A helpdesk technician received a phone call from an administrator at a remote branch office.

A helpdesk technician received a phone call from an administrator at a remote branch office. The administrator claimed to have forgotten the password for the root account on UNIX servers and asked for it. Although the technician didn't know any administrator at the branch office, the guy sounded really friendly and since he knew the root password himself, he supplied the caller with the password.

What type of attack has just occurred?