CVE-2021-0968: Google Android RCE Vulnerability

CVE-2021-0968 Overview

CVE-2021-0968 is an integer overflow vulnerability in the osi_malloc and osi_calloc functions within the allocator.cc file of Android's Bluetooth stack. This vulnerability allows for an out-of-bounds write condition that can be exploited remotely to achieve code execution without requiring additional execution privileges. Notably, user interaction is not needed for exploitation, significantly increasing the risk profile of this vulnerability.

Critical Impact

Remote code execution is possible on affected Android devices through the Bluetooth subsystem without any user interaction, potentially allowing complete device compromise.

Affected Products

• Google Android 9.0
• Google Android 10.0
• Google Android 11.0
• Google Android 12.0

Discovery Timeline

• 2021-12-15 - CVE-2021-0968 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-0968

Vulnerability Analysis

The vulnerability exists within the Bluetooth Operating System Interface (OSI) memory allocation routines in Android. The osi_malloc and osi_calloc functions in allocator.cc fail to properly validate size parameters before performing memory allocation operations. When specially crafted input causes an integer overflow during size calculations, the allocator may allocate a buffer significantly smaller than intended. Subsequent operations that write data to this undersized buffer result in an out-of-bounds write condition.

This vulnerability is classified under CWE-190 (Integer Overflow or Wraparound), which occurs when an arithmetic operation attempts to create a numeric value that exceeds the maximum value that can be stored in the allocated memory space.

Root Cause

The root cause is insufficient bounds checking on allocation size parameters within the osi_malloc and osi_calloc functions. When a size value is provided that, when multiplied or combined with other values during allocation calculations, exceeds the maximum representable integer value, the result wraps around to a small positive number. This causes the allocator to provision a much smaller memory region than the calling code expects, leading to heap corruption when data is subsequently written.

Attack Vector

The attack can be conducted remotely over a network connection targeting the Bluetooth subsystem. An attacker can craft malicious Bluetooth packets or establish a malicious Bluetooth connection that triggers the vulnerable code path in the memory allocator. The exploitation chain works as follows:

1. Attacker sends specially crafted data to the target device's Bluetooth interface
2. The data triggers a code path that calls osi_malloc or osi_calloc with attacker-controlled size values
3. Integer overflow occurs during size calculation, resulting in undersized buffer allocation
4. Subsequent data copy operations overflow the allocated buffer
5. Heap corruption enables arbitrary code execution with the privileges of the Bluetooth process

The vulnerability does not require user interaction, meaning an attacker within Bluetooth range or with network access to the device can potentially exploit this flaw without the device owner taking any action.

Detection Methods for CVE-2021-0968

Indicators of Compromise

• Unusual Bluetooth service crashes or restarts on Android devices
• Unexpected memory allocation failures logged in the Bluetooth subsystem
• System instability following Bluetooth connection attempts from unknown devices
• Anomalous heap corruption signatures in crash dumps from the Bluetooth process

Detection Strategies

• Monitor Android system logs for Bluetooth service crashes with memory-related error codes
• Implement heap integrity monitoring for the Bluetooth process to detect corruption patterns
• Deploy network-level monitoring to identify malformed Bluetooth protocol traffic
• Utilize mobile threat detection solutions capable of identifying exploitation attempts targeting Android system services

Monitoring Recommendations

• Enable verbose logging for Android Bluetooth services in enterprise environments
• Implement centralized log collection from Android devices to correlate potential exploitation attempts
• Deploy mobile device management (MDM) solutions with security monitoring capabilities to track device patch levels
• Monitor for unexpected Bluetooth service behavior patterns across managed device fleets

How to Mitigate CVE-2021-0968

Immediate Actions Required

• Apply the December 2021 Android Security Bulletin patches immediately
• Disable Bluetooth on devices that cannot be immediately patched when not in active use
• Restrict Bluetooth pairing to known and trusted devices only
• Consider network isolation for unpatched Android devices in enterprise environments

Patch Information

Google addressed this vulnerability in the Android Security Bulletin December 2021. The fix implements proper bounds checking on allocation size parameters in the osi_malloc and osi_calloc functions to prevent integer overflow conditions. The Android ID associated with this vulnerability is A-197868577.

Organizations should ensure all Android devices running versions 9.0, 10.0, 11.0, and 12.0 receive the December 2021 or later security patch level. Device manufacturers may have varying timelines for patch availability, so administrators should coordinate with device vendors for update schedules.

Workarounds

• Disable Bluetooth functionality entirely on devices where patches cannot be applied
• Use Android's Bluetooth visibility settings to minimize device exposure when Bluetooth must remain enabled
• Implement strict Bluetooth pairing policies that prevent connections from unknown devices
• Consider deploying mobile threat defense solutions that can detect exploitation attempts

# Check current Android security patch level via ADB
adb shell getprop ro.build.version.security_patch

# Disable Bluetooth via ADB for enterprise device management
adb shell settings put global bluetooth_on 0