CVE-2021-39776
📋 TL;DR
This CVE describes a use-after-free vulnerability in Android's NFC subsystem that could allow local privilege escalation. An attacker with user-level access could exploit memory corruption to gain elevated system privileges. Only Android 12L devices are affected.
💻 Affected Systems
- Android devices with NFC capability
📦 What is this software?
Android by Google
⚠️ Risk & Real-World Impact
Worst Case
Complete device compromise allowing attacker to execute arbitrary code with system privileges, potentially installing persistent malware or accessing sensitive data.
Likely Case
Local privilege escalation allowing attacker to bypass app sandboxing and access other apps' data or system resources.
If Mitigated
No impact if patched; minimal risk if device has restricted physical access and no malicious apps installed.
🎯 Exploit Status
Exploitation requires user execution privileges and knowledge of memory corruption techniques. No user interaction needed once malicious code is running.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Android Security Bulletin March 2022 or later
Vendor Advisory: https://source.android.com/security/bulletin/android-12l
Restart Required: Yes
Instructions:
1. Check for system updates in Settings > System > System update. 2. Install available security updates. 3. Reboot device after installation.
🔧 Temporary Workarounds
Disable NFC
androidTemporarily disable NFC functionality to prevent exploitation
Settings > Connected devices > Connection preferences > NFC (toggle off)
🧯 If You Can't Patch
- Restrict physical access to devices and implement application allowlisting
- Monitor for suspicious app behavior and implement mobile device management controls
🔍 How to Verify
Check if Vulnerable:
Check Android version in Settings > About phone > Android version. If it shows '12L' and security patch level is before March 2022, device is vulnerable.Check Version:
Settings > About phone > Android version and Android security updateVerify Fix Applied:
Verify Android version is 12L with security patch level March 2022 or later in Settings > About phone > Android security update.📡 Detection & Monitoring
Log Indicators:
- Unexpected NFC service crashes
- Privilege escalation attempts in system logs
- SELinux policy violations related to NFC
Network Indicators:
- None - local exploitation only
SIEM Query:
source="android_system" AND (event="nfc_crash" OR event="privilege_escalation")