Classified documents relating to the missing Malaysian Airlines Flight MH370 were stolen using a carefully-crafted spear-phishing attack, targeting 30 government officials just one day after the disappearance of the still-missing aircraft.

The Malaysian Star claims that the attack targeted officials with a PDF document which appeared to be a news report about Flight MH370, and was sent to a group of investigators. Around 30 computers were infected by the malware.

“We received reports from the administrators of the agencies telling us that their network was congested with e-mail going out of their servers,” CyberSecurity Malaysia chief exec Dr Amirudin Abdul Wahab said.

Flight MH370: ‘Confidential data’

“Those e-mail contained confidential data from the officials’ computers, including the minutes of meetings and classified documents. Some of these were related to the Flight MH370 investigation.”

Business Insider says that the attack occurred one day after the Boeing 777 went missing, and took the form of an .exe file disguised as a PDF (a common office file format).

It’s unclear who the attacker - or attackers - were, but information from infected computers was transmitted to an IP address in China. Officials in Malaysia blocked the transmission, The Star said.

‘Very sophisticated attack’

Department of Civil Aviation, the National Security Council and Malaysia Airlines were among those targeted by the hacker, the Telegraph reports. The infected machines were shut down, but “significant amounts” of information on Flight MH370 had been stolen.

“This was well-crafted malware that antivirus programs couldn’t detect. It was a very sophisticated attack,” Amirudin said.

CyberSecurity Malaysia suspects the motivation may have been curiosity about supposedly “secret” information held by the Malaysian government on Flight MH370.

“At that time, there were some people accusing the Government of not releasing crucial information,” Amirudin said.“But everything on the investigation had been disclosed.”