CVE-2021-29952
📋 TL;DR
A race condition in Firefox's Web Render components during destruction could lead to undefined behavior, potentially allowing arbitrary code execution through memory corruption. This affects Firefox desktop versions before 88.0.1 and Firefox for Android versions before 88.1.3.
💻 Affected Systems
- Mozilla Firefox
- Mozilla Firefox for Android
📦 What is this software?
Firefox by Mozilla
Firefox by Mozilla
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution allowing attacker to take full control of the affected system.
Likely Case
Browser crash or instability; successful exploitation requires significant effort.
If Mitigated
No impact if patched or if exploit attempts fail due to complexity.
🎯 Exploit Status
Exploitation requires triggering a race condition during Web Render destruction, which is complex and may require multiple attempts.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Firefox 88.0.1, Firefox for Android 88.1.3
Vendor Advisory: https://www.mozilla.org/security/advisories/mfsa2021-20/
Restart Required: Yes
Instructions:
1. Open Firefox. 2. Click menu > Help > About Firefox. 3. Allow automatic update to version 88.0.1 or later. 4. Restart Firefox. For Android: Update via Google Play Store to version 88.1.3 or later.
🔧 Temporary Workarounds
Disable Web Render
allTemporarily disable Web Render to mitigate the vulnerability (may impact performance).
Set gfx.webrender.all to false in about:config
🧯 If You Can't Patch
- Restrict browser use to trusted websites only.
- Implement application whitelisting to block Firefox execution.
🔍 How to Verify
Check if Vulnerable:
Check Firefox version via about:firefox or Help > About Firefox; if version is less than 88.0.1, it is vulnerable.Check Version:
firefox --version (Linux/macOS) or check about:firefox (all platforms)Verify Fix Applied:
Confirm version is 88.0.1 or later for Firefox, or 88.1.3 or later for Android.📡 Detection & Monitoring
Log Indicators:
- Browser crash logs referencing Web Render or race conditions
- Unexpected process termination in system logs
Network Indicators:
- Unusual outbound connections from Firefox process post-crash
SIEM Query:
source="firefox.log" AND ("crash" OR "WebRender")