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.

Note: X-Content-Type-Options only apply request-blocking due to nosniff for request destinations of "script" and "style". However, it also enables Cross-Origin Read Blocking (CORB) protection for HTML, TXT, JSON and XML files (excluding SVG image/svg+xml).

Header type Response header
Forbidden header name no

Syntax

X-Content-Type-Options: nosniff

Directives

nosniff
Blocks a request if the request destination is of type:
  • "style" and the MIME type is not text/css, or
  • "script" and the MIME type is not a JavaScript MIME type
Enables Cross-Origin Read Blocking (CORB) protection for the MIME-types:
  • text/html
  • text/plain
  • text/json, application/json or any other type with a JSON extension: */*+json
  • text/xml, application/xml or any other type with an XML extension: */*+xml (excluding image/svg+xml)

Specifications

Specification Status Comment
Fetch
The definition of 'X-Content-Type-Options definition' in that specification.		 Living Standard Initial definition

Browser compatibility

BCD tables only load in the browser

Browser specific notes

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

See also