Experian Credit Report Leak

In an article on Krebs On Security Links to an external site. Brian Krebs reported that the credit reporting agency Experian was subject to a cyber attack that potentially exposed the credit reports of their customers via their interface to the annualcreditreport.com portal.
What was the failure of the site that allowed this data exposure?
What could an attacker could have accessed and how can they target people for their information?
What other security breaches have the credit reporting agencies had?

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