Russian hackers were able to read the confidential email correspondence of US President Barack Obama during a cyber attack on the White House.

The attack on the White House email system was first reported back in October but at the time officials claimed it was not a big deal and only affected unclassified content. Now however a report in the New York Times quoting sources within the Obama regime suggests the breach was "far more intrusive and worrisome than has been publicly acknowledged".

On 22 April, security company Kaspersky Lab revealed details of the group behind the attack - a group of hackers known as CozyDuke - revealing the social engineering methods used to breach the White House systems.

While Kaspersky didn't go so far as to point the finger of blame at any one state, there was a lot of corroborating evidence which strongly suggested that the Russian government was involved in the attack.

Earlier in April, White House officials admitted that the hackers had gotten access to President Obama's schedule, which, while not classified is still highly sensitive information, seen as highly prized by foreign intelligence agencies.

The latest report suggests the hackers were not able to access the well-guarded servers which contain messages sent by Obama's BlackBerry - which he always carries with him - but they were able to access accounts which contained email correspondence of people with whom Obama communicated regularly.

"From those accounts, they reached emails that the president had sent and received, according to officials briefed on the investigation," the New York Times report says.

Particularly worrisome

It is unsurprising that the White House email systems would be a major target for hackers, but the strong indication that this is the work of Russian hackers will be hugely controversial.

Calling it "one of the most sophisticated attacks" they have seen on its network, a senior White House official added: "It's the Russian angle to this that's particularly worrisome".

Obama or his officials have been surprisingly reticent to point the finger of blame for the attack on the White House, refusing to publicly discuss the findings of its investigations.

In recent months the White House and Obama have not been shy about pointing the finger of blame when it comes to state-sponsored cyberattacks against the US. Back in May, 2014 the US government charged five Chinese military officials with carrying out cyberattacks against six US companies.

Earlier this year Obama authorised new sanctions against North Korea after the White House publicly backed claims that the dictatorship was behind a devastating cyberattack on Sony Pictures.

Spear phishing

According to the Kaspersky Lab report, the CozyDuke group goes after "blatantly sensitive high profile victims and targets" utilising "evolving crypto and anti-detection capabilities".

The main attack vector was spear phishing campaigns some of which contain links to high profile, legitimate websites such as "diplomacy.pl" which hosted a Zip archive.

Once downloaded the extracted Zip archive contains a file which installs the malware as well as a decoy file showing an empty PDF.

Another "highly successful" attack saw the hackers send a phone flash videos attached to the phishing emails, one of which was a video called "Office Monkeys LOL Video.zip". When the victim clicks on the link the video plays, but in the background the malware is installed on the system.