1. A method comprising: determining whether a user, who utilizes a computing device to interact with a computerized service, is either an authorized user or an attacker; wherein the determining comprises: (i) monitoring user interactions of at least a first user and a second user and a third user during multiple usage sessions with the computerized service; (ii) determining that the user interactions of the first user, the user interactions of the second user, and the user interactions of the third user, all exhibit a same first behavioral trait when the first, the second and the third user are utilizing a first type of computing platform to access the computerized service; (iii) determining that the user interactions of the first user and the user interactions of the second user, exhibit a same second behavioral trait when the first and the second user are utilizing a second type of computing platform to access the computerized service; (iv) upon a first-ever access of the third user to the computerized service via the second type of computing platform, determining whether or not user interactions of the third user with the computerized service via the second type of computing platform exhibit said second behavioral trait; (v) based on the determining of step (iv), detecting whether said third user is an authorized user or an attacker.