CVE-2020-3855
📋 TL;DR
This macOS vulnerability allows malicious applications to overwrite arbitrary files on the system. It affects macOS High Sierra, Mojave, and Catalina before specific security updates. Users who haven't applied Apple's security updates are vulnerable to file system manipulation by untrusted applications.
💻 Affected Systems
- macOS
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise through overwriting critical system files, privilege escalation, or persistent malware installation.
Likely Case
Data destruction, configuration tampering, or installation of malicious payloads by user-downloaded malicious applications.
If Mitigated
Limited impact if only trusted applications from App Store are installed and proper file permissions are enforced.
🎯 Exploit Status
Requires user to execute malicious application. No public exploit code available. Apple addressed before widespread exploitation.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: macOS Catalina 10.15.3, Security Update 2020-001 Mojave, Security Update 2020-001 High Sierra
Vendor Advisory: https://support.apple.com/en-us/HT210919
Restart Required: Yes
Instructions:
1. Open System Preferences > Software Update. 2. Install available security updates. 3. Restart when prompted. 4. Verify update installed via About This Mac > System Report.
🔧 Temporary Workarounds
Application Restriction
allOnly install applications from App Store or identified developers in System Preferences > Security & Privacy
Gatekeeper Enforcement
allEnsure Gatekeeper is enabled to block unsigned applications
sudo spctl --master-enable
🧯 If You Can't Patch
- Implement application allowlisting to only permit trusted applications
- Enforce principle of least privilege and monitor for unexpected file modifications
🔍 How to Verify
Check if Vulnerable:
Check macOS version: About This Mac > Overview. If version is High Sierra 10.13.x, Mojave 10.14.x, or Catalina 10.15.0-10.15.2 without security updates, system is vulnerable.Check Version:
sw_versVerify Fix Applied:
Verify macOS version is Catalina 10.15.3 or later, or that Security Update 2020-001 is installed for Mojave/High Sierra.📡 Detection & Monitoring
Log Indicators:
- Unexpected file modifications in system directories
- Application installation from untrusted sources
Network Indicators:
- Downloads of suspicious macOS applications
SIEM Query:
source="macos" AND (event="file_modification" AND path IN ("/System/*", "/Library/*", "/etc/*")) OR (event="app_install" AND developer!="Apple" AND developer!="identified")