South Korea is investigating the simultaneous shutdown of computer networks at several major broadcasters and banks, raising speculation of cyber-jousting by North Korea.
The shutdown comes just over a week after neighbouring North Korea accused the South and its US ally of carrying out cyber attacks against its internet servers, casting suspicion on the North.
Tensions between the two sides have been high since UN sanctions were tightened against North Korea following its latest nuclear test.
South Korean YTN cable news channel reported that the company's internal computer network was paralysed and local TV showed workers staring at blank computer screens, according to Sky News.
The state-run Korea Information Security Agency, which is investigating the shutdown, confirmed that at least five South Korean companies’ computers were down.
Shinhan Bank said the bank's system, including online banking and cash machines, had stopped working.
While there were no reports of government-related computer networks being affected, and while officials stated it was not yet known whether North Korea was involved, the South Korean defence ministry said it had not ruled out the possibility.
Accusations of cyber attacks on the Korean Peninsula are not new, with South Korea accusing North Korea of being responsible for cyber attacks on its companies in 2011 and 2012.
Security experts say that if the computer shutdown in South Korea is indeed a cyber attack, the choice of targets is telling of the trend that the chief candidates for attack are increasingly likely to be global financial markets and critical infrastructure systems.
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“If these systems are taken down, attackers have the power to cripple a nation,” says Jarno Limnell, director of cyber security at security firm Stonesoft and former advisor to the military and government in Finland.
“In today’s digitally interconnected world there is huge potential for unpredictable side effects and collateral damage from aggressive actions. As such, fighting fire with fire is a dangerous tactic,” he said.
Limnell notes that the defence policy of many countries is based on the assumption that if you are able to expose strong enough military capability, the likelihood of being attacked decreases.
“Testing the cyber capabilities of other nations, and the use of offensive techniques are as such an increasingly recognised part of strategic influence and combat,” he said.
Military experts believe cyber warfare is a major threat from North Korea alongside its conventional forces and weapons of mass destruction, posing a risk to South Korean critical infrastructure and communication networks.
According defectors from North Korea, the government is recruiting thousands of computer engineers to its cyber warfare unit.
Cyber attacks on critical national infrastructure is a top concern in the US, where president Barack Obama his issued an executive order requiring federal agencies to share cyber threat information with private companies.