Four unnamed Korean companies have fallen victim to a worldwide cyberattack, according to the Korea Internet & Security Agency (KISA) Sunday.
More public institutions and private businesses are expected to undergo the ransomware attack Monday as their employees return to work and reboot their computers.
Ransomware is malicious software designed to block access to a computer system or data. Hackers encrypt victims' files and demand hundreds of dollars in virtual money such as bitcoin in a ransom to release the files.
According to KISA, seven institutions, including a university hospital, made inquiries to the agency about the malware as of Sunday afternoon.
The agency offered technical support to the four affected companies, and analyzed the damages to see if they were a result of the global attack.
It is unknown whether any individual users in Korea have fallen victim to the cyberattack.
Because it began on the weekend, most businesses here have yet to see any damage so far. However, experts said that Monday morning will be a watershed for the malware to spread nationwide.
"It is feared that variants of the ransomware will infect computers on Monday as soon as workers turn on the power to their computers," Hauri cyber security firm senior official Choi Sang-myong said.
Experts recommended users to disconnect their computers from local area networks (LAN) before turning them on, and then to update their vaccine software, if they want to prevent damage.
They emphasized prevention is the only solution to the malware, as victims have no choice but to initialize their computers in most ransomware infection cases.
"Firms and individuals should not open dubious email. Previous Windows OS versions should be upgraded to Windows 7 or higher versions as well," a KISA official said. "In the case of damage, users should immediately visit boho.or.kr or call 118."
The world's worst-ever ransomware attack paralyzed over 130,000 computer systems in about 100 countries, including those in British hospitals and the Russian government, over the weekend.
Unlike previous malware which spread through email, the latest software, known as WannaCry, spreads through LAN, affecting any computers running on the Microsoft (MS) Windows operating system (OS).
In response to the vulnerability of Windows OS, MS had already offered a security patch in March. Following the attack on the weekend, the company released a security patch for Windows XP as well, even though the security service for this system was halted three years ago.