• HTML smuggling leverages the legitimate functionalities of HTML5 and JavaScript, which are widely supported by modern web browsers.
  • It circumvents security protocols such as sandboxing through an innovative approach to delivering malicious code.

According to Microsoft, there is an emerging pattern where malicious actors are utilizing HTML smuggling as a means to disseminate ransomware and other malicious code through email campaigns directed at financial services companies and a range of other organizations.

These attacks can bypass traditional cyber defenses as anti-malware software often fails to recognize the threat. Unlike typical malware, there’s no identifiable malicious executable file for detection; instead, the script assembles the payload locally on the host device behind the firewall technology.

Nation-backed hacking groups and other cyber criminals extensively use this technique to compromise governments, individuals, and organizations.

It is best to begin with a detailed definition of the concept to provide a comprehensive introduction to HTML smuggling.

What Is HTML Smuggling?

The term HTML smuggling’ is coined to describe the technique employed by attackers to discreetly embed an encoded malicious JavaScript BLOB within an HTML email attachment.

Attackers use this technique to conceal and deliver malicious code within crafted HTML documents or web pages. When a user opens it, the code is decoded and executed within the browser, enabling local payload assembly without transmitting a detectable malicious file over the network. It’s a tactic employed to bypass network security and is frequently used in targeted cyberattacks for system compromise and data theft.

It is, though not new, is gaining traction among cybercriminals. This highlights the fluid exchange of tactics, techniques, and procedures (TTPs) between various threat actors, reinforcing the commoditization of effective TTPs in the underground economy.

Before we dive into the specifics of HTML smuggling, let’s start by understanding the fundamentals of this technique and how it operates in the digital landscape.

How Does HTML Smuggling Work?

HTML smuggling harnesses the legitimate capabilities of HTML5 and JavaScript, both universally supported by contemporary web browsers. This technique constructs malicious files discreetly behind a firewall by utilizing the HTML5 “download” attribute within anchor tags and employing JavaScript BLOBs to assemble and deliver the payload to the targeted device.

Attackers hide malicious JavaScript within HTML files or webpages, which, when opened by a user via email attachments, links, or social engineering, triggers the execution of harmful actions. This JavaScript assembles and executes a malicious payload locally on the user’s device, potentially compromising it. It avoids transmitting obvious malicious files over the network by using legitimate HTML and JavaScript elements, making it challenging for standard network security measures to detect.

This method proves effective because the perimeter firewall perceives only the anticipated HTML and JavaScript traffic, with the option to obscure the JavaScript code to conceal the BLOB’s contents.

Understanding HTML Smuggling is crucial, but grasping how attackers utilize it is paramount. Let’s dive into their evasive maneuvers next.

How Attackers Employ HTML Smuggling Techniques to Bypass Traditional Security

In the business world, it’s important to understand why HTML smuggling techniques can effectively circumvent conventional security measures. Traditional security tools tend to focus primarily on detecting suspicious attachments and unusual network activity by relying on predefined signatures and behavioral patterns. Let’s delve into the topic of how HTML smuggling successfully evades sandboxing mechanisms:

  • It sidesteps security protocols like sandboxing through innovative delivery of malicious code. It embeds tiny fragments of malicious code within seemingly harmless JavaScript components, avoiding direct downloads that security measures might intercept.
  • The small, undecodable BLOBs escape sandbox analysis, showing no harmful behavior individually and avoiding sandbox alerts. However, without user involvement, they can autonomously reconstruct into a harmful executable at the local browser level.
  • Attackers employ obfuscation methods like Base64 encoding, making it challenging for security tools to spot the hidden malicious code. HTML smuggling attacks further bypass web proxy defenses by encoding as binary data within JavaScript, which is decoded into a file object when the user’s browser opens it.

These techniques exploit web browsers’ complexities and employ obfuscation, fragmentation, and dynamic loading to disguise and deliver malicious code, ultimately evading traditional security defenses designed to detect and block threats at the network or perimeter level.

As we unravel the mechanics of HTML smuggling, we must explore real-world examples to see how threat actors exploit this technique in various scenarios.

A Look Into HTML Smuggling Examples

HTML smuggling has become a favored tool of cyber threat actors, enabling them to deliver malware while evading standard security measures discreetly. This discussion explores real-world examples of such threats, highlighting the need for ongoing vigilance in the ever-changing cybersecurity landscape.

  • IcedID

The infamous IcedID malware, also called Bokbot, has been observed exhibiting a remarkably similar delivery approach to Qakbot.

It is a malicious banking Trojan designed to illicitly procure sensitive financial data, encompassing login credentials and personal information. This insidious tool is frequently harnessed by cybercriminals in the execution of phishing attacks and online fraud, and it is notorious for its adeptness in evading detection, establishing it as a major cybersecurity menace.

Since 2017, IcedID has diversified its delivery methods, primarily relying on email as the initial point of access. Initially designed as a banking trojan for financial institutions, it has evolved to deliver various malware payloads, including ransomware. Additionally, it serves as an entry point for other threat actors aiming to establish a presence on a target system.

  • Qakbot

Qakbot HTML smuggling, a well-known malware often distributed via spam, has been adopted as a delivery method since June 2022. This shift occurred in response to Microsoft tightening restrictions on using macros from the Internet.

When examining the HTML source code, we observe that the functions and methods responsible for constructing the payload are concealed within arrays. This strategy aims to hide any potentially suspicious commands and avoid detection by email gateway filters. This approach leverages JavaScript functions in a way that can be exploited.

  • Cobalt Strike

An HTML attachment was discovered in a recent spam email that deployed Cobalt Strike. The HTML bait closely resembles the tactics used in Qakbot and IcedID campaigns.

Cobalt Strike, a commercial adversary simulation software initially marketed to red teams, has unfortunately been misappropriated and widely employed by various threat actors. These actors range from ransomware operators to advanced espionage-focused Advanced Persistent Threats (APTs). While many network defenders have encountered Cobalt Strike payloads in intrusion attempts, comprehending the full scope of this framework’s components and features can be challenging, especially for those who haven’t used it as operators.

These examples clarify how HTML smuggling techniques can be leveraged for various malicious purposes, from data theft to malware delivery. To protect systems and data, it’s vital to understand and apply effective mitigation strategies against these emerging threats.

Safeguarding Against HTML Smuggling

HTML smuggling represents a sophisticated tactic cybercriminals use to infiltrate networks and compromise web applications. To fortify your organization against this evolving challenge, contemplate enacting the subsequent risk management measures:

  • CDR, or Content Disarm and Reconstruction, removes active content from files, leaving only safe, static elements. It is a strong defense against HTML smuggling for web traffic but can’t handle encrypted content.
  • Proper browser security settings, like disabling risky features and keeping up with updates, are crucial defenses against it.
  • HTML smuggling attacks often work due to user unawareness. Regular training to recognize and avoid attacks can significantly reduce risks. An informed user is the best defense against cyber threats.
  • The most effective prevention is using advanced threat protection solutions. These can detect and alert HTML smuggling techniques and block or quarantine suspicious content, keeping the client’s system safe.


Delivering malicious code through HTML represents a significant challenge in cybersecurity. This clandestine technique allows malicious actors to covertly send malware and compromise systems, often evading traditional security defenses.

As illustrated by real-world examples, organizations should prioritize effective HTML smuggling detection and subsequently implement proactive strategies, including regular user training and the reinforcement of robust browser security settings.

Staying informed and vigilant in the face of evolving cyber threats is paramount, as this smuggling continues to pose a formidable risk to the digital landscape.

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