CVE-2021-4197 in Linux
Summary
by MITRE • 03/24/2022
An unprivileged write to the file handler flaw in the Linux kernel's control groups and namespaces subsystem was found in the way users have access to some less privileged process that are controlled by cgroups and have higher privileged parent process. It is actually both for cgroup2 and cgroup1 versions of control groups. A local user could use this flaw to crash the system or escalate their privileges on the system.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 05/18/2025
The vulnerability identified as CVE-2021-4197 represents a critical flaw within the Linux kernel's control groups and namespaces subsystem that affects both cgroup1 and cgroup2 implementations. This security issue resides in the handling of file descriptors and process management within the kernel's resource control mechanisms, specifically impacting how unprivileged users can interact with processes that are managed through control groups. The flaw manifests when users have access to less privileged processes that are controlled by cgroups while those processes have higher privileged parent processes, creating an exploitable condition that can be leveraged for privilege escalation or system disruption.
The technical implementation of this vulnerability stems from improper validation of file handler access within the kernel's control groups subsystem. When processes are managed through cgroup boundaries, the kernel must properly enforce access controls to prevent unauthorized operations across privilege levels. This particular flaw allows an unprivileged user to manipulate file descriptors associated with processes running under different privilege contexts, effectively bypassing expected security boundaries. The issue is particularly concerning because it affects the fundamental kernel subsystems that manage process isolation and resource allocation, making it a core component of the system's security architecture.
The operational impact of CVE-2021-4197 extends beyond simple privilege escalation to include potential system instability and complete system compromise. An attacker with local access can exploit this vulnerability to either crash the system through carefully crafted operations that trigger kernel memory corruption or escalate their privileges to root level access, thereby gaining complete control over the affected system. This vulnerability directly violates the principle of least privilege that is fundamental to operating system security models and can be exploited in various attack scenarios including container escape, privilege escalation in shared hosting environments, and local privilege escalation attacks. The dual impact on both cgroup1 and cgroup2 implementations means that the vulnerability affects a broad range of Linux distributions and kernel versions that utilize these control group mechanisms.
Mitigation strategies for CVE-2021-4197 primarily focus on kernel updates and system hardening measures. Organizations should immediately apply the relevant kernel patches that address this specific vulnerability, as provided by their Linux distribution vendors. System administrators should also implement additional security controls such as restricting local user access where possible, monitoring for suspicious process behavior, and ensuring that control groups are properly configured with appropriate access controls. The vulnerability aligns with CWE-284 which addresses improper access control in software systems, and can be categorized under ATT&CK technique T1068 which covers local privilege escalation through kernel exploits. Additionally, the flaw demonstrates characteristics of privilege escalation through process control mechanisms, making it particularly dangerous in environments where multiple users share system resources or where containerized applications are deployed.