X-Content-Type-Options

The X-Content-Type-Options response HTTP header is a marker used by the server to indicate that the MIME types advertised in the Content-Type headers should be followed and not be changed. The header allows you to avoid MIME type sniffing by saying that the MIME types are deliberately configured.

This header was introduced by Microsoft in IE 8 as a way for webmasters to block content sniffing that was happening and could transform non-executable MIME types into executable MIME types. Since then, other browsers have introduced it, even if their MIME sniffing algorithms were less aggressive.

Starting with Firefox 72, top-level documents also avoid MIME sniffing (if Content-type is provided). This can cause HTML web pages to be downloaded instead of being rendered when they are served with a MIME type other than text/html. Make sure to set both headers correctly.

Site security testers usually expect this header to be set.