"According to South Korean experts, Pyongyang’s electronic warfare capabilities are second only to Russia and the United States."
Not sure how accurate THAT statement is, but North Korea has waged electronic warfare multiple times in the past including jamming SK's GPS systems at one point. They probably have a fully capable electronics warfare unit.
Its also quite possible that China supplies the technology/know how to conduct such attacks and uses NK as a proxy to test systems as SK military procedures are somewhat related to the United States.
Also, South Korea has REALLY bad security. For example, if you want to use some form of online banking you must use an ActiveX control with IE.