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Why are HTML Files Dangerous?
8/23/2023 - Brian O'Neill

Since the early days of the internet, Hyper-Text Markup Language (HTML) has made it easy for folks with a wide range of technical abilities to organize and present web content for client-side viewers. This simple and readable text-based language has grown considerably over the decades, gradually evolving as a form of digital scaffolding for dynamic, interactive programming languages like JavaScript to plug into.

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While most HTML files we encounter are legitimate – containing anything from web design iterations to browser-accessible versions of Office file formats – the dissemination of HTML files for the purpose of spreading malware and/or performing other common malicious web activities has grown considerably in recent years. HTML files can house a surprisingly wide variety of threats in their various elements, and as a result, it’s critical to exercise extreme caution before opening and viewing the contents of these files.

What Makes HTML Files Dangerous?

While HTML files might appear to contain inert, locally available text information that merely executes through an internet browser, the reality is that HTML files can reference externally hosted JavaScript libraries once they're launched, and they can subsequently redirect users to malicious locations. These locations can include phishing websites designed to trick users into providing sensitive information (much like a Phishing URL), and they can also trick unsuspecting users into downloading malware-infected files onto their devices.

Further, modern versions of HTML make it possible for cyber criminals to embed malware within the HTML file itself. This means that opening a malicious HTML file might directly inject the user’s device with malicious scripts and/or executable content. Malware including ransomware, spyware, viruses, trojans & more can infect a device in this way, compromising sensitive internal systems and placing the attacker in control of valuable content.

These are just a few of the more notable examples of HTML file threats. Depending on the vulnerabilities present in the victim’s environment, HTML can be used to launch Cross-Site Scripting attacks within a web application, initiate automatic file downloads, and much more.

Where can Malicious HTML Files be Shared?

In general, email is the most common method attackers use to share malicious files and URLs at scale, and attackers typically rely on social engineering techniques to encourage unsuspecting victims to access malicious content in their inbox. While many such emails are detected by anti-spam policies and siloed away from primary inboxes, emails which are more targeted (particularly those originating from compromised devices which were once trusted/corresponded with) are often capable of breaching these policies and hiding in plain sight.

It’s also possible for attackers to share HTML files through client-side file upload portals. The growth of affordable cloud-storage locations has made file upload portals drastically more popular in recent years, making it possible for a wider variety of companies to accept User-Generated Content (UGC) directly from client-side users. Latent attacks on cloud storage instances have increased as a result, often beginning with an attacker sharing a malicious file through a file upload portal and culminating in an unsuspecting internal or external user accessing that content later. It’s possible for attackers to hide malicious HTML content under the guise of different file extensions and fool weakly designed validation policies.

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Mitigating HTML Threats with Cloudmersive

The Advanced Scan iteration of the Cloudmersive Virus Scan API offers 360-degree protection against more than 17 million virus and malware signatures & a variety of hidden content threats. After deploying this API in a network proxy or adjacent to an Azure Blob, AWS S3, SharePoint Online Site Drive or Google Cloud storage instance (or in a variety of other locations via custom code deployment), you can set custom threat rules against a variety of threatening file types, including HTML files, scripts, executables, invalid files, password-protected files, macros, unsafe archives, and more. This API will perform in-depth content verification on each file it scans, ensuring the contents match the extension while allowing you to block unwanted file types entirely with comma-separated file extension whitelisting.

For more information on the Cloudmerisve Advanced Virus Scan API, please do not hesitate to reach out to a member of our sales team.

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