Aviv Raff, chief technology officer at the Israeli cybersecurity firm Seculert, said Monday the hackers gained access to 15 computers in Israel early month.
"At the beginning of this month a number of mails were sent to a number of companies in Israel, including security organizations," he said. "There was an attachment ... and whoever opened it was infected with a virus, a Trojan horse, which allowed the attackers to control those computers. One of the computers belonged to the Civil Administration."
The military said reports of the incident were being investigated.
No classified communications were compromised in the attack, the Egyptian news agency website Middle East Online reported.
"The moment that we detected it we canceled the hackers' control and notified the relevant authorities," Raff said.
In response to the hacking, Prime Minister Binyamin Netanyahu said the biggest challenge in the cyberworld "is protecting the privacy and security of the public."
"There should be a sort of U.N. for the Internet. A coalition of the leading companies in the cyberworld ... and in my opinion Israel is the most advanced," he was quoted by the Jerusalem Post as saying.