What is "X-Content-Type-Options=nosniff"?

Question

I am doing some penetration testing on my localhost with OWASP ZAP, and it keeps reporting this message:

The Anti-MIME-Sniffing header X-Content-Type-Options was not set to 'nosniff'

This check is specific to Internet Explorer 8 and Google Chrome. Ensure each page sets a Content-Type header and the X-CONTENT-TYPE-OPTIONS if the Content-Type header is unknown

I have no idea what this means, and I couldn't find anything online. I have tried adding:

<meta content="text/html; charset=UTF-8; X-Content-Type-Options=nosniff" http-equiv="Content-Type" />

but the I still get the alert.

What is the correct way of setting the parameter?

Answer 1

It prevents the browser from doing MIME-type sniffing. Most browsers are now respecting this header, including Chrome/Chromium, Edge, IE >= 8.0, Firefox >= 50 and Opera >= 13. See :

https://blogs.msdn.com/b/ie/archive/2008/09/02/ie8-security-part-vi-beta-2-update.aspx?Redirected=true

Sending the new X-Content-Type-Options response header with the value nosniff will prevent Internet Explorer from MIME-sniffing a response away from the declared content-type.

EDIT:

Oh and, that's an HTTP header, not a HTML meta tag option.

See also : http://msdn.microsoft.com/en-us/library/ie/gg622941(v=vs.85).aspx