What Is x-content-type-options: nosniff Response Header?

Response headers contain a lot of useful details about a web document/file that is sent over the internet. A very known example is a text/html content type, or in other words, a regular HTML page, as the non-technical folks would say. The information inside a response header has nothing to do with what you see on the page, but rather its attributes such as age, server, content type, etc.

You can think of it as something similar to the surrounding particulars of an image file, such as size, dimensions, whether it’s a PNG or JPG, etc.

Out of the many values within a response header, one of them can be–>x-content-type-options: nosniff. To understand what exactly it does and how it helps, we first need to get a quick, easy primer on MIME Types.

A Simple Succinct Explanation on MIME Types

MIME stands for Multipurpose Internet Mail Extensions, but in less fancy terms, it helps categorize what kind of content is being served (such as HTML, PDF, XML, etc.), so the web browsers can correctly interpret and process it. There are numerous content types out there, and there is a technical method to represent what it is. This post won’t cover all; nevertheless, a few common ones are:

• text/html for normal web pages.
• text/xml for XML URLs. A widely recognized example would be XML sitemap URLs in an uncompressed format.
• application/pdf for online .pdf URLs.
• Etc.

So What Does “x-content-type-options: nosniff” Do, After All?

Some web browsers can practice what is called MIME sniffing. Translated, these browsers on their own — try to figure out the content classification of the transmitted file/page/document.

The intent behind it is pure as browsers don’t want to decode a file format incorrectly; however, it can prove dangerous as malicious actors or hackers can take advantage of this unseemly routine exercise. Put another way, MIME sniffing leaves a door open for a security breach.

An Example of How a MIME Sniffing Attack Can Take Place

One of the vulnerabilities that MIME sniffing poses is that cybercriminals can mislabel or misrepresent a file type in an entirely different format than what it actually is. For instance, an executable program can be deliberately miscategorized as an html document — allowing itself to circumvent the security mechanisms associated with executables, such as asking the user to download something or popping up a dialog box.

The Role of “x-content-type-options: nosniff” Against MIME Sniffing

Plainly described, x-content-type-options: nosniff counters the ability of browsers to MIME sniff by forcing them to use the MIME Type declared by the server, rather than relying on its own capacity. It essentially tells the browser to trust the provided MIME Type and only use that; no other.

By compelling the browsers only to digest the given MIME Type, one can be protected against certain varieties of MIME sniffing attacks. That is, if the browser you’re using acknowledges that command in the first place.

Online Security Risks Are Real

Hearing about an online web-based attack has become so recurrent for us these days that it almost feels like a regular part of our lives.

At the end of the day, it really is a battle between the good and the bad, with the hope that the existing security systems, methodologies, and philosophies will prevail. But apart from that wishful thinking, it is also said that when it comes to safeguarding against vicious online exploitative attempts, security starts with you and starts with being educated about things.

Fingers-crossed, this post has helped in some manner to equip oneself with more knowledge about online security susceptibility associated with MIME sniffing. Or at the least, enough to conduct further research.

Expert Sources

• The WebPlatform project
• Keycdn