Threat Hunter’s Corner: Smuggling with HTML

Welcome 

Welcome back to the Threat Hunters Corner! I’m Josh Shepard, Principal Threat Hunter at Radical, and in this episode, we’ll be going over a fun Defense Evasion technique known as HTML smuggling.

RadBot Generated Text Summary 

What is HTML Smuggling? 

HTML smuggling is a technique that allows malicious payloads to be delivered to a victim's endpoint using HTML files. As the name suggests, this method involves "smuggling" malware through seemingly benign HTML content. When a victim opens an infected HTML file or visits a compromised website, the browser interprets the HTML or embedded JavaScript, ultimately constructing a malicious binary that is saved to the user's device, typically in the default download folder. 

How Does HTML Smuggling Work? 

Threat actors typically employ two primary methods to trick victims into executing infected HTML files: 

1. Drive-by Downloads: In this scenario, an attacker compromises a legitimate website or hosts their own website, which then serves malicious HTML or JavaScript to unsuspecting visitors. When users load the page, the malicious payload is downloaded without their knowledge.
   
   
2. Phishing Emails: Attackers may send spear-phishing or generic phishing emails containing links to compromised websites or attachments that are HTML files. Since the HTML file itself is not inherently malicious, it can bypass many spam filters and web security tools, making it a favored method for attackers. 

Detecting HTML Smuggling 

Detecting HTML smuggling can be challenging due to the nature of the technique. However, there are several indicators that security teams can monitor: 

1. Unusual HTML Downloads
   
   Monitoring for unusual downloads of HTML (.html or .htm) files is a critical first step. If a user downloads an HTML file from an email or an unexpected source, it warrants further investigation. While there are legitimate reasons for receiving HTML files, they are relatively uncommon in typical user behavior. 

2. Command Line Arguments
   
   When a user opens an HTML file, the default browser may be executed with command line arguments that include the file name. Monitoring for unusual command line activity involving HTML files can help identify potential exploitation attempts. For example, if a browser such as Chrome or Edge is invoked with a non-baselined HTML file as an argument, this could indicate suspicious behavior. 

3. Malicious File Downloads
   
   The ultimate goal of HTML smuggling is to download a malicious payload. Security teams should look for unusual file downloads that occur shortly after an HTML file interaction. Common file types associated with this technique include password-protected ZIP files or ISO files. These formats are often used to evade detection by security tools, as password protection can prevent scanning. 

4. Baseline Behavior Analysis
   
   Establishing a baseline of normal user behavior is essential for detecting anomalies. If a user typically does not download ISO files, any instance of such a download should be flagged for review. Security teams can employ automated tools to assist in identifying deviations from established norms. 

Conclusion 

HTML smuggling represents a sophisticated defense evasion tactic that leverages the inherent trust users place in HTML files. By understanding how this technique operates and implementing robust detection strategies, organizations can enhance their defenses against this evolving threat. 

As always, staying informed and vigilant is key to maintaining a strong security posture. If you have questions about hunting for HTML smuggling or other cybersecurity topics, feel free to reach out. Until next time, stay safe and happy hunting!