Phishing attackers recently utilized an HTML smuggling technique to disseminate a malicious payload, marking the beginning of an attack chain triggered by a phishing email that impersonated an American Express notification. The deceptive email led recipients through a series of redirects until reaching a Cloudflare R2 public bucket that hosted an HTML file. This file launched an external JavaScript code containing a Base64-encoded string that, upon decoding, unveiled the actual phishing page, showcasing how HTML smuggling can effectively hide malicious content.

This JavaScript code operated by waiting for the page to fully load before unleashing its functionality. It then decoded a Base64-encoded HTML string into plain text, likely revealing a fraudulent phishing page designed to dupe users into divulging confidential information. The code’s primary function involved creating a hidden iframe within the webpage and loading the decoded phishing content into it, effectively concealing the malicious activity from the user.

An essential component of this code was the openFileURL function, which crafted a downloadable or viewable file from the decoded HTML content. This function created a blob object using the decoded data and a specified content type, generating a URL link to access this blob. Subsequently, the function directed the browser to this URL, loading and displaying the content. To prevent memory leaks, the function revoked the blob URL after a brief delay, ensuring a smoother operation.

It’s crucial to note that blob URLs are transient web addresses directing to binary data stored within the browser. Malicious actors exploit this feature to create harmful files locally, sidestepping conventional security protocols. These files serve as vehicles for delivering detrimental payloads directly to users, making it challenging to detect and trace these attacks. By crafting files on the client side, cybercriminals can embed them in seemingly innocuous web pages or exploit browser vulnerabilities, significantly heightening security risks.

The sophisticated HTML smuggling technique displayed in these phishing pages involves concealing malicious code within seemingly legitimate HTML elements. By mimicking reputable services like DocuSign and Microsoft, cybercriminals aim to deceive users into disclosing sensitive information, highlighting the importance of vigilance and the necessity for advanced threat detection measures to combat evolving phishing attacks.

The rise of HTML smuggling in phishing attacks poses a substantial threat due to its ability to evade traditional security measures. This tactic involves camouflaging malicious content within seemingly harmless HTML files, often utilizing obfuscation methods like blob URLs to reference obscured data. As phishing attacks grow more sophisticated, the prevalence of HTML smuggling is expected to increase, underscoring the urgency for organizations to implement advanced security solutions capable of identifying and neutralizing such threats, as highlighted by Trustwave in their analysis of this evolving cyber threat landscape.

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