CVE-2025-6435: Firefox Vulnerability Leading to Potential System Compromise and Data Leakage

Overview

CVE-2025-6435 is a serious security vulnerability affecting versions of Firefox prior to 140. It stems from an issue in the browser’s Devtools, specifically within the Network tab’s ‘Save As’ context menu option. This flaw could lead to a user inadvertently running a potentially malicious executable, thereby compromising their system or leading to data leakage. Given the popularity of Firefox and the potential widespread impact of this vulnerability, it is crucial for users and administrators to understand the nature of this flaw and take necessary steps to mitigate it.

Vulnerability Summary

CVE ID: CVE-2025-6435
Severity: High (CVSS score 8.1)
Attack Vector: Network
Privileges Required: None
User Interaction: Required
Impact: Potential system compromise or data leakage

Affected Products

Product | Affected Versions

Firefox | < 140 How the Exploit Works

The exploit leverages the ‘Save As’ function in the Network tab of Firefox’s Devtools. When a user saves a response using this function, the resulting file may not have the ‘.download‘ extension. This could lead the user to mistake the file’s nature, potentially opening an executable file believing it to be an innocuous download. If this file contains malicious code, it could then be executed on the user’s system, leading to compromise or data leakage.

Conceptual Example Code

A conceptual example of how this might be exploited could involve a malicious actor crafting a payload to be delivered through the browser’s network responses. The user, believing they’re saving a harmless file, could inadvertently save and execute this malicious payload. For illustrative purposes, a sample HTTP response might look like this:

HTTP/1.1 200 OK
Date: Mon, 23 May 2025 22:38:34 GMT
Content-Type: application/octet-stream
Content-Disposition: attachment; filename="example"
{ "malicious_payload": "..." }

In this example, the malicious payload is embedded within the HTTP response. If saved and executed by the user, it could lead to the exploitation of CVE-2025-6435.

Mitigation

To mitigate this vulnerability, users should apply the vendor-supplied patch for Firefox. If for some reason this is not possible, using a Web Application Firewall (WAF) or Intrusion Detection System (IDS) could serve as a temporary mitigation measure. These systems could potentially identify and block malicious network responses based on known patterns of malicious activity. However, these should only be seen as temporary measures, and applying the official patch should be a top priority.