South Korea has arrested one of its generals (a two star officer only identified as Kim) for being a North Korean spy. Kim is believed to have passed on South Korean plans for how the South Korean military would respond to an all-out war with North Korea. Kim last did this in 2007, and the plan is revised every two years. So the North Koreans don't have the latest version.
It's rare for South Korean officers, especially generals, to spy for North Korea. But for the last few years, North Korea has been increasing its espionage efforts in the south. A lot. Earlier this year, South Korean police arrested two North Korea agents who had entered the country pretending to be refugees from North Korea. The two were on an assassination mission. Other such "refugees" have been caught spying lately. For decades, South Korea believed that North Korea would try to sneak spies into South Korea by having agents pretend to be refugees. But until two years ago, none of these agents was never caught. Most of those caught since then appear to have been sent to locate and kill refugees who were officials in the North Korean government, and were a constant source of embarrassment to North Korea because of all that inside knowledge.
The first of these assassins was discovered back in 2008. A 35 year old North Korean woman, Jong Hwa Won was arrested after being observed by South Korean intelligence for three years. The South Koreans were hoping Won would lead them to other North Korean spies, but she appeared to be operating alone.
There are over 18,000 North Korean refugees living in South Korea, and the number arriving each week has gone from 30 to nearly a 100 in the last five years. There are over 2,000 North Koreans who have obtained asylum in other countries. Many more are getting out of North Korea, but it's difficult to get from China to South Korea. This is usually done by travelling across China to a Southeast Asian nation, like Thailand, and asking for political asylum there. That usually results in the South Korean government stepping in and transporting the North Korean refugees to South Korea. China does not want to encourage North Koreans to sneak into China, by making it easy to get to South Korea from China.
There are believed to be at over 400,000 North Korean refugees in northern China, nearly all of them there illegally. A survey of these revealed that 40 percent of them had never encountered any foreign food aid, and that nearly all of them left North Korea because of food shortages.
Attracted by the opportunity to settle in prosperous South Korea, many of the millions of Chinese, of Korean ancestry, try to sneak in as North Korea refugees. There are schools in northern China that will train these Chinese, who already speak Korean (with a North China accent) to sound and act like North Koreans, and pass the intense interrogation South Korean intelligence officials give each refugee who makes it to South Korea. Most of these Chinese appear to succeed. That is worrisome, as North Korea also runs training programs to help their agents get past the South Korean security screening. Thus it is likely that many North Korean agents have got past the screening and are operating. But are they? It's just as likely that many of these agents realized, once in South Korea, that all the North Korean propaganda about South Korea and the West was a lie. At that point, all they have to do is go dark and hope for the best, or become double agents for South Korea. Perhaps in light of all this, South Korean intelligence has recently increased the screening process from 90 days, to 180.