An ethical hacker has been hired to conduct a physical penetration test of a company. During the first day of the test, the ethical hacker dresses up like a plumber and waits in the building's main lobby until an employee goes through the main turnstile. As soon as the employee enters his access number and proceeds to go through the turnstile, the ethical hacker follows them through the access gate. What type of attack did the ethical hacker utilize to access the restricted area of the building?