FileFix Exploit Bypasses Mark-of-the-Web Protections via Windows Browser Flaw

A recent analysis has uncovered a significant security flaw affecting the way Google Chrome and Microsoft Edge handle the “Save As” functionality for HTML content on Windows.

When users save web pages as “Webpage, Single File” (.mhtml) or “Webpage, Complete” (.html) files, the downloaded file does not receive a Mark-of-the-Web (MOTW) attribute provided the MIME type of the resource is set to text/html or application/xhtml+xml.

Crucially, other content types, such as images or SVG files, still receive the MOTW tag. MOTW is a security mechanism in Windows that designates files originating from the internet, prompting further scrutiny or security prompts before execution.

Browsers Ignore MOTW for Saved HTML

This discrepancy opens the door for attackers to sidestep MOTW-based protections. When HTML pages are saved in the “Webpage, Complete” format, the page structure is preserved with only a minimal addition a “saved from url” HTML comment.

This means attackers can craft HTML content under their control, prompting the user to save it with a .hta extension.

Since .hta (HTML Application) files are processed by Microsoft’s mshta.exe, they can execute embedded scripts with far fewer restrictions than regular HTML files opened in the browser.

The exploit relies primarily on social engineering: an attacker lures users into saving a web page as a backup, guide, or some other innocent-looking file using the browser’s “Save As” option.

By instructing the user to modify the file extension from .html to .hta before saving, the file escapes MOTW tagging and, when executed, runs with full permissions.

For example, a malicious HTML page could display legitimate-looking instructions and back-up codes, encouraging the target to save the page as “MyBackup.hta”.

Attackers Can Weaponize Social Engineering

After saving, double-clicking the file launches it via mshta.exe, immediately executing attacker-supplied scripts such as commands to open a reverse shell or contact a remote server without any security warnings, since the MOTW is absent.

To further enhance this trick, the attacker can manipulate the page’s <title> element to suggest a name ending in .hta.

If the user’s file explorer hides known file extensions, this increases the chance the file will be saved as an executable HTA without the victim’s awareness.

Additionally, Data URIs with a MIME type of text/html are also saved without the MOTW attribute, providing another technical vector for the exploit.

The risk is amplified by the default configuration of browsers and Windows, requiring neither special exploits nor privilege escalation on the attacker’s part.

The most direct countermeasure is to block or remove mshta.exe’s ability to run HTA files, or restrict user permissions to prevent execution of downloaded HTAs steps which, while straightforward, may impact certain legacy applications.

According to the Report, Security researchers have flagged the issue as a prime example of how browser-user interaction and OS-level file handling can be exploited in real-world, low-complexity attacks.

Until Chrome, Edge, or Windows itself enforces stricter tagging of saved files originating from the internet, especially for potentially executable content like HTA, this vulnerability will remain a tempting target for threat actors leveraging social engineering to bypass MOTW-based protections.

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