CVE-2025-6427: Mozilla Firefox XSS Vulnerability

CVE-2025-6427 Overview

CVE-2025-6427 is a security policy bypass vulnerability in Mozilla Firefox and Thunderbird. Attackers can bypass the connect-src directive of a Content Security Policy (CSP) by manipulating subdocuments. The bypass also hides the unauthorized connections from the Network tab in Developer Tools, reducing the chance of detection during debugging or incident response. Mozilla fixed the issue in Firefox 140 and Thunderbird 140. The vulnerability is tracked under CWE-693 (Protection Mechanism Failure) and is documented in Mozilla Security Advisory MFSA-2025-51.

Critical Impact

Attackers can exfiltrate data or contact unauthorized endpoints from a victim's browser while bypassing CSP enforcement and evading detection in DevTools.

Affected Products

• Mozilla Firefox versions prior to 140
• Mozilla Thunderbird versions prior to 140
• Web applications relying on connect-src CSP directives for outbound request control

Discovery Timeline

• 2025-06-24 - CVE-2025-6427 published to NVD
• 2026-04-13 - Last updated in NVD database

Technical Details for CVE-2025-6427

Vulnerability Analysis

The vulnerability resides in Firefox's enforcement of the connect-src Content Security Policy directive. The connect-src directive restricts the origins that scripts can contact via interfaces such as fetch(), XMLHttpRequest, WebSocket, EventSource, and navigator.sendBeacon(). By manipulating subdocuments such as nested iframes, an attacker can issue network connections that should be blocked by the parent document's CSP. The browser fails to consistently propagate or evaluate the policy boundary for these subdocument-initiated requests. Network requests issued through this bypass also do not appear in the Network tab of Developer Tools, complicating forensic review and runtime monitoring.

Root Cause

The root cause is a protection mechanism failure [CWE-693] in how Firefox's CSP engine handles subdocument contexts. The browser does not enforce the parent frame's connect-src policy on requests routed through manipulated child documents. The associated logging path to DevTools is also skipped, indicating that the affected code path circumvents both enforcement and instrumentation hooks.

Attack Vector

Exploitation requires a victim to load attacker-influenced content in a page that relies on CSP for protection. This typically involves a stored or reflected cross-site scripting (XSS) primitive, a compromised third-party script, or a malicious advertisement embedded as a subdocument. The attacker constructs a subdocument that triggers outbound connections to attacker-controlled endpoints. Because the connections bypass connect-src and remain invisible to DevTools, the attacker can exfiltrate session data, tokens, or form contents without triggering CSP violation reports.

No public proof-of-concept or exploit code is currently available. For technical context, refer to Mozilla Bug Report #1966927.

Detection Methods for CVE-2025-6427

Indicators of Compromise

• Outbound browser connections to unexpected domains that are not declared in the deployed connect-src policy
• Absence of expected CSP violation reports on endpoints protected by report-uri or report-to directives
• Subdocuments (iframes) loaded from third-party origins that initiate cross-origin requests
• Firefox or Thunderbird clients reporting versions below 140 in user-agent telemetry

Detection Strategies

• Inspect network egress at the proxy or DNS layer for connections to domains absent from approved CSP allowlists
• Correlate browser version inventory data with the Firefox 140 and Thunderbird 140 fix baseline to surface vulnerable endpoints
• Review CSP report-only telemetry for gaps where in-page requests are observed by server logs but not reported as violations

Monitoring Recommendations

• Forward CSP violation reports to a centralized SIEM or data lake for longitudinal analysis
• Monitor browser process telemetry for outbound connections initiated by firefox.exe or thunderbird.exe to uncategorized domains
• Track patch deployment status across managed endpoints to confirm coverage of Firefox 140 and Thunderbird 140 or later

How to Mitigate CVE-2025-6427

Immediate Actions Required

• Upgrade Firefox to version 140 or later on all managed endpoints
• Upgrade Thunderbird to version 140 or later for users running the mail client
• Audit web applications that depend on connect-src for data exfiltration protection and add defense-in-depth controls
• Validate browser version compliance through endpoint management tooling

Patch Information

Mozilla released fixes in Firefox 140 and Thunderbird 140. Refer to Mozilla Security Advisory MFSA-2025-51 and Mozilla Security Advisory MFSA-2025-54 for full advisory text and update instructions. Enterprise deployments should distribute the update through standard software management workflows and verify installation on every host.

Workarounds

• Restrict loading of untrusted subdocuments by tightening frame-src and child-src CSP directives
• Apply the sandbox attribute to iframes containing third-party content to limit network and script capabilities
• Enforce egress filtering at the network perimeter to restrict browser-originated traffic to approved domains
• Disable execution of untrusted JavaScript and third-party advertising frames on sensitive applications until patches are deployed

# Configuration example: tighten CSP to limit subdocument exposure
# Apply via HTTP response header on protected web applications
Content-Security-Policy: default-src 'self'; \
  connect-src 'self' https://api.example.com; \
  frame-src 'self'; \
  child-src 'none'; \
  sandbox allow-scripts allow-same-origin; \
  report-to csp-endpoint