The U.S. National Security Agency, which has a cybersecurity mission in addition to surveillance, has disputed a report that it knew about the Heartbleed security vulnerability for at least two years before other researchers disclosed the flaw this month. The NSA used Heartbleed to gather intelligence, according to a report from Bloomberg, quoting two anonymous sources. Heartbleed is a flaw in OpenSSL that could allow attackers to monitor all information passed between a user and a Web service.

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 agency’s reported 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. The NSA, after declining to comment on the report, subsequently denied that it was aware of Heartbleed until the vulnerability was made public by a private security report earlier this month.