IP/domain/URL reputation.
Web of Trust API is a free Anti-Malware API that requires apiKey authentication, is served over HTTPS and has unconfirmed CORS support.

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IP/domain/URL reputation
Yes. The Web of Trust API is listed in the Anti-Malware 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.
Yes. The Web of Trust API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the Web of Trust API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Web of Trust API is served over HTTPS, so it's safe to use in production.