Last Thursday, JPMorgan, the biggest US bank by assets, said the names, addresses, telephone numbers and emails of 76m households had been compromised by a cyber attack, making it one of the largest such thefts on record.
The US Secret Service and Federal Bureau of Investigation are leading an investigation into the attacks, which are now believed to involve more than a dozen targets.
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People familiar with the matter said not all the institutions' security infrastructure was breached. JPMorgan is the only company to have confirmed that information was stolen.
"We have no indication that any Fidelity customer sites, accounts, information, services or systems were affected by this matter," said a Fidelity spokesman.
"We take security very seriously and closely monitor the online environment. Fidelity has a range of safeguards and multiple layers of security in place to protect customer accounts and information, our sites, and systems. For security reasons, some of these protections are visible, some are not. Beyond that, for security reasons, it's our practice not to comment on details of specific matters," the spokesman said.