A cyberattack this summer on JPMorgan Chase compromised the accounts of 76 million households and seven million small businesses, a tally that dwarfs previous estimates by the bank and makes the intrusion among the largest ever. Until just a few weeks ago, JP Morgan had said it believed only one million accounts had been affected. Hackers operating overseas gained access to names, addresses, phone numbers and emails of JPMorgan account holders. The details of the breach were disclosed in a securities filing Thursday and emerge at a time when consumer confidence in the digital operations of corporate America has already been shaken. Target, Home Depot and a number of other retailers have experienced major data breaches. The hit on Target compromised the information of 40 million cardholders and 70 million others. The Home Depot attack affected 56 million cards.