HTTP Observatory

developer.mozilla.org/en-US/observatory

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Scans a website's HTTP security headers, such as CSP, HSTS and cookie flags, then grades the result and explains how to fix each issue.

Why it is useful

A fast way to find missing security headers on a site you run. Each failed test comes with a short explanation and guidance, so it works as a checklist as well as a score. Run by MDN, formerly hosted by Mozilla.