FileFix Attack Exploits Windows Browser Loophole to Evade Mark-of-the-Web Security

A newly identified attack vector, dubbed the “FileFix Attack,” has surfaced, exploiting a subtle yet critical loophole in how modern browsers like Google Chrome and Microsoft Edge handle saved web content.

This technique cunningly sidesteps the Windows Mark-of-the-Web (MOTW) security feature, which typically flags downloaded files as potentially unsafe and prompts users with warnings before execution.

A Clever Bypass of Windows Security Mechanisms

By leveraging specific MIME types and user behavior, attackers can deliver malicious content that executes without triggering these essential safeguards.

According to the Report, the core of the FileFix Attack lies in its manipulation of how browsers save HTML content.

When a user saves a webpage using Ctrl+S or the “Save as” option in formats like “Webpage, Single File” (.mhtml) or “Webpage, Complete” (.html), and the content is served with a MIME type of text/html or application/xhtml+xml, the resulting file is not tagged with MOTW.

This contrasts with other MIME types like image/png or image/svg+xml, where MOTW is applied.

Social Engineering Meets Technical Exploitation

Attackers exploit this by crafting HTML files often disguised as benign content such as backup codes and embedding malicious scripts within.

When saved and renamed with a .hta (HTML Application) extension, these files can execute scripts without security prompts, thanks to the .hta format’s ability to process HTML and scripts natively.

The attack’s sophistication is amplified through social engineering. A typical scenario involves a phishing page styled to mimic a legitimate service, prompting users to save “backup codes” using Ctrl+S.

The page might include instructions to name the file with a .hta extension, such as MfaBackupCodes2025.hta.

Upon saving and executing, the embedded JScript (e.g., spawning a command shell to ping a domain) runs unchecked.

Furthermore, attackers manipulate the