(Bloomberg) -- The number of hacked U.S. credit cards whose information was offered for sale to other criminals on the dark web jumped by about two-thirds in the first half of this year, according to a cybersecurity research firm.

More than 4,000 credit cards per bank were on sale in the first half, up from about 2,400 in the second half of 2017, New York-based IntSights said in a report released on Wednesday. IntSights said its researchers analyzed data from the “top 50” banks and financial firms in the U.S. and Europe for its report on the dark web, where anonymous browsing allows illegal activity to flourish.

The number of cards up for sale has almost quadrupled in the last two years, IntSights said. That will probably increase the losses banks and insurance companies face this year. Hackers are also targeting bank-account data, IntSights’ Itay Kozuch said in an interview.

“To get credit-card or bank-account credentials you don’t need to hack the bank, but you just trick the customer into giving up his credentials on a fake site,” said Kozuch, director of threat research. “As banks fortify their defenses, hackers go for the weaker link: the customer.”

Bloomberg News