The Horizon Connection Server must prevent MIME type sniffing.

MIME types define how a given type of file is intended to be processed by the browser. Modern browsers are capable of determining the content type of a file by byte headers and content inspection and can then override the type dictated by the server. An example would be a ".js" that was sent as the "jpg" mime type vs the JavaScript mime type. The browser would "correct" this and process the file as JavaScript. The danger is that a given file could be disguised as something else on the server, like JavaScript, opening up the door to cross-site scripting. To disable browser "sniffing" of content type, the Connection Server sends the "x-content-type-options: nosniff" header by default. This configuration must be validated and maintained over time.