To help its hundreds of millions of users prevent unauthorised access to their accounts, Facebook has added an optional verification step to its log-in process.
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, the company said Thursday in a blog post.
In this manner, an account would be protected against unauthorised 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.
If a user has activated Login Approvals, attempts to access the account from an unrecognised device and doesn't have the authorised cell phone at hand, he would remain locked out until he logs in from an approved device.
Facebook may consider incorporating other technology to Login Approvals beyond SMS, but it is holding off on it until those other forms of two-factor authentication become simpler.
"One challenge in building login approvals was balancing security and usability. Similar features on other websites require you to download authentication apps or purchase physical tokens to act as your second factor. These are good approaches, and we're considering incorporating them in the future, but they require a lot from the user before being able to turn on the feature," wrote Andrew Song, a Facebook engineering intern, in the blog post.
People's user names and passwords are stolen in a variety of ways, including through phishing scams, by intercepting unsecured Wi-Fi communications and with key-logging malware.
The Login Approvals feature can be turned on by going to the Account Security section of the Facebook account settings page, the company said.