Seoul, South Korea (CNN) -- A cyber attack Friday morning hit 40 South Korean websites, including those of several government agencies and major banks, the country's internet security agency said.
The government detected the first wave of the attack Thursday. It was coordinated from outside South Korea using "zombie" computers -- computers which have downloaded malicious codes -- to target designated websites, slow them down or paralyze their operation, the internet security agency said.
Infected computers directed a giant traffic surge to targeted websites. That's called a DDoS attack, short for distributed denial-of-service -- and it's hard for most sites to defend against.
By Friday, South Korea's leading security software company had developed a free anti-virus program to treat infected computers.
Software company AhnLab said it developed the anti-virus program after receiving reports of the attack from the South Korean government Thursday. The company warned that another attack would hit websites at 6:30 p.m. Friday.
Targeted sites include South Korea's presidential office, the foreign ministry, the defense ministry and main banking websites, according to a statement issued by AhnLab.
No severe damage has been reported so far, but the government said it was monitoring the attack.
During a similar attack in 2009, 17 South Korean websites were hit over a three-day period.