5 of the biggest-ever credit card hacks

Target (TGT) got hit at the worst possible time in 2013: peak holiday shopping season.

Hackers managed to break into Target's payment system. They stole computer logins from a company that, at the time, did heating and air-conditioning services for Target.

Criminals quietly collected debit and credit cards that customers swiped at Target stores between Nov. 27 and Dec. 15. In just under three weeks, they grabbed data on 40 million cards.

The terrible surprise? Target should have been able to stop it, according to an investigative piece in Bloomberg Businessweek. Cybersecurity alarms went off -- but they were ignored.

Target's CEO was fired, and the incident has cost the company $146 million so far.

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