U.S. National Vulnerability Database.
National Vulnerability Database API is a free Security API that requires no API key, is served over HTTPS and has unconfirmed CORS support.

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Programmatic interfaces to engage with the Microsoft Security Response Center (MSRC)
Yes. The National Vulnerability Database API is listed in the Security category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The National Vulnerability Database API requires no authentication, so you can start making requests without signing up.
CORS support for the National Vulnerability Database API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The National Vulnerability Database API is served over HTTPS, so it's safe to use in production.