The U.S. National Security Agency (NSA) has discovered a major security flaw in Microsoft's Windows 10 operating system that could let hackers intercept seemingly secure communications.

But rather than exploit the flaw for its own intelligence needs, the NSA tipped off Microsoft so that it can fix the system for everyone.

Microsoft released a free software patch to fix the flaw Tuesday and credited the intelligence agency for discovering it. The company said it has not seen any evidence that hackers have used the technique.

Amit Yoran, CEO of security firm Tenable, said it is "exceptionally rare if not unprecedented" for the U.S. government to share its discovery of such a critical vulnerability with a company.

Yoran, who was a founding director of the Department of Homeland Security's computer emergency readiness team, urged all organizations to prioritize patching their systems quickly.

An advisory sent by the NSA on Tuesday said "the consequences of not patching the vulnerability are severe and widespread."

Microsoft said an attacker could exploit the vulnerability by spoofing a code-signing certificate so it looked like a file came from a trusted source. 

"The user would have no way of knowing the file was malicious, because the digital signature would appear to be from a trusted provider," the company said.

If successfully exploited, attackers would have been able to conduct "man-in-the-middle attacks" and decrypt confidential information they intercept on user connections, the company said. 

"The biggest risk is to secure communications," said Adam Meyers, vice president of intelligence for security firm CrowdStrike.

Some computers will get the fix automatically, if they have the automatic update option turned on. Others can get it manually by going to Windows Update in the computer's settings.

Microsoft typically releases security and other updates once a month and waited until Tuesday to disclose the flaw and the NSA's involvement. Microsoft and the NSA both declined to say when the agency privately notified the company.