New documents leaked by NSA leaker Edward Snowden reveal that the British spy agency GCHQ — and not the NSA — was behind a cyberattack against Belgian telecommunications company Belgacom. The telecom counts the European Commission and the European Parliament among its customers.

The documents reveal "Operation Socialist," a malware campaign spanning over several years to infect the computers of Belgacom employees who had access to important parts of the company's infrastructure, as reported by the German magazine Der Spiegel, thanks to new top secret documents leaked from the GCHQ.

See also: NSA Contract With French Hacking Company Revealed

The attack was labeled by Belgian authorities as an act of "state-sponsored espionage," a serious allegation considering that both Belgium and the United Kingdom are allies and members of the European Union.

On Monday, Belgian authorities had started discussing a possible attack against Belgacom, the country's former state monopoly company (now still partially owned by the government of Belgium), as reported by Reuters.

"The inquiry has shown that the hacking was only possible by an intruder with significant financial and logistic means," federal prosecutors said. "This fact, combined with the technical complexity of the hacking and the scale on which it occurred, points towards international state-sponsored cyber espionage."

This led De Standaard, one of the major Belgian newspapers, to point the finger towards the NSA.

A GCHQ spokesperson told UK newspaper The Independent that the agency has a "longstanding practice" of not commenting on leaks, but also added that "All GCHQ’s work is carried out in accordance with a strict legal and policy framework which ensures that its activities are authorized, necessary and proportionate, and that there is rigorous oversight."

Belgacom has announced that they have eradicated the virus from their networks and the matter is now in the hands of federal prosecutors.

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