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 not be changed and be followed. This is a way to opt out of MIME type sniffing, or, in other words, to say 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, the opting out of MIME sniffing is also applied to top-level documents if a 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.

X-Content-Type-Options: nosniff

nosniff

Blocks a request if the request destination is of type style and the MIME type is not text/css, or of type script and the MIME type is not a JavaScript MIME type.

• Firefox 72 enables X-Content-Type-Options: nosniff for top-level documents

• Content-Type
• The original definition of X-Content-Type-Options by Microsoft.
• The Mozilla Observatory tool testing the configuration (including this header) of Web sites for safety and security
• Mitigating MIME Confusion Attacks in Firefox
• Cross-Origin Read Blocking (CORB)
• Google Docs CORB explainer