CVE-2021-39653 in Android
Summary
by MITRE • 12/15/2021
In (TBD) of (TBD), there is a possible way to boot with a hidden debug policy due to a missing warning to the user. This could lead to local escalation of privilege after preparing the device, hiding the warning, and passing the phone to a new user, with no additional execution privileges needed. User interaction is not needed for exploitation.Product: AndroidVersions: Android kernelAndroid ID: A-193443223References: N/A
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Analysis
by VulDB Data Team • 12/18/2021
The vulnerability identified as CVE-2021-39653 represents a critical security flaw in the Android kernel that allows for unauthorized privilege escalation through improper debug policy handling during device boot processes. This weakness specifically affects the kernel-level implementation of debug policy enforcement mechanisms, where the system fails to properly warn users about potential security implications when transitioning to debug modes. The vulnerability stems from insufficient user awareness protocols during device preparation phases, creating a scenario where security warnings can be effectively bypassed or hidden from end users.
The technical implementation of this flaw occurs at the kernel level where debug policy enforcement mechanisms lack proper user interaction requirements for critical security transitions. When a device is prepared for use, the system should enforce explicit user confirmation before activating debug policies that could compromise security boundaries. However, the missing warning mechanism allows for automatic activation of debug modes without user acknowledgment, effectively creating a backdoor pathway for privilege escalation. This issue is particularly concerning because it operates entirely within the kernel space, making it invisible to standard user-level security controls and detection mechanisms.
The operational impact of this vulnerability extends beyond simple privilege escalation to encompass complete system compromise potential. An attacker who gains physical access to a prepared device can exploit this flaw to gain root-level access without requiring any additional execution privileges or user interaction. The vulnerability becomes particularly dangerous when considering the scenario where a device is passed to a new user after preparation, as the hidden debug policy remains active and functional. This creates a persistent security risk that can be exploited multiple times without detection, as the original user would have no knowledge of the compromised state. The attack vector is particularly stealthy since it requires no user interaction and operates entirely through legitimate system preparation processes.
The security implications of CVE-2021-39653 align with CWE-693 (Protection Mechanism Failure) and can be mapped to ATT&CK technique T1068 (Local Privilege Escalation) within the adversary tactics framework. The vulnerability represents a fundamental failure in the security design principle of least privilege, where debug policies that should require explicit user consent are automatically enabled without proper warning mechanisms. This flaw violates core security assumptions about user awareness and explicit consent for critical system changes, creating an environment where security controls can be bypassed through simple preparation procedures.
Mitigation strategies for this vulnerability must focus on implementing mandatory user warnings and explicit consent mechanisms for debug policy activation. System administrators and device manufacturers should ensure that all debug policy transitions require explicit user acknowledgment before proceeding, with clear warnings about potential security implications. The kernel implementation should be modified to enforce these requirements regardless of preparation state, preventing automatic activation of debug modes without user interaction. Additionally, regular security audits should verify that debug policy warnings are properly displayed and cannot be bypassed through configuration modifications or system preparation processes. Organizations should also implement monitoring solutions to detect unauthorized debug policy activation attempts and establish procedures for device reconfiguration when such vulnerabilities are discovered.