Threat Spotlight: Proportion of Malicious HTML Attachments Doubles Within a Year

PRESS RELEASE

The security industry has been highlighting the cybercriminal misuse of HTML for years — and evidence suggests it remains a successful and popular attack tool. Last year we reported that around one-in-five (21%) of all HTML attachments scanned by Barracuda in May 2022 were malicious. Ten months on, that figure has more than doubled — 45.7% of scanned HTML files were found to be malicious in March 2023.

The Legitimate Use of HTML

HTML stands for Hypertext Markup Language, and it is used to create and structure content that is displayed online. HTML is also commonly used in email communication — for example in automated reports that users might be receiving on regular basis, such as newsletters, marketing materials, and more. In many cases, reports are attached to an email in HTML format (with the file extension .html, .htm or .xhtml, for example).

If the communication appears to come from a known or trusted brand, the recipient is unlikely to be suspicious. 

The Malicious use of HTML

However, attackers can successfully leverage HTML as an attack technique by using well-crafted messages and/or compromised websites and malicious HTML file attachments to trick users.

This approach is used by attackers to conceal malicious intentions such as phishing and credential theft, and more.

If recipient opens the HTML file, multiple redirects via JavaScript libraries hosted elsewhere will take them to a phishing site or other malicious content controlled by the attackers. Users are then asked to enter their credentials to access information or download a file that may contain malware.

However, in some of cases seen by Barracuda researchers, the HTML file itself includes sophisticated malware which has the complete malicious payload embedded within it, including potent scripts and executables. This attack technique is becoming more widely used than those involving externally hosted JavaScript files.

Protection against malicious HTML- based attacks should take into account the entire email carrying HTML attachments, looking at all redirects and analyzing the content of the email for malicious intent. More on that below.

Recent Examples of Malicious HTML Attachments Are Often Similar to Those Seen in the Past.

For example, the following phishing attachment that looks like a Microsoft login has been popular for some years, but their continued and widespread use in attacks suggests attackers remain successful in trapping victims.

Proportion of Unique Attacks

If you compare the total number of malicious HTML detections to how many different (unique) files were detected, it becomes clear that the growing volume of malicious files detected is not simply the result of a limited number of mass attacks, but the result of many different attacks each using specially crafted files.

For example, daily detection data for the three months from January to March 2023 reveals two significant attack peaks, on March 7 and March 23. 

On March 7, there were 672,145 malicious HTML artifacts detected in total, comprising 181,176 different items. This means that around a quarter (27%) of the detected files were unique and the rest were repeat or mass deployments of those files.

However, on March 23, almost nine in ten (405,438 — 85%) of the total 475,938 malicious HTML artefacts were unique ― which means that almost every single attack was different. 

HTML Attachments Continue to Dominate the List of File Types Used for Malicious Purposes

Barracuda analysis further shows that not only is the overall volume of malicious HTML attachments increasing, nearly a year on from our last report, HTML attachments remain the file type most likely to be used for malicious purposes.

When it comes to attack tactics and tools, the fact that something has been around for a while doesn’t appear to make it any less potent. Malicious HTML is still being used by attackers because it works. Getting the right security in place is as important now as it has ever been, if not more so.

How to Protect Against Malicious HTML Attachments

Barracuda has identified 13 email threat types, and published a guide explaining how they target and compromise victims, and how to defend against them.