The U.S. National Security Agency knew for at least two years about a flaw in the way that many websites send sensitive information, now dubbed the Heartbleed bug, and regularly used it to gather critical intelligence, two people familiar with the matter said. The NSA’s decision to keep the bug secret in pursuit of national security interests threatens to renew the rancorous debate over the role of the government’s top computer experts.

Heartbleed appears to be one of the biggest glitches in the Internet’s history, a flaw in the basic security of as many as two-thirds of the world’s websites. Its discovery and the creation of a fix by researchers five days ago prompted consumers to change their passwords, the Canadian government to suspend electronic tax filing and computer companies including Cisco Systems Inc. to Juniper Networks Inc. to provide patches for their systems.