The names, Social Security numbers, birth dates and drivers license numbers of more than 143 million people in the U.S. have been exposed as a result of a cyber attack at credit reporting agency Equifax Inc. (EFX) that was revealed Thursday.

Equifax said in an announcement that criminals gained access to the information through its website in breaches between May and July. The company learned of the attack on July 29, it said. The information accessed was not from Equifax's consumer and commercial credit databases, meaning that credit scores don't appear to have been exposed.

However, in addition to the personal information of its customers, the credit card numbers of 209,000 U.S. consumers and dispute documents belonging to an additional 182,000 people were accessed

Some limited information from U.K. and Canadian residents was also part of the incident, the company said.

Events like this, exposing massive amounts of customer's personal data stored in online databases, have become increasingly common in recent years. A series of data breaches at Yahoo Inc. in which the information of more than one billion users was exposed is the largest cyber attack reported to date. 

This latest incident at Equifax is "reason Number 10,000 to check your online bank statements and credit card statements on a regular basis, ideally weekly," said industry analyst Matt Schulz of CreditCards.com.

"When breaches like these happen, consumers need to be diligent -- and not just in the short term. Just because nothing looks amiss on your bank statements or your credit report now, that doesn't mean you haven't been compromised," Schultz added.

Equifax, one of three credit monitoring agencies in the U.S., has set up a website to allow consumers to check whether they are among the customers affected by the breach: www.equifaxsecurity2017.com. The company is also offering complimentary enrollment in its identity theft protection and credit file monitoring service.

After-hours trading of Equifax dropped 12.4% on the news to $124.85, after closing at $142.72. Shares closed up slightly after the regular trading session.

The company said it didn't yet have an estimate for investors regarding what the cost of the incident to the company will be.

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