WASHINGTON — The 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.

Putting the Heartbleed bug in its arsenal, the NSA was able to obtain passwords and other basic data that are the building blocks of the sophisticated hacking operations at the core of its mission, but at a cost. Millions of ordinary users were left vulnerable to attack from other nations’ intelligence arms and criminal hackers.

“It flies in the face of the agency’s comments that defense comes first,” said Jason Healey, director of the cyber statecraft initiative at the Atlantic Council and a former Air Force cyber officer. “They are going to be completely shredded by the computer security community for this.”

Vanee Vines, an NSA spokeswoman, declined to comment on the agency’s knowledge or use of the bug. Experts say the search for flaws is central to NSA’s mission, though the practice is controversial. The NSA and other elite intelligence agencies devote millions of dollars to hunt for common software flaws that are critical to stealing data from secure computers. Open-source protocols like OpenSSL, where the flaw was found, are primary targets.