1. A method comprising: (a) detecting that within a pre-defined period of time, a single user is attempting to access, via a single computing device, a first account that belongs to a first account-owner, and a second account that belongs to a second account-owner; (b) querying a database that stores data about real-life relationship between people, and determining existence of a real-life family linkage or a real-life business linkage, between the first account owner and the second account-owner; (c) based on said real-life linkage, determining that said single user is a legitimate human user that is genuinely associated with each one of said accounts, and is not a cyber-attacker that is illegitimately attempting to access multiple accounts.