CVE-2025-6427 in Firefox
Summary
by MITRE • 06/24/2025
An attacker was able to bypass the `connect-src` directive of a Content Security Policy by manipulating subdocuments. This would have also hidden the connections from the Network tab in Devtools. This vulnerability affects Firefox < 140.
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Analysis
by VulDB Data Team • 07/30/2025
This vulnerability represents a critical bypass of Firefox's Content Security Policy implementation, specifically targeting the connect-src directive which is designed to restrict from where scripts can establish connections. The flaw allows attackers to circumvent security controls by manipulating subdocuments, effectively undermining the browser's security model. The vulnerability affects Firefox versions prior to 140, indicating a significant window of exposure where users were potentially vulnerable to sophisticated attacks that could bypass network restriction policies.
The technical nature of this flaw involves the manipulation of subdocuments within the browser's rendering engine, where the CSP enforcement mechanism fails to properly validate connection attempts originating from nested document contexts. This bypass occurs at the policy enforcement layer, where the browser's interpretation of the connect-src directive does not adequately account for subdocument connection attempts. The vulnerability specifically impacts the Network tab in browser developer tools, meaning that security monitoring and debugging activities would be rendered ineffective as connections would be hidden from normal inspection mechanisms.
From an operational perspective, this vulnerability creates a significant risk for organizations relying on CSP as a defense-in-depth mechanism to prevent unauthorized network communications. Attackers could establish connections to malicious domains without detection, potentially exfiltrating data or establishing command and control channels while evading traditional network monitoring tools. The hidden nature of these connections from Devtools makes incident response and forensic analysis particularly challenging, as security professionals would not be able to identify the malicious activity through standard browser inspection techniques.
The vulnerability aligns with CWE-16 Software Fault, specifically related to improper enforcement of security policies, and can be mapped to ATT&CK technique T1071.004 Application Layer Protocol: DNS, as it enables covert communication channels that bypass normal network monitoring. Organizations should immediately update to Firefox version 140 or later to remediate this vulnerability. Additional mitigations include implementing additional network monitoring solutions beyond browser-based CSP, using web application firewalls, and conducting regular security assessments to detect potential bypasses of security controls. Security teams should also consider implementing network traffic analysis tools that can detect anomalous connection patterns independent of browser-based CSP enforcement mechanisms.