SEOUL—South Korean authorities said they are investigating an executive at a local technology company suspected of helping North Korea in cyberattacks on the South.

Identified only by the surname Kim, the man is in his 50s and is suspected of helping the North infiltrate about 110,000 South Korean PCs, including by providing usernames and passwords, a spokesman for the National Intelligence Service said Wednesday. The infiltration would allow them to be used as "zombie PCs" in launching attacks on government and commercial...