CVE-2021-39793: Google Android Privilege Escalation Flaw

CVE-2021-39793 Overview

CVE-2021-39793 is an out-of-bounds write vulnerability in the ARM Mali GPU kernel driver affecting Android devices. The flaw exists in the kbase_jd_user_buf_pin_pages function within mali_kbase_mem.c, where a logic error in the code allows for memory corruption. This vulnerability enables local privilege escalation without requiring any additional execution privileges or user interaction.

Critical Impact

This vulnerability is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, indicating active exploitation in the wild. Attackers can leverage this flaw to gain elevated privileges on affected Android devices.

Affected Products

• Google Android (all versions with vulnerable Mali GPU kernel driver)
• Android devices using ARM Mali GPUs
• Android kernel with affected Mali driver components

Discovery Timeline

• 2022-03-16 - CVE-2021-39793 published to NVD
• 2025-10-23 - Last updated in NVD database

Technical Details for CVE-2021-39793

Vulnerability Analysis

This vulnerability stems from a logic error in the Mali GPU kernel driver's memory management subsystem. The kbase_jd_user_buf_pin_pages function is responsible for pinning user buffer pages in memory for GPU operations. Due to a coding error in the bounds checking logic, the function can write data beyond the allocated buffer boundaries.

The out-of-bounds write condition occurs during the handling of user-supplied buffer requests. When processing these requests, the driver fails to properly validate the boundaries, allowing memory corruption in adjacent kernel memory regions. Since this vulnerability exists at the kernel driver level, successful exploitation grants attackers kernel-level privileges.

The local attack vector means that an attacker must have some level of access to the device, but the lack of required privileges and user interaction makes this vulnerability particularly dangerous for exploitation through malicious applications.

Root Cause

The root cause is a logic error (CWE-787: Out-of-bounds Write) in the kbase_jd_user_buf_pin_pages function within the Mali GPU kernel driver. The function improperly calculates buffer boundaries when pinning user pages for GPU operations. This calculation error allows writes to memory locations outside the intended buffer, leading to kernel memory corruption and potential code execution with elevated privileges.

Attack Vector

The attack vector is local, requiring an attacker to execute code on the target Android device. This can be achieved through:

1. A malicious application installed on the device that interacts with the Mali GPU driver through standard Android APIs
2. The application crafts specific GPU memory requests that trigger the vulnerable code path
3. By exploiting the out-of-bounds write, the attacker can corrupt kernel memory structures
4. This corruption can be leveraged to escalate privileges from a normal application context to kernel-level access

The vulnerability is particularly concerning because no special permissions are needed beyond what a standard Android application possesses, and no user interaction is required to trigger the exploit.

Detection Methods for CVE-2021-39793

Indicators of Compromise

• Unusual GPU driver activity or crashes in system logs related to Mali kernel module
• Unexpected privilege escalation events from standard applications
• Anomalous memory access patterns in kernel space associated with mali_kbase_mem.c
• Applications making suspicious ioctl calls to the Mali GPU driver

Detection Strategies

• Monitor for applications attempting to interact with the Mali GPU driver in unusual patterns
• Implement kernel-level monitoring for out-of-bounds memory access attempts
• Deploy mobile threat detection solutions capable of identifying privilege escalation attempts
• Analyze application behavior for signs of kernel exploitation techniques

Monitoring Recommendations

• Enable detailed logging for GPU driver operations where available
• Monitor for new applications requesting GPU access without apparent legitimate need
• Implement behavioral analysis to detect post-exploitation activity such as unauthorized data access
• Review security logs for evidence of kernel memory corruption or instability

How to Mitigate CVE-2021-39793

Immediate Actions Required

• Update affected Android devices to the March 2022 security patch level or later immediately
• Review installed applications and remove any suspicious or unnecessary apps
• Enable automatic security updates on all Android devices
• Consider restricting installation of applications from unknown sources

Patch Information

Google addressed this vulnerability in the Android Security Bulletin March 2022. The patch corrects the logic error in the kbase_jd_user_buf_pin_pages function to ensure proper bounds checking when pinning user buffer pages.

Organizations should verify that all Android devices in their environment are updated to at least the 2022-03-01 security patch level. This can be verified in device settings under "About phone" > "Android security patch level".

For additional information on this actively exploited vulnerability, refer to the CISA Known Exploited Vulnerabilities Catalog.

Workarounds

• Limit application installation to trusted sources only (Google Play Store with Play Protect enabled)
• Implement mobile device management (MDM) solutions to enforce security policies
• Consider enterprise mobility management to restrict vulnerable devices from accessing sensitive resources
• Isolate devices that cannot be immediately patched from critical network resources

# Verify Android security patch level via ADB
adb shell getprop ro.build.version.security_patch
# Expected output should be 2022-03-01 or later