HTML smuggling is an advanced method employed by cybercriminals to distribute malware by incorporating harmful JavaScript into seemingly innocuous HTML files. This technique leverages HTML5 and JavaScript functionalities, enabling attackers to generate payloads directly on the victim’s device upon opening the HTML file. Recent findings by Trustwave SpiderLabs reveal that hackers are increasingly utilizing HTML smuggling tactics to deploy sophisticated phishing pages.
Hackers Exploit HTML Smuggling Techniques for Malicious Delivery
Researchers have uncovered a sophisticated phishing campaign utilizing HTML smuggling. The attack commenced with an email impersonating American Express, containing a clickable link that functions as a redirector. This initial redirect leads to a secondary redirector, ultimately directing users to a Cloudflare R2 public bucket that hosts an HTML file.
The JavaScript employed HTML smuggling by encoding the phishing page as a lengthy Base64 string. When executed, the script utilizes the atob() function to decode the Base64 string back into standard HTML.
Subsequently, it creates a Blob object from the decoded HTML, using window.URL.createObjectURL() to generate a blob URL, which loads this content into the current browser window via window.location.href.
By delivering the malicious payload as seemingly innocuous HTML and JavaScript, attackers bypass certain security measures. This mechanism allows them to unveil the actual phishing page upon client-side execution.
The entire operation illustrates a multi-stage attack chain crafted to evade detection and deliver a convincing phishing experience to potential victims.
Blob URLs and URIs are temporary web addresses that reference binary data stored in blob objects, enabling threat actors to handle files and media within web browsers flexibly.
However, malicious actors exploit this technology through HTML smuggling to generate harmful files directly in the user’s browser instead of downloading them from a server. This method creates client-side files, helping them evade security measures monitoring incoming server-side content.
Furthermore, HTML smuggling allows for the covert distribution of harmful payloads disguised as benign data. Utilizing blob URLs to create and manage files locally enables attackers to conduct discreet operations that are difficult to detect and trace.
This technique is especially effective in the cloud era, as it bypasses email scanners, endpoint protection, and other security tools by concealing phishing content within seemingly harmless HTML files.
Typically, this process involves embedding obfuscated JavaScript code, which, when executed, utilizes blob URLs to generate and deploy a malicious payload, significantly complicating detection efforts.
