LONDON (AP) — The cyberattack that spread malicious software around the world, shutting down networks at hospitals, banks and government agencies, was stemmed by a young British researcher and an inexpensive domain registration, with help from another 20-something security engineer in the U.S.

Britain's National Cyber Security Center and others were hailing the cybersecurity researcher, a 22-year-old identified online only as MalwareTech, who — unintentionally at first — discovered a "kill switch" that halted the unprecedented outbreak.

By then, the "ransomware" attack had hobbled Britain's ho...