"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.
Technology news site ZDNet separately reported that Anonymous appeared to have published information allegedly containing the log-in information, credentials, internet protocol addresses and contact information of more than 4,000 U.S. bankers on Sunday night.
The claim was made via Twitter over an account registered to OpLastResort, which is linked to Anonymous, a loosely organized group of hacker activists who have claimed responsibility for scores of attacks on government and corporate sites over the past several years.
OpLastResort is a campaign that some hackers linked to Anonymous have started to protest government prosecution of computer prodigy Aaron Swartz, who committed suicide on Jan. 11.
The Fed declined to identify which website had been hacked. But information that it provided to bankers indicated that the site, which was not public, was a contact database for banks to use during a natural disaster.
A copy of the message sent by the Fed to members of its Emergency Communication System (ECS), which was obtained by Reuters, warned that mailing address, business phone, mobile phone, business email, and fax numbers had been published.
"Some registrants also included optional information consisting of home phone and personal email. Despite claims to the contrary,passwords were not compromised," the Fed said.
The central bank separately confirmed the authenticity of the message to ECS members.