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
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
text/html. Make sure to set both headers correctly.
Site security testers usually expect this header to be set.
X-Content-Type-Options only apply request-blocking
nosniff for request
destinations of "
script" and "
style". However, it also
Cross-Origin Read Blocking (CORB) protection for HTML, TXT, JSON and XML
files (excluding SVG
|Header type||Response header|
|Forbidden header name||no|
- Blocks a request if the request destination is of type:
style" and the MIME type is not
- Enables Cross-Origin Read Blocking (CORB) protection for the MIME-types:
application/jsonor any other type with a JSON extension:
application/xmlor any other type with an XML extension:
The definition of 'X-Content-Type-Options definition' in that specification.
|Living Standard||Initial definition|
BCD tables only load in the browser
- Firefox 72 enables
X-Content-Type-Options: nosnifffor top-level documents