The United States government reportedly hacked into the office of former President Nicolas Sarkozy of France shortly before he was voted out of office earlier this year.
L'Express magazine said a "powerful worm" had been found in the computers of the Elysee Palace by a cyber war agency.
The palace has since confirmed a big cyber attack did take place.
The US embassy in Paris "categorically denied" the United States had been involved in any cyber attack on the French government.
The malware the agency is said to have discovered is called Flame. The BBC reports it can collect files on a machine, take screen shots and even activate the microphone on a computer to record conversations.
According to L'Express, even the computer of Mr Sarkozy's chief of staff, Xavier Musca, was hacked.
The magazine said the worm was probably introduced to the Elysee network through Facebook.
It was found shortly after the second round vote of the presidential election on 6 May. L'Express reports it took agents three days to cleanse the system.