CVE-2021-0675: Google Android Privilege Escalation Flaw

CVE-2021-0675 Overview

CVE-2021-0675 is an out-of-bounds write vulnerability in the MediaTek ALAC (Apple Lossless Audio Codec) decoder component affecting Android devices powered by MediaTek chipsets. The vulnerability exists due to an incorrect bounds check in the audio decoder, which allows an attacker to write data beyond the allocated buffer boundaries. This flaw enables local privilege escalation without requiring user interaction, making it particularly dangerous for affected Android devices.

Critical Impact

Successful exploitation allows local privilege escalation on affected Android devices with MediaTek chipsets, potentially granting attackers elevated system access without any user interaction required.

Affected Products

• Google Android 8.1, 9.0, 10.0, and 11.0
• MediaTek MT6xxx series chipsets (including MT6570, MT6580, MT6735, MT6737, MT6739, MT6750, MT6753, MT6755, MT6757, MT6758, MT6761, MT6763, MT6765, MT6768, MT6771, MT6779, MT6781, MT6785, MT6797, MT6799, MT6833, MT6853, MT6873, MT6877, MT6883, MT6885, MT6889, MT6893)
• MediaTek MT8xxx series chipsets (including MT8163, MT8167, MT8168, MT8173, MT8175, MT8176, MT8183, MT8185, MT8195, MT8321, MT8362A, MT8365, MT8385, MT8765, MT8766, MT8768, MT8786, MT8788, MT8789, MT8791, MT8797)

Discovery Timeline

• December 15, 2021 - CVE-2021-0675 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-0675

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-Bounds Write), affecting the ALAC decoder component in MediaTek's audio processing subsystem. The ALAC decoder fails to properly validate buffer boundaries when processing audio data, allowing an attacker with local access to trigger memory corruption by providing specially crafted input.

The vulnerability is exploitable locally, requiring low privileges to initiate the attack. No user interaction is needed for successful exploitation, which significantly increases the risk profile. When triggered, the out-of-bounds write can corrupt adjacent memory regions, potentially overwriting critical data structures or function pointers.

Root Cause

The root cause of CVE-2021-0675 lies in an incorrect bounds check within the ALAC decoder implementation. When processing audio frames, the decoder calculates buffer sizes and offsets but fails to properly validate that write operations remain within allocated memory boundaries. This coding error allows crafted audio data to cause writes beyond the intended buffer, leading to memory corruption.

The vulnerability is tracked internally by MediaTek under Patch ID ALPS06064258 and Issue ID ALPS06064258.

Attack Vector

The attack vector for this vulnerability is local, meaning an attacker must have some level of access to the target device to exploit it. The exploitation scenario typically involves:

1. An attacker delivers a malicious audio file or media content containing crafted ALAC audio data to the target device
2. When the MediaTek audio subsystem processes this content, the ALAC decoder processes the malformed data
3. The incorrect bounds check allows the attacker-controlled data to be written outside the allocated buffer
4. By carefully crafting the overflow, an attacker can overwrite adjacent memory structures
5. This memory corruption can be leveraged to escalate privileges on the device

Since no additional execution privileges are required and user interaction is not needed, a malicious application with minimal permissions could potentially exploit this vulnerability to gain elevated system access.

Detection Methods for CVE-2021-0675

Indicators of Compromise

• Unexpected crashes or instability in media playback services or audio-related system components
• Unusual process behavior from media codec services, particularly those handling audio decoding
• Evidence of privilege escalation attempts from applications that normally run with limited permissions
• Suspicious ALAC or M4A audio files present on the device that may have been used as exploit payloads

Detection Strategies

• Monitor for abnormal memory access patterns in media decoder processes using memory protection mechanisms
• Implement behavioral analysis to detect applications attempting privilege escalation after audio file processing
• Use mobile device management (MDM) solutions to track and alert on unexpected system permission changes
• Deploy endpoint detection solutions capable of monitoring Android system component behavior

Monitoring Recommendations

• Enable logging for media framework components to capture decoder errors and crashes
• Monitor for applications requesting unusual permissions or attempting to access privileged system resources
• Implement application vetting processes to identify potentially malicious media files before they reach devices
• Use SentinelOne Mobile Threat Defense to detect exploitation attempts and privilege escalation activities on Android endpoints

How to Mitigate CVE-2021-0675

Immediate Actions Required

• Apply the December 2021 security updates from MediaTek and your device manufacturer immediately
• Ensure all affected Android devices are updated to the latest available security patch level
• Restrict installation of applications from untrusted sources to reduce the risk of malicious apps exploiting this vulnerability
• Implement mobile device management policies to enforce security updates across your organization's device fleet

Patch Information

MediaTek has released a security patch addressing this vulnerability as part of their December 2021 security bulletin. The fix is identified by Patch ID ALPS06064258. Device manufacturers and carriers should incorporate this patch into their Android security updates for devices using affected MediaTek chipsets.

Organizations should refer to the MediaTek Security Bulletin - December 2021 for detailed patch information. Android device users should check with their device manufacturer for the availability of security updates containing this fix.

Workarounds

• Avoid opening audio files from untrusted or unknown sources until the patch is applied
• Use mobile security solutions that can detect and block exploitation attempts targeting media codecs
• Consider restricting the use of affected devices for sensitive operations until updates are available
• Implement network-level controls to scan media files before they reach endpoints

# Check Android security patch level on device
adb shell getprop ro.build.version.security_patch

# Verify MediaTek chipset on device
adb shell cat /proc/cpuinfo | grep Hardware