The most common security risk of intrusion through an access control system is by simply following a legitimate user through a door, and this is referred to as tailgating. Often the legitimate user will hold the door for the intruder. This risk can be minimized through security awareness training of the user population or more active means such as turnstiles. In very high-security applications this risk is minimized by using a sally port, sometimes called a security vestibule or mantrap, where operator intervention is required presumably to assure valid identification.