CVE-2021-3347 in Linux
Summary
by MITRE • 01/29/2021
An issue was discovered in the Linux kernel through 5.10.11. PI futexes have a kernel stack use-after-free during fault handling, allowing local users to execute code in the kernel, aka CID-34b1a1ce1458.
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Analysis
by VulDB Data Team • 02/25/2026
The vulnerability identified as CVE-2021-3347 represents a critical use-after-free condition within the Linux kernel's futex implementation, specifically affecting versions through 5.10.11. This flaw exists in the PI futexes (Priority Inheritance futexes) subsystem which is designed to handle priority inheritance protocols for thread synchronization. The vulnerability stems from improper memory management during fault handling scenarios where kernel stack memory is accessed after it has been freed, creating a potential exploitation vector for local attackers. The issue manifests as a kernel stack use-after-free condition that can be leveraged to execute arbitrary code with kernel privileges, fundamentally compromising system security.
The technical implementation of this vulnerability involves the interaction between the futex subsystem and the kernel's memory management during page fault handling. When a process attempts to access a futex structure that has been freed due to a race condition or improper cleanup, the kernel's fault handler may attempt to access freed memory on the kernel stack. This use-after-free condition occurs in the context of priority inheritance mechanisms where the kernel must manage complex synchronization states between threads. The flaw is particularly dangerous because it operates within kernel space, allowing attackers to escalate privileges from user mode to kernel mode without requiring special permissions or external exploitation vectors. The vulnerability is classified under CWE-416 as a use-after-free error, which is a well-known class of memory safety issues that frequently leads to privilege escalation and system compromise.
From an operational impact perspective, this vulnerability enables local users to achieve kernel code execution, which provides complete system compromise capabilities. Attackers can leverage this flaw to escalate privileges, install rootkits, modify system files, or extract sensitive information from the kernel memory space. The exploitation requires local access to the system, making it less severe than remote exploits but still highly dangerous in environments where untrusted users have access to system resources. The vulnerability affects systems using the Linux kernel with PI futexes enabled, which is standard for most modern distributions and applications relying on thread synchronization mechanisms. Organizations running affected kernel versions face significant risk, particularly in multi-user environments or systems with shared resources where privilege escalation could lead to complete system takeover.
Mitigation strategies for CVE-2021-3347 primarily involve applying kernel updates to versions that contain the necessary patches addressing the use-after-free condition in the futex subsystem. System administrators should prioritize updating to kernel versions 5.10.12 or later, which contain the appropriate fixes for this vulnerability. Additionally, implementing proper access controls and limiting local user privileges can reduce the attack surface, though this does not eliminate the vulnerability itself. Organizations should also monitor for any signs of exploitation attempts, as the use-after-free condition may manifest through system instability or unexpected behavior. The ATT&CK framework categorizes this vulnerability under privilege escalation techniques, specifically targeting the kernel mode execution capability. Regular system auditing and kernel security hardening measures should be implemented alongside patch management to maintain system integrity. The vulnerability demonstrates the critical importance of memory safety in kernel code and highlights the need for comprehensive security testing of core system components.