The new security feature, called Login Approvals, is a form of two-factor authentication, as it adds a step to the basic user name and password verification.
With Login Approvals, Facebook users can now opt to have the company require that a one-time numeric code be entered along with their user name and password when a log-in attempt is made from devices that users haven't saved as approved ones.
Facebook sends the numeric code as an SMS message to the mobile phone the user has linked to the account, .
In this manner, an account would be protected against unauthorized access by someone who stole the user's user name and password.
The next time he tries to log in, the legitimate user would be notified that someone had tried to access the account and would be asked to change the compromised password.