A real-world example of cross-site information aggregation. The target patient “Jean” has profiles on two online medical social networking sites (1) and (2). By comparing the attributes from both profiles, the adversary can link the two with high confidence. Furthermore, the attacker can use the attribute values to get more profiles of the target through searching the Web (3) and other online public data sets (4 and 5). By aggregating and associating the five profiles, Jean’s full name, date of birth, husband’s name, home address, home phone number, cell phone number, two email addresses, occupation, medical information including lab test results are disclosed unintendedly.