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UPDATE 2-Equifax shares slump after massive data breach

(Updates shares, adds background on recent cyber attacks)

Sept 8 (Reuters) - Equifax Inc's 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 drivers' license numbers.

"Obviously the size and scope of this breach will likely drive a number of negative headlines for EFX that will weigh on its brand for the foreseeable future," Barclays analyst Manav Patnaik wrote in a note.

Several U.S. companies have been hit by sophisticated cyber attacks, with hackers stealing and selling personal information.

Companies such as eBay Inc, Yahoo, Target Corp and Marriott International Inc have been victims of cyber attacks.

Atlanta-based Equifax's larger rival Experian Plc reported a data breach two years ago that exposed sensitive personal data of some 15 million people.

The U.S. government has been pushing for stringent laws to protect online data from phishing attempts.

Equifax said hackers breached the accounts between mid-May and July, potentially accessing information of 143 million Americans. Accounts of some UK and Canadian residents were also compromised.

Britain's Information Commissioner's Office (ICO) said the breach "gives us cause for concern". ICO Deputy Commissioner James Dipple-Johnstone said the office would advise Equifax to alert affected UK customers at the earliest opportunity.

Equifax shares, which have gained 21 percent this year, were down 13 percent at $123.18. The shares earlier touched $117.25, their lowest in more than seven months.

Shares of rival TransUnion were down 4 percent, while Experian shares were down 1.3 percent.

"We expect Equifax and, to a lesser extent, TransUnion and Experian, to face pressure near-term given the magnitude of the breach and attention on data integrity," analyst Michael Tarkan from Compass Point wrote in a note.

Equifax handles data on more than 820 million consumers and more than 91 million businesses worldwide and manages a database with employee information from more than 7,100 employers, according to its website. (Reporting by Aishwarya Venugopal and Sweta Singh in Bengaluru; Editing by Maju Samuel and Saumyadeb Chakrabarty)