A surprise distributed denial-of-service (DDoS) attack briefly took down 29 web sites belonging to the South Korean government and local businesses.
The attack targeted government ministries, the National Assembly, local and resident US military systems and banking services, resulting in the brief shut down of an online stock exchange.
All sites are now back online but experts are reporting that another attack will occur at the same time tomorrow.
"There was a DDoS attack, but no damage was done," an official from the presidential office told the BBC.
There are similarities with the DDoS campaign that took down web sites in July 2009. These have since been attributed to North Korean hacking.
However the Korea Internet Security Agency said that the attack used around 21,000 infected PCs, compared to the estimated 115,000 that were used in 2009, according to local media reports.
The government has begun issuing free anti-virus software to deal with any local infections, but said that many of the attacks came from outside South Korea.
"Whether or not this was a politically motivated attack remains to be seen," said Mark Darvill, director of security firm AEP Networks.
"The fact that every one of the organisations hit plays a key role in the national security of South Korea and its economy suggests that this is a targeted attack. Governments should sit up, take note and be prepared."
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