US officials said they have ordered a security review after hackers managed to break into the Senate website at the weekend.
An official said the incident had been "inconvenient", but had not compromised the security of the staff.
The confirmation came after Lulz Security, a loosely aligned group of hackers, said it had carried out the attack for fun and posted files online.
Lulz has previously targeted Sony, Nintendo and Fox News.
Senate Deputy Sergeant-at-Arms Martina Bradford said the hacking had been noticed at the weekend, and that officials were now reviewing all the sites hosted on Senate.gov.
"Specifically, there is no individual user account information on the server supporting senate.gov that could have been compromised," she said in a statement.
Lulz Security posted files online which indicated they had been in the Senate network. However, none of the files appeared to be sensitive.
"We don't like the US government very much," Lulz Security said at the top of a release.
"This is a small, just-for-kicks release of some internal data from Senate.gov - is this an act of war, gentlemen? Problem?" it added.
The US recently said it would consider cyber attacks an "act of war", and reserved the right to respond to attacks with conventional means if they were found to have been orchestrated by a nation state.
Lulz is a reference to internet-speak for "laugh out loud".