HTML smuggling has been around for years, and it continues to gain traction, with threat actors still deploying malware and Trojans using the method. This post will tackle this highly evasive payload delivery method, giving insights into its impact, detection, and prevention.
What Is an HTML Smuggling Attack ?
An HTML smuggling attack is a malicious method attackers use to secretly deliver malicious files to a victim’s device without being detected by network firewalls, antivirus software, and other security solutions.
Imagine a magician pulling a rabbit out of a previously empty, normal-looking hat. In reality, the hat has a secret compartment where the magician can hide the rabbit. In the same way, HTML smuggling involves threat actors hiding malicious code within a seemingly innocent HTML file or webpage.
How Does an HTML Smuggling Attack Work?
Here is a simplified overview of how HTML smuggling works.
1. Malicious Website Creation
The attacker begins by crafting a malicious HTML webpage that contains JavaScript code designed to appear legitimate. In the past, for example, threat actors mimicked trusted websites and services, such as Adobe Acrobat, Google Drive, and the U.S. Postal Service to smuggle malicious code into user devices. These websites were also hosted on web servers the attackers operated.
2. Blob Creation
The attacker then encodes the malicious payload and creates a Blob object using JavaScript in the user’s browser. This Blob stores binary data, such as images, audio files, videos, documents, and executable files. In HTML smuggling’s case, attackers store malicious payloads in the Blob.
3. User Interaction
Like most attack methods, threat actors need a victim to act. They often lure victims to malicious websites through phishing or social engineering. When unsuspecting users visit the page, they would see familiar and legitimate-looking content, leading them to trust and interact with it.
User interaction can occur when the victim clicks a button or an image on the malicious website.
4. Payload Download
The user’s browser would then trigger the JavaScript code to decode the Blob and execute the embedded script. This action downloads the malicious file directly to the victim’s device.
Since this seems like a legitimate process, the user’s security solutions would not flag the download and the malicious content with the payload gets delivered successfully.
5. Malicious Code Execution
Attackers may set the payloads to execute automatically. Others may require further user action, such as opening a file. Either way, the malicious code can steal sensitive data, encrypt files for ransom, or create a backdoor for a more nefarious attack.
The Dangers of HTML Smuggling
HTML smuggling is highly evasive because it is executed on the client side, which means it relies on vulnerabilities in a user’s browser and system. Since most regular Internet users only employ basic security, attackers can exploit these vulnerabilities more easily.
Aside from being evasive, this attack can pave the way for more damaging attacks. Attackers can use it as an initial attack that can be used to install more persistent malware, such as ransomware, Trojans, and spyware. These can cause significant damage to the victims’ systems.
HTML smuggling can also be used to escalate privileges and gain deeper access to a victim’s system. A successful attack on an employee, for instance, can ultimately give attackers access to an organization’s critical systems.
How to Detect HTML Smuggling Attacks
HTML smuggling hides malicious code behind seemingly ordinary web content, making it difficult to detect using traditional tools. That said, detection is still possible using a combination of the following:
- Static analysis: This technique involves both manual and automated inspection of HTML code to detect suspicious patterns, obfuscation, vulnerabilities, and other potentially malicious code constructs.
- Browser sandboxing: This method creates a controlled environment for web browsers to run so HTML files are contained for analysis, preventing malicious code from accessing a user’s operating system (OS).
- Behavior-based analysis: Monitor files downloaded from the Internet and identify behavior patterns instead of analyzing individual files or events. For example, observe if a downloaded file attempts to contact external servers, executes unusual commands, or interacts with other system components.
Preventing HTML Smuggling
As discussed, HTML smuggling is difficult to detect because of its evasive nature. Therefore, nipping it in the bud could be a better resort. This means employing proactive measures to prevent this type of attack from happening in the first place.
Maintaining strong security hygiene is one such measure. Examples include using strong passwords, implementing multifactor authentication (MFA), and practicing safe browsing habits.
Employee training is also crucial, enabling employees to learn about the risks of HTML smuggling and other attacks and how to spot suspicious emails and websites.
