CVE-2021-39642 in Android
Summary
by MITRE • 12/15/2021
In synchronous_process_io_entries of lwis_ioctl.c, there is a possible out of bounds write due to a race condition. This could lead to local escalation of privilege with System execution privileges needed. User interaction is not needed for exploitation.Product: AndroidVersions: Android kernelAndroid ID: A-195731663References: N/A
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Analysis
by VulDB Data Team • 12/18/2021
The vulnerability identified as CVE-2021-39642 represents a critical race condition flaw within the Android kernel's lightweight synchronous I/O implementation. This issue exists in the synchronous_process_io_entries function located in the lwis_ioctl.c source file, where improper synchronization mechanisms fail to prevent concurrent access to shared memory regions during I/O operations. The race condition occurs when multiple threads or processes attempt to modify the same memory locations simultaneously without adequate locking mechanisms, creating opportunities for memory corruption and unauthorized privilege escalation.
The technical nature of this vulnerability stems from inadequate input validation and memory boundary checking within the kernel's I/O processing subsystem. When the system handles asynchronous I/O operations through the Lightweight I/O subsystem, the race condition manifests during the processing of I/O entry structures where the kernel fails to properly validate array indices or buffer boundaries before writing data. This flaw is categorized under CWE-367, which specifically addresses Time-of-Check to Time-of-Use (TOCTOU) vulnerabilities, though the exact implementation suggests a more fundamental race condition pattern. The vulnerability allows for out-of-bounds memory writes that can potentially overwrite critical kernel data structures, function pointers, or privilege levels, enabling malicious code execution with system-level privileges.
From an operational perspective, this vulnerability poses significant risks to Android devices as it enables local privilege escalation without requiring user interaction or specialized attack vectors. An attacker with local access to an Android device can exploit this race condition to elevate their privileges from standard user level to system level execution privileges, effectively bypassing the kernel's security model. The exploitation requires only that the attacker can execute code on the target device, making it particularly dangerous in scenarios where malicious applications or compromised user accounts exist. The impact extends beyond simple privilege escalation as successful exploitation could lead to complete system compromise, data exfiltration, or persistent backdoor installation. This vulnerability directly maps to ATT&CK technique T1068, which covers 'Local Privilege Escalation' and specifically targets the kernel-level execution environment where such attacks are most effective.
Mitigation strategies for CVE-2021-39642 should focus on implementing proper synchronization mechanisms within the kernel's I/O subsystem and applying the latest security patches from Android vendors. System administrators should ensure all Android devices receive timely security updates from their manufacturers, particularly those addressing kernel-level vulnerabilities. The fix typically involves adding proper mutex or spinlock mechanisms around critical sections of code that handle I/O entry processing, ensuring that only one thread can modify shared data structures at any given time. Additionally, implementing bounds checking and input validation for all array operations within the lwis_ioctl.c module would prevent the out-of-bounds write conditions that enable privilege escalation. Organizations should also consider deploying kernel hardening techniques such as stack canaries, kernel address space layout randomization, and control flow integrity checks to reduce the effectiveness of potential exploitation attempts. Regular security auditing of kernel modules and proactive monitoring for similar race condition patterns in other subsystems can help identify and remediate analogous vulnerabilities before they can be exploited in the wild.