CVE-2026-0877
📋 TL;DR
This CVE describes a mitigation bypass vulnerability in the DOM Security component of Mozilla products. It allows attackers to circumvent security protections, potentially leading to arbitrary code execution. Affected users include those running vulnerable versions of Firefox, Firefox ESR, and Thunderbird.
💻 Affected Systems
- Firefox
- Firefox ESR
- Thunderbird
📦 What is this software?
Firefox by Mozilla
Firefox by Mozilla
Firefox by Mozilla
Thunderbird by Mozilla
Thunderbird by Mozilla
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise through arbitrary code execution, potentially leading to data theft, ransomware deployment, or lateral movement within networks.
Likely Case
Browser sandbox escape leading to privilege escalation, installation of malware, or credential theft from the compromised browser session.
If Mitigated
Limited impact with proper network segmentation, application whitelisting, and endpoint protection that can detect exploitation attempts.
🎯 Exploit Status
Exploitation requires user interaction (visiting malicious website or opening malicious email), but no authentication is needed once the user interacts with the malicious content.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Firefox 147+, Firefox ESR 115.32+, Firefox ESR 140.7+, Thunderbird 147+, Thunderbird 140.7+
Vendor Advisory: https://www.mozilla.org/security/advisories/
Restart Required: Yes
Instructions:
1. Open Firefox/Thunderbird. 2. Click menu → Help → About Firefox/Thunderbird. 3. Allow automatic update to complete. 4. Restart the application when prompted.
🔧 Temporary Workarounds
Disable JavaScript
allTemporarily disable JavaScript to prevent exploitation through malicious websites
about:config → javascript.enabled = false
Use Content Security Policy
allImplement strict CSP headers to limit script execution
Content-Security-Policy: script-src 'self'
🧯 If You Can't Patch
- Block access to untrusted websites and email attachments
- Implement network segmentation to isolate vulnerable systems
🔍 How to Verify
Check if Vulnerable:
Check browser version in About dialog and compare against affected versionsCheck Version:
firefox --version or thunderbird --versionVerify Fix Applied:
Verify version is equal to or greater than patched versions listed in fix_official📡 Detection & Monitoring
Log Indicators:
- Unexpected browser crashes
- Suspicious process creation from browser
- Unusual network connections from browser process
Network Indicators:
- Traffic to known malicious domains
- Unusual outbound connections from browser
SIEM Query:
process_name:firefox.exe AND (event_id:1 OR event_id:4688) AND parent_process_name:explorer.exe🔗 References
- https://bugzilla.mozilla.org/show_bug.cgi?id=1999257
- https://www.mozilla.org/security/advisories/mfsa2026-01/
- https://www.mozilla.org/security/advisories/mfsa2026-02/
- https://www.mozilla.org/security/advisories/mfsa2026-03/
- https://www.mozilla.org/security/advisories/mfsa2026-04/
- https://www.mozilla.org/security/advisories/mfsa2026-05/
