HTML attachments are more of a security risk than ever - here's what you need to know

A growing number of emails are arriving loaded with malicious or harmful HTML attachments, new research has warned.

A report from Barracuda found almost half (46%) of HTML attachment in emails it scanned was found to be malicious. Barracuda says the Hypertext Markup Language (HTML) is growing increasingly popular in phishing, credential theft, and other forms of cyberattacks. 

“If a 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,” Barracuda CTO, Fleming Shi, said in a blog post.

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“However, in some 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.”

The CTO also said that the HTML threats are being distributed via countless individual attacks, rather than a handful of mass events. 

“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,” Shi said. “However, on March 23, almost nine in ten (85%) of the total 475,938 malicious HTML artifacts were unique – which means that almost every single attack was different.”

The figures are pointing to HTML attachments remaining one of the most common ways to deliver malware through email, the blog concludes, saying that it’s pivotal for businesses to have the right security solutions set up. “This means having effective, AI-powered email protection in place that can evaluate the content and context of an email beyond scanning links and attachments,” it was said.

Multi-factor authentication, zero-trust access controls, as well as automation in response and attack remediation, is also essential to any organization’s cybersecurity tech stack, right next to employee training, Shi concluded.

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