CVE-2021-39635 in Android
Summary
by MITRE • 02/11/2022
ims_ex is a vendor system service used to manage VoLTE in unisoc devices?But it does not verify the caller's permissions?so that normal apps (No phone permissions) can obtain some VoLTE sensitive information and manage VoLTE calls.Product: AndroidVersions: Android SoCAndroid ID: A-206492634
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Analysis
by VulDB Data Team • 02/17/2022
The vulnerability identified as CVE-2021-39635 affects the ims_ex system service component within unisoc Android SoC devices, representing a critical authorization flaw that undermines the security boundaries of VoLTE (Voice over LTE) functionality. This service operates as a vendor-specific system component responsible for managing VoLTE operations, yet fails to properly validate caller permissions before exposing sensitive telephony functions. The flaw exists within the Android operating system framework where the ims_ex service lacks proper access control mechanisms, allowing unauthorized applications to interact with VoLTE management functions despite not possessing the necessary phone permissions typically required for such operations.
The technical implementation of this vulnerability stems from inadequate permission checking within the ims_ex service daemon, which operates at a system level with elevated privileges. When normal applications attempt to communicate with this service through inter-process communication mechanisms, the service fails to validate whether the calling application has legitimate authorization to access VoLTE management functions. This permission bypass enables malicious or benign applications to query and manipulate VoLTE call parameters, potentially accessing sensitive information such as call status, session details, and other telephony-related data that should only be accessible to system-level components or applications with explicit phone permissions.
The operational impact of this vulnerability extends beyond simple information disclosure, as it creates a potential attack surface for adversaries to manipulate VoLTE services without proper authorization. An attacker could leverage this flaw to monitor call activities, intercept VoLTE communications, or even disrupt ongoing calls by manipulating the underlying telephony infrastructure. The vulnerability affects Android SoC devices specifically, with the Android ID A-206492634 indicating a system-level issue within the unisoc chipset implementation. This presents a significant risk to user privacy and communication security, as sensitive VoLTE information becomes accessible to any application capable of reaching the ims_ex service, potentially exposing personal communication patterns and telephony data to unauthorized access.
Security implications of CVE-2021-39635 align with CWE-284 (Improper Access Control) and represent a direct violation of the principle of least privilege within the Android security model. The vulnerability enables privilege escalation through service misconfiguration, allowing applications to perform actions typically restricted to system components or applications with elevated permissions. From an ATT&CK framework perspective, this flaw maps to T1068 (Local Privilege Escalation) and T1566 (Phishing) as attackers could exploit this to gain unauthorized access to sensitive telephony functions. The issue demonstrates a failure in Android's security architecture where vendor-specific system services do not properly enforce the permission model that normally protects sensitive telephony functions from unauthorized access by regular applications.
Mitigation strategies should focus on implementing proper access control validation within the ims_ex service, requiring explicit phone permissions or system-level privileges before allowing access to VoLTE management functions. Device manufacturers should ensure that all system services properly validate caller identities and permissions through Android's permission system. Security updates should include code modifications to enforce mandatory access controls, ensuring that only applications with legitimate phone permissions or system-level components can interact with the ims_ex service. Additionally, regular security audits of vendor-specific system services should be conducted to identify similar permission bypass vulnerabilities that could compromise the integrity of telephony services and user communication privacy.