CVE-2021-39802 in Android
Summary
by MITRE • 04/12/2022
In change_pte_range of mprotect.c , there is a possible way to make a shared mmap writable due to a permissions bypass. 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-213339151References: Upstream kernel
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Analysis
by VulDB Data Team • 04/19/2022
The vulnerability identified as CVE-2021-39802 resides within the Android kernel's memory management subsystem, specifically in the change_pte_range function located in mprotect.c. This flaw represents a critical permissions bypass that allows malicious actors to manipulate memory mapping permissions for shared memory regions. The vulnerability stems from improper validation of memory protection flags during page table entry modifications, creating an avenue for unauthorized access to shared memory mappings that should remain read-only or protected from write operations. The issue affects Android kernel implementations and was tracked under Android ID A-213339151, highlighting its significance within the mobile operating system ecosystem.
The technical implementation of this vulnerability occurs when the kernel processes memory protection changes through the mprotect system call. During the change_pte_range function execution, the kernel fails to properly validate whether the requested memory mapping permissions align with the existing shared memory characteristics. This validation gap allows an attacker to bypass normal memory protection mechanisms and elevate write permissions on shared memory mappings that were originally configured as read-only or read-write protected. The flaw essentially enables a malicious process to modify memory regions that should be protected from write access, potentially leading to data corruption or code injection opportunities. This represents a direct violation of the kernel's memory management security model and can be exploited without requiring any additional privileges or user interaction.
The operational impact of CVE-2021-39802 extends beyond simple privilege escalation, as it provides a pathway for local attackers to gain elevated system privileges without requiring additional execution privileges or user interaction. This vulnerability directly maps to the attack pattern described in the MITRE ATT&CK framework under privilege escalation techniques, specifically targeting kernel-level vulnerabilities that can be exploited for system compromise. The implications are particularly severe in Android environments where the kernel serves as the primary security boundary between user applications and system resources. An attacker could leverage this vulnerability to modify critical system memory regions, potentially gaining access to sensitive data, bypassing security controls, or executing arbitrary code with kernel-level privileges. The vulnerability's classification aligns with CWE-284 (Improper Access Control) and CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization) as it involves improper permission handling and memory sharing mechanisms within the kernel's memory management subsystem.
Mitigation strategies for CVE-2021-39802 should focus on implementing proper kernel memory protection validation and ensuring that shared memory mappings maintain their intended access controls. System administrators should prioritize applying the upstream kernel patches that address this specific vulnerability in change_pte_range function. Additionally, organizations should consider implementing memory protection mechanisms such as kernel page table isolation, memory management unit (MMU) hardening, and runtime memory access monitoring. The vulnerability's exploitation requires no user interaction, making it particularly dangerous as it can be triggered automatically when processes attempt to modify shared memory mappings. Security teams should monitor for suspicious memory management activities and implement comprehensive logging of mprotect system calls to detect potential exploitation attempts. The fix should be applied across all Android kernel versions and devices that are susceptible to this specific permissions bypass vulnerability.