The unclassified email system used by the Joint Chiefs of Staff was hijacked in the attack, leaving data of nearly 3,500 military personnel and civilians vulnerable to exposure.

It was believed to have been an attempt at crippling the Pentagon's systems, instead of a cyberespionage campaign, but the attack did not exactly come off as planned.

Chairman of the Joint Chiefs Martin Dempsey told CBS News that the NSA director Admiral Mike Rogers alerted him about the attack in an early morning phone call. The attack saw passwords and electronic signatures of Dempsey and hundreds of other senior Army officials obtained by the hackers.

The attackers reportedly seized control of the Pentagon's email system in under an hour. The only way to mitigate it was to allegedly take down the entire network and conduct a full hardware and software systems overhaul, which took around two weeks.

The attack was motivated by Russia's umbrage at economic sanctions imposed by the Obama administration, which was in response to Russian president Vladimir Putin's appropriation of Crimea and interference in Ukraine.

This is not the only time that Putin’s hackers have been in the news lately. The Office of the Director of National Intelligence (ODNI), which oversees 16 US intelligence agencies, including the NSA and the CIA, is slated to publicly release its report on foreign governments' attempts at influencing US elections "in the coming weeks".