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I entered the last four digits of my SSN and provided basic DOB and full name. Then it asked me three security questions that only I would know the answers to. For example, it asked where I was in 1989-1990 and it asked where something was.
From this article airbnb is using an identity verification service from a company called IDology. This company provides a service to verify your identity:
Derived from information in public data records, ExpectID IQ serves up non-intrusive, intelligent questions relating to that person’s history such as something involving a previous address or an associated person.
Quanto è sicuro? Immagino che un aggressore intraprendente possa rispondere alle domande di verifica di chiunque, sia ricercando personalmente le registrazioni pubbliche, sia sottoscrivendo l'IDologia e richiedendo serie di domande-risposte sul bersaglio fino a quando non è venuta fuori la stessa domanda.