Facebook engineers have modified a controversial feature to prevent it from exposing the phone numbers users must provide to receive an additional level of security against account takeovers.

The change, made over the weekend, tweaks a recently added reverse phone number lookup service (which, as you'd expect, allows users to enter an unknown phone number to see who it belongs to). The service no longer includes phone numbers users provide when signing up for two-factor authentication protection known as login approvals. Login approvals require users to provide a one-time password sent to their mobile device when logging into their accounts from new computers or smartphones. Previously, those numbers were automatically included in the reverse lookup database. Users who wanted to avail themselves of the two-factor protection ran the risk of exposing their phone numbers to the world at large or their Facebook friends, depending on how privacy settings were configured.

A Facebook spokesman on Sunday told Ars the change is temporary. Engineers are in the process of creating a system that will allow people who sign up for login approvals to opt in or out of the reverse lookup feature.

The change comes after a researcher created a program that allowed people to cycle through tens of thousands of phone numbers to identify the names of each corresponding owner. Facebook engineers responded by vastly reducing the number of queries a given account holder could make in a given time period. But even then, the researcher said he was able to procure 300 results before being locked out of his account for 24 hours. Other researchers also criticized the rate limit as being ineffective.

The reverse lookup feature still provides results for numbers that are included in users' account profiles, and by default, the names of the number holders are provided to all Facebook users. Those who want to restrict that ability only to Facebook friends must change default configurations included in the Facebook privacy settings.