Cyber attacks have become a grave threat to international peace and security. Northeast Asia is a critical point of many of these cyber operations. First, South Korea has been the target of cyber attacks from North Korea. Second, there are harsh debates on this matter between the US and China. While the United States have expressed their concerns about the growing threat of cyber intrusions from China, the People’s Republic of China has blamed the US for attacks against their respective computer networks. From the perspective of the jus ad bellum, potential cyber attacks raise a number of difficult and complex issues. The following article examines which cyber operations amount to the use of force as stipulated in Article 2(4) of the UN Charter and discusses the conditions under which type of cyber attacks could trigger the right to self-defense. In addition, other available remedies outside the framework of Article 51 of the UN Charter will be discussed.