October 08, 2012, 3:49 PM — Attackers can abuse Facebook's phone search feature to find valid phone numbers and the name of their owners, according to security researchers.
The attack is possible because Facebook doesn't limit the number of phone number searches that can be performed by a user via the mobile version of its website, Suriya Prakash, an independent security researcher said Friday in a blog post.
Facebook allows users to associate their phone numbers with their accounts. If fact, a mobile phone number is required to verify any new Facebook account and unlock features like video uploading or timeline URL personalization.
When adding phone numbers in the "Contact info" section of their respective Facebook profile pages, users can choose if they want to make this information visible to the general public, only to their friends or if they want to keep it to themselves, which is a good privacy option.
Facebook also allows users to find other people on the website by searching for those people's phone numbers in international format.
Users can control who can locate them using this method through an option under "Privacy Settings" > "How You Connect" > "Who can look you up using the email address or phone number you provided?" which is set by default to "Everyone."
This means that even if you set your phone number's visibility to "Me only" on your profile page, anyone who knows your phone number will still be able to find you on Facebook unless you change the second setting to "Friends" or "Friends of friends." There is no option to prevent everyone from locating your profile using your phone number.
Since most people don't change the default value of this setting it is possible for an attacker to generate a list of sequential phone numbers within a chosen range -- for example from a specific operator -- and use Facebook's search box to discover who they belong to, Prakash said. Connecting a random phone number to a name is every advertiser's dream and these sort of lists would fetch a large price on the black market, he said.
Prakash claims that he shared this attack scenario with Facebook's security team in August and after an initial response on Aug. 31 all of his emails went unanswered until Oct. 2, when a Facebook representative responded and said that the rate at which users can be found on the website via any means, including phone numbers, is restricted.
However, the mobile version of Facebook's website -- m.facebook.com -- doesn't appear to have any search rate limitation, Prakash said.