CVE-2021-3739 in Linux
Summary
by MITRE • 03/10/2022
A NULL pointer dereference flaw was found in the btrfs_rm_device function in fs/btrfs/volumes.c in the Linux Kernel, where triggering the bug requires ‘CAP_SYS_ADMIN’. This flaw allows a local attacker to crash the system or leak kernel internal information. The highest threat from this vulnerability is to system availability.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 06/17/2025
The vulnerability identified as CVE-2021-3739 represents a critical NULL pointer dereference flaw within the Linux kernel's btrfs file system implementation. This issue resides in the btrfs_rm_device function located in the fs/btrfs/volumes.c source file, making it a targeted weakness within the kernel's storage subsystem. The vulnerability specifically affects systems utilizing the btrfs file system and demonstrates how even seemingly isolated kernel components can pose significant threats to overall system stability and security.
The technical nature of this flaw stems from improper validation of pointer values within the device removal process of the btrfs file system. When the btrfs_rm_device function executes, it fails to properly check whether certain pointer variables contain valid memory references before attempting to dereference them. This condition creates a scenario where a NULL pointer is accessed, leading to an immediate system crash or kernel panic. The vulnerability requires the attacker to possess CAP_SYS_ADMIN capabilities, which represents a privileged user context typically associated with system administration tasks, but this does not mitigate the severity of the potential impact.
From an operational perspective, this vulnerability presents a significant availability threat to affected systems. The local attacker with appropriate privileges can trigger a system crash that results in complete service disruption, forcing administrators to perform system restarts and potentially leading to data loss or corruption. Additionally, the flaw may enable information leakage mechanisms that could expose kernel memory contents to unauthorized parties, potentially revealing sensitive system internals that could be leveraged in subsequent attacks. The nature of the vulnerability means that any system running the affected kernel version with btrfs file systems is at risk when a privileged user attempts to execute the specific device removal operations.
The impact of CVE-2021-3739 aligns with CWE-476 which identifies NULL pointer dereference as a fundamental programming error that can lead to system instability and potential information disclosure. This vulnerability also maps to ATT&CK technique T1068 which describes the use of local system privileges to execute malicious code that can cause system compromise. Organizations should prioritize patching affected systems as the fix requires kernel updates that address the pointer validation logic in the btrfs subsystem. System administrators should also implement monitoring to detect unauthorized access attempts to system administration capabilities, as the vulnerability requires elevated privileges to exploit effectively. The patching process should be coordinated with business continuity planning to minimize disruption, particularly in mission-critical environments where btrfs file systems are actively utilized.