication channels and domains. An online system initially authenticates a user for access to the online system over a network. The online system provides a user identifier for the user to an authentication service. The authentication service generates a non-repeatable challenge from the aggregated user data for the user identifier and provides the non-repeatable challenge to the online system. The online system provides the challenge to the user and receives a response from the user. The online system provides the response to the authentication service and the authentication sends a success or failure back to the online system based on the response to the challenge, and based on the success or failure the online system makes a final determination for authenticating the user for accessing to the online system.