France's electoral commission has ordered media not to publish contents of Emmanuel Macron's leaked campaign emails to avoid influencing the election.
It warned news outlets in France that journalists could face criminal charges for publishing or republishing the material, under laws that came into effect at midnight forbidding any commentary liable to affect the presidential race.
There were fears the hack could swing Sunday’s final vote, where Mr Macron was expected to comfortably beat far-right candidate Marine Le Pen.
As much as 9GB gigabytes of data were posted on a profile called EMLEAKS to an anonymous document sharing site under two days before the final vote.
While French election rules forbid the media from publishing the emails, they also ban Mr Macron or his team from commenting on or denying any allegations.
His En Marche! party said it had “been the victim of a massive and coordinated hack” on Friday evening, adding that it had “given rise to the diffusion on social media of various internal information”.
A spokesperson said the communications only showed the normal functioning of a presidential campaign, but that authentic documents had been mixed on social media with fake ones to sow “doubt and misinformation”.
“This operation is obviously an attempt at destabilising democracy, as has already been seen in the US during the last presidential campaign,” he added.
“The ambition of the authors of this leak is obviously to harm the En Marche! movement within hours of the second round of the French presidential election.”
En Marche! previously complained about attempts to hack its emails, blaming Russian interests in part for the cyber attacks despite denials from the Kremlin.
Officials said it had been the target of failed attempts to steal email credentials dating back to January, identifying a hacking group operating in Ukraine.
Vitali Kremez, director of research with US-based cyber intelligence firm Flashpoint, told Reuters APT 28, a group tied to Russia’s military intelligence directorate, was behind the leak.
The collective, also known as “Fancy Bear” and “Sofacy”, has been linked to cyber attacks on the Democratic National Committee during the US election, the White House, German Parliament, Nato and French media.
Last month, APT 28 hackers registered decoy internet addresses to mimic the name of En Marche!, which were used to send corrupted emails to hack into the campaign’s computers, Mr Kremez said.
“If indeed driven by Moscow, this leak appears to be a significant escalation over the previous Russian operations aimed at the US presidential election, expanding the approach and scope of effort from simple espionage efforts towards more direct attempts to sway the outcome,” he added.
Far-right American activists are believed to be behind early efforts to spread the documents on social media, before they were picked up by Ms Le Pen’s supporters in France.
The leaks emerged on 4chan, where an anonymous poster provided links to documents on Pastebin with the message: “This was passed on to me today so now I am giving it to you, the people.”
The hashtag #MacronLeaks was spread by prominent Twitter accounts including that of Jack Posobiec, a pro-Donald Trump activist and employee of the far-right site Rebel TV.
The cyber attack came after repeated allegations of Russian interference in elections across Europe and the US, with Mr Macron previously targeting state media including Russia Today and Sputnik for spreading “fake news” to damage his campaign.
The two government-owned news outlets has announced legal action against Mr Macron over his allegations, which came after the politician denied unsubstantiated reports of an alleged offshore bank account.
Margarita Simonyan, the editor of both RT and Sputnik, said: “We are tired of their lies. We will sue them.”
The Paris’ prosecutor’s office said no one was named in the complaint, which has triggered an inquiry into the suspected spread of false stories aimed at influencing the election.
Vladimir Putin has dismissed allegations of interfering in foreign elections including the US and Germany, hitting out at unproven "rumours".
“We never interfere in other countries’ politics and we want no one to meddle in ours,” the Russian President said during a tense press conference with Angela Merkel.
“Unfortunately, we have seen the opposite happening for years. We have seen attempts to influence political processes in Russia through the so-called NGOs and directly.
“Realising the futility of such efforts, it has never occurred to us to interfere."
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