An international cyberattack struck computers in Ukraine, Australia and the United States Tuesday, in the latest instance of hackers unleashing an attack using a cyberweapon stolen from the National Security Agency. The cyberattack shut down the radiation monitoring systems at the Chernobyl nuclear plant, stopped dozens of Ukrainian ATMs from working, and affected companies’ computer systems worldwide, from a U.S. drug manufacturer to a Danish shipping company to an Australian chocolate factory. The telecommunications company IDT said, “The N.S.A. needs to take a leadership role in working closely with security and operating system platform vendors such as Apple and Microsoft to address the plague that they’ve unleashed.”