Being "under constant attack" supposedly from hackers Nasdaq plans to increase its expenses on cyber security.

"As we sit here, there are people trying to slam into our system every day," Robert Greifeld, chief executive officer of Nasdaq OMX Group, said in an interview on Wednesday. "So we have to be ever-vigilant against an ever-changing foe."

Last year Nasdaq’s systems were attacked by perpetrators who left "suspicious files" on the exchange's servers which incited an investigation involving the FBI. Trading platforms were not compromised, the exchange said, although some Internet-based client applications were vulnerable.

"We're (now) in normal state in that we resolved that particular issue," Greifeld said.

On Wednesday Nasdaq said it would increase its costs on ensuring high level of security of sensitive data kept on its servers. Now total expenses will account for $950 million to $965 million this year, up from the $920 million to $940 million estimated in February.