The National Security Agency has discovered a major security flaw in Microsoft's Windows operating system and tipped off the company rather than exploit it for its own intelligence needs.

Microsoft made a software patch to fix it available Tuesday and credited the agency for discovering the flaw. The company said it has not seen any evidence that hackers have used the technique discovered by the NSA.

Microsoft said the flaw affected Windows 10, the newest version of its operating system. 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 from a trusted provider," the company said.

If successfully exploited, an attacker would have been able to conduct "man-in-the-middle attacks" and decrypt confidential information on user connections to the affected software, the company said.

Some computers will get the free update automatically if they have the option turned on. Others can get it manually. Microsoft typically releases security and other updates once a month and waited until Tuesday to disclose the flaw and the NSA's involvement.

Priscilla Moriuchi, who retired from the NSA in 2017 after running its East Asia and Pacific operations, said this is a good example of the “constructive role” that the NSA can play in improving global information security. Moriuchi, now an analyst at the U.S. cybersecurity firm Recorded Future, said it’s likely a reflection of changes made in 2017 to how the U.S. determines whether to disclose a major vulnerability or exploit it for intelligence purposes.

The revamping of what’s known as the “Vulnerability Equities Process” put more emphasis on disclosing unpatched vulnerabilities whenever possible to protect core internet systems and the U.S. economy and general public.

Those changes happened after a group calling itself “Shadow Brokers” released a trove of high-level hacking tools stolen from the NSA.