South Korean officials say no evidence has been found to blame North Korea for Wednesday's computer network failure in the country.
South Korea's Defense Ministry said it could not rule out the involvement of Pyongyang, which has a history of carrying out cyber warfare against the South. But South Korea's Internet and Security Agency said it knew of no cyber attack coming from outside the country.
The South Korean news agency Yonhap quoted government officials as saying a malicious code from unknown hackers caused the cyber attack, which crippled the networks of major broadcasters and top banks.
South Korean officials are investigating the cause of the attack, but officials said it could take several days to identify who was behind it.
In Washington, Jason Healey of the Atlantic Council said such attacks tend to happen when tensions between the two Koreas are high, as has been the case in recent weeks.
"A few of us had been warning as early as last week that this was… that this kind of attack on South Korea would be likely since the North Koreans have been quite belligerent lately and even renounced the armistice with South Korea and the United States. The North Koreans are always wanting to be about bad behavior and intensification of their tantrums, so it’s entirely possible that this was North Korea,” said Healey.
Another Washington-based cyber analyst, Jessica Herrera-Flanigan of the Monument Policy Group, said North Korean involvement in the attack would not be surprising. She also said South Koreans who are sympathetic to the North's government could have carried it out.
"Broadcasters are an easy target because they carry news, they carry information, and we’ve seen those types of cyber attacks increasingly occurring," she said. "Banks, it’s a disruption of the economic system, and it’s a statement about the economic system and trying to attack those systems.”
South Korea's military raised its alert level following the attack, which paralyzed the computers of at least three broadcasters [KBS, MBC and YTN]. Computers at two banks, the Shinhan Bank and the Nonghyup Bank, also were hit, as were some of the country's ATMs. South Korean stocks fell in trading following the news.
North Korea has used increasingly violent rhetoric against the U.S.-backed South in recent weeks, threatening to wage all-out war after the United Nations passed sanctions in response to the North's latest nuclear test.
Pyongyang is believed to have an elite cyber warfare unit that reportedly was behind computer attacks on South Korean government agencies and financial institutions in 2009 and 2011. South Korea's military said it was not affected by the outage.
Wednesday's attack came just days after North Korea accused the United States and South Korea of launching a computer attack on some of its websites, which suffered a prolonged outage last week.