CVE-2021-39741
📋 TL;DR
This vulnerability in Android's Keymaster component allows local attackers to write data beyond allocated memory boundaries due to missing bounds checking. It enables local privilege escalation to System-level access on affected Android devices without requiring user interaction. Only Android 12L devices with Keymaster implementations are affected.
💻 Affected Systems
- Android devices with Keymaster implementation
📦 What is this software?
Android by Google
⚠️ Risk & Real-World Impact
Worst Case
Complete device compromise with System privileges, allowing installation of persistent malware, data theft, and bypassing security controls.
Likely Case
Local attacker gains System privileges to install malicious apps, modify system files, or access protected data.
If Mitigated
Attack fails due to patched systems or lack of local access; minimal impact with proper access controls.
🎯 Exploit Status
Requires System execution privileges for initial access; out-of-bounds write could be leveraged for privilege escalation.
🛠️ 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 Android system updates in Settings > System > System update. 2. Install March 2022 or later security patch. 3. Reboot device after installation.
🔧 Temporary Workarounds
Restrict local access
allLimit physical and network access to devices to reduce attack surface
Disable unnecessary privileges
androidReview and remove System execution privileges from unnecessary apps
🧯 If You Can't Patch
- Isolate affected devices from critical networks and data
- Implement strict app installation policies and monitor for suspicious activity
🔍 How to Verify
Check if Vulnerable:
Check Android version in Settings > About phone > Android version. If it shows Android 12L and security patch level is before March 2022, device is vulnerable.Check Version:
adb shell getprop ro.build.version.release && adb shell getprop ro.build.version.security_patchVerify Fix Applied:
Verify Android version is 12L with security patch level March 2022 or later in Settings > About phone.📡 Detection & Monitoring
Log Indicators:
- Unusual Keymaster service crashes
- Privilege escalation attempts in system logs
- Unexpected System-level process creation
Network Indicators:
- None - local exploitation only
SIEM Query:
source="android_system" AND (event="keymaster_crash" OR process="escalation_attempt")