Chapter 10: What is HTML smuggling and how adversaries use this technique

Evasion at its core: Understanding the malicious power of HTML smuggling

Adversaries are constantly seeking new ways to bypass security controls and deliver their malicious payloads undetected. One such technique that has gained prominence is HTML smuggling. This highly evasive malware delivery method leverages legitimate HTML and JavaScript features to successfully distribute banking malware, remote access trojans, and various other payloads, especially targeting banks and financial institutions.

HTML smuggling allows attackers to encode a malicious script within a specially crafted HTML attachment or web page. When the target user opens the HTML in their web browser, the browser decodes the malicious script, assembling the payload on the host device. Unlike traditional malware delivery methods, HTML smuggling ensures that the malicious executable is built locally behind a firewall, evading network-based security measures.

What makes HTML smuggling particularly challenging to detect is its ability to bypass standard security controls, including web proxies and email gateways. These security solutions often focus on checking for suspicious attachments or traffic based on known signatures and patterns. Since the malicious files are created only after the HTML file is loaded on the endpoint through the browser, protection solutions may initially see benign HTML and JavaScript traffic, which can also be obfuscated to further conceal their true purpose.

Threat actors employing HTML smuggling exploit the legitimate uses of HTML and JavaScript in daily business operations, making it difficult for organizations to distinguish between malicious and benign activities. Disabling JavaScript, for example, could mitigate HTML smuggling that relies on JavaScript blocks, but this would also disrupt the rendering of legitimate web pages essential for business operations.

Furthermore, there are multiple ways to implement HTML smuggling, including using alpha scan and various coding techniques for JavaScript, making the technique highly evasive against content inspection. To effectively defend against HTML smuggling and similar threats, organizations require a true defense-in-depth strategy and a multilayered security solution. This approach involves inspecting email delivery, monitoring network activity, analyzing endpoint behavior, and investigating follow-on attacker activities.

Without comprehensive security solutions in place, organizations may find themselves manually searching for indicators of HTML smuggling and other sophisticated threats. Security Operation Center (SOC) analysts equipped with sufficient information about specific threats can conduct manual investigations, but relying solely on manual efforts can be time-consuming and resource-intensive.

To effectively combat the risks associated with HTML smuggling and other advanced attack techniques, organizations should prioritize the implementation of advanced security solutions that provide holistic protection across multiple layers of the IT infrastructure. By adopting a proactive approach and investing in robust security measures, organizations can safeguard their systems, data, and operations against the evolving threat landscape.