CVE-2020-26950
📋 TL;DR
This vulnerability is a use-after-free condition in Firefox's JavaScript engine that allows remote code execution when exploited. Attackers can craft malicious web content to trigger the vulnerability, potentially taking control of affected browsers. It impacts Firefox, Firefox ESR, and Thunderbird users running outdated versions.
💻 Affected Systems
- Mozilla Firefox
- Mozilla Firefox ESR
- Mozilla Thunderbird
📦 What is this software?
Firefox by Mozilla
Firefox Esr by Mozilla
Thunderbird by Mozilla
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution leading to complete system compromise, data theft, or ransomware deployment.
Likely Case
Browser compromise allowing session hijacking, credential theft, and installation of malware.
If Mitigated
Limited impact with proper browser sandboxing and security controls, potentially only browser crash.
🎯 Exploit Status
Proof-of-concept code is publicly available. Exploitation requires JavaScript execution but no authentication.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Firefox 82.0.3, Firefox ESR 78.4.1, Thunderbird 78.4.2
Vendor Advisory: https://www.mozilla.org/security/advisories/mfsa2020-49/
Restart Required: Yes
Instructions:
1. Open affected application. 2. Go to Help > About Firefox/Thunderbird. 3. Allow automatic update. 4. Restart when prompted. For enterprise: Deploy updated packages via management tools.
🔧 Temporary Workarounds
Disable JavaScript
allTemporarily disable JavaScript execution in browser to prevent exploitation.
about:config -> javascript.enabled = false
Use NoScript Extension
allInstall NoScript to selectively block JavaScript execution on untrusted sites.
🧯 If You Can't Patch
- Isolate vulnerable browsers from internet access using network segmentation.
- Implement application whitelisting to prevent execution of unknown processes.
🔍 How to Verify
Check if Vulnerable:
Check browser version in Help > About Firefox/Thunderbird. Compare against affected versions.Check Version:
firefox --version (Linux) or check About dialog (Windows/macOS)Verify Fix Applied:
Confirm version is Firefox ≥82.0.3, Firefox ESR ≥78.4.1, or Thunderbird ≥78.4.2.📡 Detection & Monitoring
Log Indicators:
- Browser crash reports with memory access violations
- Unexpected JavaScript engine errors
Network Indicators:
- Connections to known exploit servers
- Unusual JavaScript payloads in web traffic
SIEM Query:
source="browser_logs" AND (event="crash" OR error="use-after-free")🔗 References
- http://packetstormsecurity.com/files/166175/Firefox-MCallGetProperty-Write-Side-Effects-Use-After-Free.html
- https://bugzilla.mozilla.org/show_bug.cgi?id=1675905
- https://www.mozilla.org/security/advisories/mfsa2020-49/
- http://packetstormsecurity.com/files/166175/Firefox-MCallGetProperty-Write-Side-Effects-Use-After-Free.html
- https://bugzilla.mozilla.org/show_bug.cgi?id=1675905
- https://www.mozilla.org/security/advisories/mfsa2020-49/
