HTML smuggling, a highly evasive malware delivery technique that leverages legitimate HTML5 and JavaScript features, is increasingly used in email campaigns that deploy banking malware, remote access Trojans (RATs), and other payloads related to targeted attacks. Notably, this technique was observed in a spear-phishing campaign from the threat actor NOBELIUM in May. More recently, we have also seen this technique deliver the banking Trojan Mekotio, as well as AsyncRAT/NJRAT and Trickbot, malware that attackers utilize to gain control of affected devices and deliver ransomware payloads and other threats.
As the name suggests, HTML smuggling lets an attacker “smuggle” an encoded malicious script within a specially crafted HTML attachment or web page. When a target user opens the HTML in their web browser, the browser decodes the malicious script, which, in turn, assembles the payload on the host device. Thus, instead of having a malicious executable pass directly through a network, the attacker builds the malware locally behind a firewall.