CVE-2021-29968
📋 TL;DR
This vulnerability allows an attacker to read memory outside the bounds of a canvas buffer when Firefox renders text with WebRender disabled, potentially leaking sensitive information. It only affects Firefox versions below 89.0.1 running on Windows operating systems. Other browsers and operating systems are not impacted.
💻 Affected Systems
- Mozilla Firefox
📦 What is this software?
Firefox by Mozilla
⚠️ Risk & Real-World Impact
Worst Case
Arbitrary memory read leading to information disclosure, potentially exposing sensitive data like passwords, session tokens, or other application memory contents.
Likely Case
Limited information disclosure from browser memory, possibly revealing fragments of user data or browser state.
If Mitigated
No impact if Firefox is patched to version 89.0.1 or higher, or if running on non-Windows systems.
🎯 Exploit Status
Exploitation requires user interaction (visiting malicious website) but no authentication. The specific memory read patterns would need to be crafted by an attacker.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 89.0.1
Vendor Advisory: https://www.mozilla.org/security/advisories/mfsa2021-27/
Restart Required: Yes
Instructions:
1. Open Firefox. 2. Click menu → Help → About Firefox. 3. Firefox will automatically check for and download updates. 4. Restart Firefox when prompted.
🔧 Temporary Workarounds
Enable WebRender
windowsForce enable WebRender rendering engine which is not vulnerable to this issue
Set 'gfx.webrender.all' to true in about:config
Disable JavaScript
allPrevent malicious JavaScript from exploiting the canvas rendering vulnerability
Set 'javascript.enabled' to false in about:config
🧯 If You Can't Patch
- Switch to alternative browser on Windows systems
- Implement network filtering to block access to untrusted websites
🔍 How to Verify
Check if Vulnerable:
Check Firefox version and operating system: version < 89.0.1 on Windows indicates vulnerabilityCheck Version:
firefox --versionVerify Fix Applied:
Verify Firefox version is 89.0.1 or higher via Help → About Firefox📡 Detection & Monitoring
Log Indicators:
- Unusual memory access patterns in browser crash reports
- Multiple canvas rendering errors in browser console
Network Indicators:
- Requests to websites with complex canvas rendering scripts
- Unusual data exfiltration patterns from browser processes
SIEM Query:
source="firefox" AND (event_type="crash" OR message="*canvas*" OR message="*out of bounds*")