CVE-2021-1876
📋 TL;DR
This is a use-after-free vulnerability in macOS that allows arbitrary code execution when processing malicious web content. Attackers can exploit this by tricking users into visiting specially crafted websites, potentially taking full control of affected systems. It affects macOS Mojave, Catalina, and Big Sur before specific security updates.
💻 Affected Systems
- macOS
📦 What is this software?
Macos by Apple
macOS is Apple's desktop and laptop operating system powering Mac computers used by millions of professionals, developers, creative professionals, and enterprise users worldwide. Built on a Unix foundation with the Darwin kernel and modern Cocoa frameworks, macOS delivers a seamless ecosystem integr...
Learn more about Macos →⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise with attacker gaining root privileges, data theft, ransomware deployment, and persistent backdoor installation.
Likely Case
Browser-based exploitation leading to user-level code execution, credential theft, and lateral movement within the network.
If Mitigated
Limited impact with proper network segmentation, application sandboxing, and user privilege restrictions preventing full system takeover.
🎯 Exploit Status
Exploitation requires user interaction (visiting malicious website) but no authentication. Use-after-free vulnerabilities typically require precise memory manipulation.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: macOS Big Sur 11.3, Security Update 2021-002 Catalina, Security Update 2021-003 Mojave
Vendor Advisory: https://support.apple.com/en-us/HT212325
Restart Required: Yes
Instructions:
1. Open System Preferences > Software Update. 2. Install available updates. 3. Restart when prompted. For managed environments, deploy updates via MDM or patch management tools.
🔧 Temporary Workarounds
Browser Restrictions
allRestrict web browsing to trusted sites only and disable JavaScript for untrusted sources
Network Segmentation
allIsolate vulnerable systems from critical network segments and implement web filtering
🧯 If You Can't Patch
- Implement application whitelisting to prevent unauthorized code execution
- Deploy endpoint detection and response (EDR) solutions to monitor for exploitation attempts
🔍 How to Verify
Check if Vulnerable:
Check macOS version: if running Mojave, Catalina, or Big Sur without the specified security updates, system is vulnerable.Check Version:
sw_versVerify Fix Applied:
Verify macOS version matches patched versions: Big Sur 11.3+, Catalina with Security Update 2021-002, Mojave with Security Update 2021-003.📡 Detection & Monitoring
Log Indicators:
- Unusual browser crashes, unexpected process creation from browser processes, memory access violations in system logs
Network Indicators:
- Connections to suspicious domains followed by unusual outbound traffic, web requests to known exploit kits
SIEM Query:
source="macos_system_logs" AND (process="Safari" OR process="WebKit") AND (event="crash" OR event="memory_access")