SecurityTrails
Domain and IP related information such as current and historical WHOIS and DNS records
Search engine for Internet connected devices.
Shodan API is a free Security API that requires apiKey authentication, is served over HTTPS and has unconfirmed CORS support.
Complete reference documentation for the Shodan API
Details from developer.shodan.io.

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Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
Domain and IP related information such as current and historical WHOIS and DNS records
SSL certificate validity, domain WHOIS status, and DNS record checks for any domain
The Shodan API is provided by Shodan Developer (developer.shodan.io). Complete reference documentation for the Shodan API
Yes. The Shodan 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.
Yes. The Shodan API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the Shodan API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Shodan API is served over HTTPS, so it's safe to use in production.