CVE-2025-6427
📋 TL;DR
This vulnerability allows attackers to bypass Content Security Policy connect-src directives by manipulating subdocuments, enabling unauthorized connections to malicious servers while hiding these connections from browser developer tools. It affects Firefox versions below 140 and Thunderbird versions below 140.
💻 Affected Systems
- Mozilla Firefox
- Mozilla Thunderbird
📦 What is this software?
Firefox by Mozilla
⚠️ Risk & Real-World Impact
Worst Case
Attackers can exfiltrate sensitive data, establish persistent backdoors, or redirect users to malicious sites while evading CSP protections and detection tools.
Likely Case
Data exfiltration from web applications, bypassing CSP restrictions to load malicious resources, and evading security monitoring.
If Mitigated
Limited impact if proper network segmentation, additional security layers, and monitoring are in place, though CSP bypass remains concerning.
🎯 Exploit Status
Exploitation requires web page access but no authentication; technical details are disclosed in bug reports.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Firefox 140, Thunderbird 140
Vendor Advisory: https://www.mozilla.org/security/advisories/mfsa2025-51/
Restart Required: Yes
Instructions:
1. Open Firefox/Thunderbird. 2. Go to Help > About Firefox/Thunderbird. 3. Allow automatic update to version 140 or higher. 4. Restart the application.
🔧 Temporary Workarounds
Disable JavaScript
allPrevents exploitation by disabling JavaScript execution, but breaks most web functionality.
Use Alternative Browser
allSwitch to a non-affected browser until patched.
🧯 If You Can't Patch
- Implement network-level restrictions to block unauthorized outbound connections.
- Deploy additional web application firewalls (WAF) with CSP validation rules.
🔍 How to Verify
Check if Vulnerable:
Check browser version in Help > About Firefox/Thunderbird; if version is below 140, it is vulnerable.Check Version:
firefox --version (Linux) or check via GUI on Windows/macOSVerify Fix Applied:
Confirm version is 140 or higher in Help > About Firefox/Thunderbird after update.📡 Detection & Monitoring
Log Indicators:
- Unexpected outbound connections in proxy/firewall logs
- CSP violation reports showing bypass attempts
Network Indicators:
- Unusual traffic to unknown domains from browsers
- Connections bypassing expected CSP restrictions
SIEM Query:
source="firewall" dest_ip!=allowed_domains AND user_agent="Firefox/*"