The stolen information was contained in a statement by a group with an Iranian-sounding name calling for an inquiry into Israel's nuclear activities. The International Atomic Energy Agency (IAEA) is investigating Iran's nuclear programme.
The IAEA said the theft concerned "some contact details related to experts working" with the Vienna-based agency but it did not say who might have been behind the action.
A Western diplomat said the stolen data was not believed to include information related to confidential work carried out by the IAEA. One of the agency's tasks is preventing the spread of nuclear weapons.
The statement posted in the name of Parastoo (swallow in Farsi) included a large number of e-mail addresses and called for the people to whom they belonged to sign a petition for an "open" IAEA investigation into Israel's Dimona reactor.
The statement dated November 25 and headlined "Parastoo Hacks IAEA" said: "Israel owns a practical nuclear arsenal, tied to a growing military body."
Israel is widely believed to have the Middle East's only atomic arsenal but neither confirms nor denies this under a "strategic ambiguity" policy to deter Arab and Iranian foes.
The country is outside the Non-Proliferation Treaty that would require it to forswear nuclear weapons and open up its reactor in the southern desert town of Dimona.
Israel and the United States accuse Iran of seeking to develop a nuclear weapons capability, a charge Tehran denies, and says the Islamic state is the main proliferation threat.
Iran and Arab states say Israel's assumed atomic arsenal threatens peace and security in the Middle East.
IAEA spokeswoman Gill Tudor said the agency "deeply regrets this publication of information stolen from an old server that was shut down some time ago".
Measures had been taken to address concern over possible vulnerability in the server, she said.
"The IAEA's technical and security teams are continuing to analyse the situation and do everything possible to help ensure that no further information is vulnerable," Tudor said.
© Thomson Reuters 2012