Threat actors are increasingly banking on the technique of HTML smuggling in phishing campaigns as a means to gain initial access and deploy an array of threats, including banking malware, remote administration trojans (RATs), and ransomware payloads.
Microsoft 365 Defender Threat Intelligence Team, in a new report published Thursday, disclosed that it identified infiltrations distributing the Mekotio banking Trojan, backdoors such as AsyncRAT and NjRAT, and the infamous TrickBot malware. The multi-staged attacks — dubbed ISOMorph — were also publicly documented by Menlo Security in July 2021.
|Threat behavior observed in the Mekotio campaign|
"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," the researchers said. "Thus, instead of having a malicious executable pass directly through a network, the attacker builds the malware locally behind a firewall."
HTTP Smuggling's ability to bypass web proxies and email gateways have made it a lucrative method among state-sponsored actors and cybercriminal groups to deliver malware in real-world attacks, Microsoft noted.
Nobelium, the threat group behind the SolarWinds supply chain hack, was found leveraging this very tactic to deliver a Cobalt Strike Beacon as part of a sophisticated email-based attack aimed at government agencies, think tanks, consultants, and non-governmental organizations located across 24 countries, including the U.S., earlier this May.
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|HTML smuggling attack chain in the Trickbot spear-phishing campaign|
"The surge in the use of HTML smuggling in email campaigns is another example of how attackers keep refining specific components of their attacks by integrating highly evasive techniques," Microsoft noted. "Such adoption shows how tactics, techniques, and procedures (TTPs) trickle down from cybercrime gangs to malicious threat actors and vice versa. It also reinforces the current state of the underground economy, where such TTPs get commoditized when deemed effective."