CVE-2020-26959
📋 TL;DR
This is a use-after-free vulnerability in Mozilla Firefox, Firefox ESR, and Thunderbird that occurs during browser shutdown. Attackers could exploit this to cause memory corruption and potentially execute arbitrary code. Users running affected versions of these applications are at risk.
💻 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 malware installation.
Likely Case
Browser crash or denial of service, potentially allowing limited information disclosure.
If Mitigated
No impact if patched or if exploit attempts are blocked by security controls.
🎯 Exploit Status
Exploitation requires triggering the use-after-free condition during browser shutdown, which may require specific timing or conditions.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Firefox 83, Firefox ESR 78.5, Thunderbird 78.5
Vendor Advisory: https://www.mozilla.org/security/advisories/mfsa2020-50/
Restart Required: Yes
Instructions:
1. Open the affected application. 2. Go to Help > About Firefox/Thunderbird. 3. Allow the application to check for and install updates. 4. Restart the application when prompted.
🔧 Temporary Workarounds
Disable JavaScript
allTemporarily disable JavaScript to reduce attack surface while patching.
about:config -> javascript.enabled = false
🧯 If You Can't Patch
- Restrict browser usage to trusted websites only
- Implement application whitelisting to prevent execution of malicious code
🔍 How to Verify
Check if Vulnerable:
Check application version in Help > About Firefox/Thunderbird. If version is below patched versions, system is vulnerable.Check Version:
firefox --version or thunderbird --version on Linux/macOSVerify Fix Applied:
Confirm version is Firefox ≥83, Firefox ESR ≥78.5, or Thunderbird ≥78.5 after update.📡 Detection & Monitoring
Log Indicators:
- Application crash logs with memory corruption signatures
- Unexpected browser shutdown events
Network Indicators:
- Unusual web requests preceding crashes
SIEM Query:
source="*firefox*" OR source="*thunderbird*" AND (event="crash" OR event="shutdown")🔗 References
- https://bugzilla.mozilla.org/show_bug.cgi?id=1669466
- https://www.mozilla.org/security/advisories/mfsa2020-50/
- https://www.mozilla.org/security/advisories/mfsa2020-51/
- https://www.mozilla.org/security/advisories/mfsa2020-52/
- https://bugzilla.mozilla.org/show_bug.cgi?id=1669466
- https://www.mozilla.org/security/advisories/mfsa2020-50/
- https://www.mozilla.org/security/advisories/mfsa2020-51/
- https://www.mozilla.org/security/advisories/mfsa2020-52/
