Facebook patches security hole that allowed mass harvesting of phone numbers

IDG News Service - Facebook has restricted the rate at which users can perform phone number searches on its mobile website in order to block a recently disclosed method of harvesting phone numbers.

"The ability to search for a person by phone number is intentional behavior and not a bug in Facebook," Facebook said Tuesday in an emailed statement. "By default, your privacy settings allow everyone to find you with search and friend finder using the contact info you have provided, such as your email address and phone number. You can modify these settings at any time from the Privacy Settings page."

"Facebook has developed an extensive system for preventing the malicious usage of our search functionality and the scenario described by the researcher was indeed rate-limited and eventually blocked," the company said. "We are constantly updating these systems to improve their effectiveness and address new kinds of attacks."

On Friday, independent security researcher Suriya Prakash disclosed that Facebook's phone number search feature can be abused to find the names of people who own randomly generated phone numbers.

Facebook requires users to associate phone numbers with their accounts in order to gain access to additional features, including enhanced account security options like two-factor authentication. The website also allows users to locate other people's profiles by searching for their phone number.

However, without a strict limit on how many searches a user can perform, attackers could generate sets of thousands of sequential phone numbers and use the website's search feature to discover if any of them are associated with Facebook accounts.

This would allow them to build lists of phone numbers coupled with the names of their owners extracted from Facebook profiles. Such information could be sold to advertisers or used in phone-based scams.

Suriya claims that he privately contacted Facebook in August and presented this attack scenario and that the company's security team dismissed the threat by saying that phone number-based searches are rate-limited on the website.

However, after additional testing, the researcher found that the mobile version of Facebook appeared to have no search-rate limit implemented. He claims that subsequent emails sent to Facebook went unanswered and this led to his decision to go public with the issue.

After the public disclosure, other security researchers independently verified the vulnerability. Tyler Borland, a security researcher from security firm Alert Logic even released a script that took advantage of multicore CPUs to run multiple Facebook phone number search processes concurrently.

Security researchers confirmed that Facebook started limiting the number of searches that can be performed through its mobile website on Monday. However, they doubt that any limitation existed before that time, as suggested by the company's statement.