CVE-2021-39686 in Android
Summary
by MITRE • 03/16/2022
In several functions of binder.c, there is a possible way to represent the wrong domain to SELinux due to a race condition. This could lead to local escalation of privilege with no additional execution privileges needed. User interaction is not needed for exploitation.Product: AndroidVersions: Android kernelAndroid ID: A-200688826References: Upstream kernel
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Analysis
by VulDB Data Team • 03/19/2022
The vulnerability identified as CVE-2021-39686 resides within the Android kernel's binder driver implementation, specifically in the binder.c file where multiple functions exhibit a race condition that can result in incorrect SELinux domain representation. This flaw represents a critical security weakness that allows for local privilege escalation without requiring any additional execution privileges or user interaction for exploitation. The issue stems from the improper handling of security contexts during concurrent operations within the binder subsystem, which is responsible for inter-process communication in Android systems.
The technical root cause of this vulnerability involves a race condition that occurs when multiple threads or processes access the binder driver functions simultaneously, leading to inconsistent SELinux security context management. When the kernel processes multiple concurrent binder operations, the security domain information can become corrupted or improperly set, allowing processes to potentially operate under incorrect security contexts. This misrepresentation of security domains creates an opening for privilege escalation attacks where malicious code can leverage the compromised security context to gain elevated privileges within the system. The vulnerability specifically affects the Android kernel implementation and was tracked under Android ID A-200688826, with upstream kernel references indicating the issue originates from core kernel security mechanisms.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it fundamentally undermines the security model of Android systems by allowing processes to bypass intended security boundaries. Attackers can exploit this race condition to execute code with higher privileges than originally intended, potentially gaining access to sensitive system resources, modifying critical system files, or even establishing persistent backdoors within the device. The lack of user interaction requirements makes this vulnerability particularly dangerous as it can be exploited automatically without any user involvement, and the fact that no additional execution privileges are needed means that even unprivileged processes can potentially leverage this flaw. This vulnerability directly relates to CWE-362, which describes race conditions in security-critical operations, and aligns with ATT&CK technique T1068, which covers local privilege escalation through system weaknesses.
Mitigation strategies for this vulnerability require immediate kernel updates from device manufacturers and system administrators to address the underlying race condition in the binder driver. The recommended approach involves applying the upstream kernel patches that fix the concurrent access issues within binder.c functions, ensuring proper synchronization mechanisms are implemented to prevent the race condition from occurring. Additionally, system administrators should consider implementing additional security controls such as kernel address space layout randomization and other hardening measures to reduce the attack surface. Organizations should also monitor for any potential exploitation attempts and implement proper logging mechanisms to detect unusual privilege escalation activities that might indicate exploitation of this vulnerability. The fix typically involves implementing proper locking mechanisms and ensuring that security context information is consistently maintained across all concurrent binder operations.