The admission, which raises questions about cyber security at the Fed, follows a claim that hackers linked to the activist group Anonymous had struck the Fed on Sunday, accessing personal information of more than 4,000 U.S. bank executives, which it published on the Web.

"The Federal Reserve system is aware that information was obtained by exploiting a temporary vulnerability in a website vendor product," a Fed spokeswoman said.

"Exposure was fixed shortly after discovery and is no longer an issue. This incident did not affect critical operations of the Federal Reserve system," the spokeswoman said, adding that all individuals effected by the breach had been contacted.