The Washington Post – along with a great many other newspapers - reports that a North Korean defector, identified only as Ahn, was recently arrested for an assassination attempt on Park Sang-hak.  According to WP:

Ahn’s arrest comes as North Korea harshly criticizes activists like Park, threatening to fire artillery shells at them for sending leaflets critical of Pyongyang across the heavily armed border between the countries.

Park said Ahn tried to arrange a meeting earlier this month. Ahn was arrested shortly afterward. Park said South Korean authorities told him that Ahn planned to either poison his drink or jab him with a poison needle.

Park is head of Fighters for Free North Korea, an activist group that routinely flies balloons containing leaflets, DVDs, radios and U.S. dollar bills to the North. The group has recently focused its message on a hereditary power succession under way in North Korea.

Park said Ahn once served in North Korea’s special forces, citing a conversation they had several years ago. Officials said Ahn is in his 40s.

If charged under the South’s National Security Law, Ahn could face the death penalty.

The New York Times added some additional details:

Mr. Park told reporters that after several years of staying out of contact, Mr. Ahn approached him again in early September proposing to arrange a meeting with a Japanese man willing to support Mr. Park’s anti-Pyongyang campaign. He said that spy agents alerted him and intercepted Mr. Ahn at a subway station where they were supposed to meet.

The recent assassination attempt may only be the tip of the iceberg.  As reported by the Korea Herald:

The arrest came less than two weeks after a South Korean missionary died in the Chinese border city of Dandong just across from North Korea after suddenly collapsing in the street.

Some South Koreans claimed that the missionary was poisoned to death by a North Korean agent, though an autopsy found no poison in his body.

The AFP also adds:

Another South Korean activist elsewhere in northeast China said he himself was stabbed with a poison-tipped needle in a separate non-fatal incident.

The two incidents are better described in this article – which is worth a read.