"Don't infer the MIME type" middleware
Some browsers will try to "sniff" mimetypes. For example, if my server serves file.txt with a text/plain content-type, some browsers can still run that file with
Browsers' same-origin policies generally prevent remote resources from being loaded dangerously, but vulnerabilities in web browsers can cause this to be abused. Some browsers, like Chrome, will further isolate memory if the
X-Content-Type-Options header is seen.
There are some other vulnerabilities, too.
This middleware prevents Chrome, Opera 13+, IE 8+ and Firefox 50+ from doing this sniffing. The following example sets the
X-Content-Type-Options header to its only option,
const nosniff =app
MSDN has a good description of how browsers behave when this header is sent.