MIME Sniffing: What Is It? What Are the Security Implications?

5 min read
May 9, 2023 at 2:45 PM

Multipurpose Internet Mail Extension (MIME) sniffing has been in use for decades to allow a browser to render content when there is some question about what type of data the content contains. However, MIME sniffing can also open your organization and end users up to serious cybersecurity risks. Although it is a rare occurrence, it is still important to resolve browser-based vulnerabilities to prevent criminals from exploiting them in MIME sniffing attacks.

What Is MIME Sniffing?

When you use a browser to access the internet, site content is rendered via hypertext transfer protocol (HTTP). Your browser (the client) renders the content, and the server sends a response containing either:

  • A status update about the request, like an error message
  • The requested content

Sometimes, that response is missing the MIME header, which is the label that tells the browser what type of content it contains. A MIME sniffer is a browser algorithm that inspects the HTTP response's content to infer content type regardless of whether it is labeled.

If the label is missing, the algorithm scans the beginning of the content's code to determine whether it matches known file type structures. For example, if the content appears to be an HTML file, the sniffer will render it as HTML.

If the server has declared a content type that does not match what the sniffer finds, or if the content is missing crucial metadata, the sniffer algorithm will override the server and allow the browser to render the content according to the results of its investigation.

As browsers have become stricter about content labeling requirements, this kind of sniffing has become less necessary. However, some browsers still use it.

What Is a MIME Type?

A MIME type is a string of code attached to a file that helps a browser determine what type of content a user uploads or downloads. For example, a MIME type for JavaScript text would be “Content-type: text/javascript.”

Each MIME type has a type and subtype, which are separated by a slash:

  • Type: The general category of the file data, such as text, video, or image.
  • Subtype: The specific kind of data in the category. For example, a text MIME type could be plain text (text/plain), HTML (text/HTML) or calendar (text/calendar).

Both the type and subtype must match the file content. However, if they differ, the browser will prioritize the MIME type over the file extension. For example, if a plain text file had a MIME type of “image/jpeg,” the browser would interpret it as a JPEG image.

How Does MIME Sniffing Create Vulnerabilities?

Although the method is useful for determining an asset's correct file format, MIME sniffing can open the door to serious security risks for website owners and visitors.

MIME sniffing algorithms are notoriously easy to trick due to the lack of standardization across browsers. When a sniffer scans a piece of unlabeled content to see if it matches known file types, it can interpret the content as a different MIME type than what the server response indicated.

This becomes a risk when users can upload data to servers. Malicious actors can exploit MIME sniffing to launch a cross-site scripting (XSS) attack, which injects malicious script into your website. When your end users access content on your website, the script will run and potentially harm the user.

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Because there is no established standard, every browser uses a different algorithm to understand uploaded files. Attackers base MIME sniffing attacks on a particular browser's sniffing algorithm, so the risk is only present for users of that browser.

Therefore, cybercriminals usually base their strategies on the most commonly used browsers, like Google Chrome. Modern browsers have issued protections against content sniffing in recent years, which have significantly reduced the frequency of sniffing attacks.

Additionally, while MIME sniffing of unlabeled content can be risky, it is less risky for browsers to use sniffing algorithms to verify that the label matches the content type — this kind of sniffing helps prevent MIME confusion, which can cause serious damage.

What Is a MIME Confusion Attack?

MIME confusion is a type of cyberattack that exploits the MIME sniffing algorithm to launch an XSS attack and inject malicious code into your website. While they used to be fairly common, browsers have strengthened their defenses against these attacks over the years.

Attackers can upload malicious code or other harmful content to your site by disguising their HTML files as other MIME types. For example, an attacker could hide malicious HTML code in an upload by labeling it as something benign, like an image file. The sniffer algorithm will interpret the content according to the label, so it will not enable the security protocols it usually would for uploaded HTML.

The end user's browser will then execute the malicious script, giving it access to sensitive data such as session tokens, cookies, usernames, and passwords. It may also rewrite your page's HTML content, which can have disastrous consequences for both your end users and your company's reputation.

Taking proactive steps to stop MIME sniffing attacks is critical for mitigating risks.

How to Prevent MIME Sniffing Attacks

There are several ways you can prevent MIME sniffing attacks.

  • Disable MIME sniffing: You can force browsers to stop sniffing content types with this X-Content-Type-Options (XCTO) HTTP header — “X-Content-Type-Options: nosniff.” Setting this flag on your server makes the server add it to every HTTP response it sends out. As a result, the browser must use the MIME type in the header rather than the one inferred by the sniffing algorithm.
  • Create a separate subdomain: Host all user-uploaded content on a separate subdomain to prevent it from coming into contact with content on your primary web domain. Because malicious actors can no longer inject script into your main system, you can limit the damage of an XSS attack.
  • Use a content security policy: You can mitigate XSS attacks on your website by deploying a content security policy (CSP) in conjunction with the XCTO header. A CSP is an HTTP response header that restricts the types of resources a browser can load, which enables server administrators to manage attack vectors like MIME sniffers.

If your IT team needs additional security resources or recommendations, it may be beneficial to seek outside help. Working with a trustworthy cybersecurity consulting firm can help you determine the most effective solution for your company's website.

Leverage Cybersecurity Services from Compass IT Compliance, LLC

When you take action to manage vulnerabilities in your system, you prevent attackers from exploiting them and causing serious damage. Working with a reliable cybersecurity consulting company like Compass IT Compliance can help you secure your web application environment and remain in full compliance with regulatory requirements.

We can run a vulnerability assessment on your organization's computers and networks to identify any existing large-scale vulnerabilities and policy weaknesses. Additionally, our web application scanning service can help you find and remediate potential vulnerabilities in your organization's applications. Upon completion, we will provide a detailed report of our findings as well as a remediation strategy to mitigate the risk of each vulnerability.

Contact Compass IT Today for Your Cybersecurity Needs

While attacks are rare, MIME sniffing can be a serious risk vector for companies of all sizes. Take steps to protect your web application environment with vulnerability assessment and management services from Compass IT Compliance.

For more information about our services, submit our contact us form. Our experts are happy to explain our offerings and answer any questions you may have!

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