CVE-2026-2766
📋 TL;DR
A use-after-free vulnerability in Firefox's JavaScript JIT engine allows attackers to execute arbitrary code by tricking users into visiting malicious websites. This affects Firefox versions below 148 and Firefox ESR versions below 140.8. Users who haven't updated their browsers are vulnerable to exploitation.
💻 Affected Systems
- Mozilla Firefox
- Mozilla Firefox ESR
📦 What is this software?
Firefox by Mozilla
Firefox by Mozilla
Thunderbird by Mozilla
Thunderbird by Mozilla
⚠️ Risk & Real-World Impact
Worst Case
Full remote code execution leading to complete system compromise, data theft, and persistent malware installation.
Likely Case
Arbitrary code execution within browser context leading to session hijacking, credential theft, and further exploitation.
If Mitigated
Limited impact with proper browser sandboxing, but still potential for data exfiltration and limited system access.
🎯 Exploit Status
Exploitation requires JavaScript execution but no authentication. Complexity involves triggering specific JIT conditions.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Firefox 148, Firefox ESR 140.8
Vendor Advisory: https://www.mozilla.org/security/advisories/mfsa2026-13/
Restart Required: Yes
Instructions:
1. Open Firefox menu > Help > About Firefox. 2. Allow automatic update check and download. 3. Restart Firefox when prompted. 4. Verify version is 148 or higher (or ESR 140.8+).
🔧 Temporary Workarounds
Disable JavaScript
allTemporarily disable JavaScript execution to prevent exploitation
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
- Restrict browser usage to trusted websites only
- Implement network segmentation to limit browser access to sensitive systems
🔍 How to Verify
Check if Vulnerable:
Check Firefox version in menu > Help > About Firefox. If version is below 148 (or ESR below 140.8), system is vulnerable.Check Version:
firefox --version (Linux/macOS) or check About dialog (Windows)Verify Fix Applied:
After update, verify version shows 148 or higher (or ESR 140.8+). Test with known safe JavaScript-heavy sites to ensure functionality.📡 Detection & Monitoring
Log Indicators:
- Browser crash reports with JIT-related stack traces
- Unusual JavaScript execution patterns
Network Indicators:
- Requests to known malicious domains from Firefox instances
- Unusual outbound connections after visiting websites
SIEM Query:
source="firefox.log" AND ("JIT" OR "javascript" OR "crash") AND severity=HIGH