CVE-2021-39685 in Android
Summary
by MITRE • 03/16/2022
In various setup methods of the USB gadget subsystem, there is a possible out of bounds write due to an incorrect flag check. 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-210292376References: Upstream kernel
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Analysis
by VulDB Data Team • 05/18/2025
The vulnerability identified as CVE-2021-39685 represents a critical out-of-bounds write flaw within the Android kernel's USB gadget subsystem. This issue manifests during various setup methods of the USB gadget functionality, where an incorrect flag check creates a condition that allows malicious code to write beyond the boundaries of allocated memory regions. The vulnerability specifically affects Android kernel implementations and has been catalogued with the Android ID A-210292376, indicating its severity and the need for immediate attention from device manufacturers and security teams. The flaw exists in the kernel-level USB gadget subsystem which handles the configuration and operation of USB devices when connected to host systems, making it a fundamental component of Android's device connectivity infrastructure.
The technical nature of this vulnerability stems from improper validation of flags used during USB gadget setup procedures. When the kernel processes USB gadget configuration requests, it fails to properly validate certain flag values before proceeding with memory operations. This inadequate flag checking allows an attacker to manipulate these values in such a way that subsequent memory writes exceed the intended buffer boundaries. The out-of-bounds write condition creates an opportunity for privilege escalation because the kernel's memory management mechanisms can be manipulated to overwrite critical kernel data structures or function pointers. This type of vulnerability falls under CWE-787: Out-of-bounds Write, which is classified as a high-severity issue in the Common Weakness Enumeration catalog and represents a well-known attack vector that has been exploited in numerous security incidents.
The operational impact of this vulnerability is significant as it enables local privilege escalation without requiring any additional execution privileges or user interaction for exploitation. This means that any process running with standard user privileges on an affected Android device could potentially leverage this flaw to gain kernel-level privileges, effectively compromising the entire system. The lack of user interaction requirement makes this vulnerability particularly dangerous as it can be exploited automatically without the need for social engineering or user deception. Attackers could potentially use this vulnerability to install persistent backdoors, modify system files, access sensitive data, or disable security features. The implications extend beyond individual device compromise to potential supply chain attacks where malicious actors could exploit this vulnerability in devices before they reach end users. According to ATT&CK framework, this vulnerability maps to T1068: Exploitation for Privilege Escalation, where the attacker leverages a software vulnerability to escalate privileges.
Mitigation strategies for this vulnerability should focus on immediate kernel updates and patches provided by device manufacturers and the Android team. The upstream kernel fix addresses the root cause by implementing proper flag validation before memory operations are performed. Organizations should prioritize applying the latest security patches to all affected Android devices and ensure that the kernel version running on these systems includes the necessary fixes. Additionally, system administrators should monitor for any unusual system behavior that might indicate exploitation attempts and implement network-based monitoring to detect potential abuse of this vulnerability. The fix typically involves strengthening input validation mechanisms within the USB gadget subsystem to prevent malformed flag values from triggering the out-of-bounds write condition. Regular security audits of kernel components and adherence to secure coding practices should be maintained to prevent similar vulnerabilities from emerging in the future. Device manufacturers should also consider implementing additional runtime protections and memory safety checks to further reduce the attack surface and provide defense-in-depth measures against similar exploitation techniques.