(Reuters) - Equifax Incís shares tumbled 18 percent on Friday after the provider of consumer credit scores revealed that hackers may have stolen personal details of nearly half of the American adult population in one of the largest data breaches.

Equifax said on Thursday it discovered the breach on July 29 and that criminals exploited a vulnerability in a website application to gain access to certain files that included names, Social Security numbers and driversí license numbers.