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 due to
for request destinations of "
style". However, it also
enables 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
styleand the MIME type is not
text/css, or of type
|Fetch Standard (Fetch)|
BCD tables only load in the browser
Firefox 72 enables
X-Content-Type-Options: nosnifffor top-level documents