CVE-2009-4141 in Linux
Summary
by MITRE
Use-after-free vulnerability in the fasync_helper function in fs/fcntl.c in the Linux kernel before 2.6.33-rc4-git1 allows local users to gain privileges via vectors that include enabling O_ASYNC (aka FASYNC or FIOASYNC) on a locked file, and then closing this file.
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Analysis
by VulDB Data Team • 04/11/2025
The vulnerability described in CVE-2009-4141 represents a critical use-after-free condition within the Linux kernel's file descriptor management subsystem. This flaw exists in the fasync_helper function located in fs/fcntl.c, which handles asynchronous notification mechanisms for file operations. The vulnerability specifically impacts Linux kernel versions prior to 2.6.33-rc4-git1, making it a significant concern for systems running older kernel versions. The issue manifests when a local attacker manipulates file locking mechanisms combined with asynchronous notification settings, creating a scenario where freed memory can be accessed after deallocation.
The technical exploitation of this vulnerability involves a specific sequence of operations that triggers the use-after-free condition. An attacker must first establish a locked file descriptor and then enable the O_ASYNC flag, which activates asynchronous notification capabilities. When the file descriptor is subsequently closed, the underlying memory structures are freed but remain accessible through dangling pointers. This creates a window where malicious code can manipulate the freed memory, potentially leading to arbitrary code execution. The vulnerability is classified as a CWE-416 Use After Free, which is a well-documented class of memory safety issues that has been the subject of numerous security advisories and research papers.
From an operational perspective, this vulnerability presents a serious privilege escalation risk for local attackers who have access to the system. The use-after-free condition can be exploited to execute arbitrary code with kernel privileges, effectively allowing attackers to bypass user-space restrictions and gain root access to the compromised system. The attack requires local system access but does not need network connectivity, making it particularly dangerous in environments where local access is possible. The vulnerability's impact extends beyond simple privilege escalation as it can be leveraged to establish persistent backdoors, modify system files, or compromise the integrity of the entire operating system.
The mitigation strategies for CVE-2009-4141 primarily focus on kernel version updates and system hardening measures. Organizations should prioritize upgrading to Linux kernel versions 2.6.33-rc4-git1 or later, which contain the necessary patches to address the use-after-free condition. Additionally, system administrators should implement proper access controls to limit local user privileges and monitor for unusual file descriptor operations. The vulnerability aligns with ATT&CK technique T1068, which covers local privilege escalation through kernel vulnerabilities, and demonstrates how seemingly minor kernel flaws can have catastrophic security implications. Security monitoring should include detection of abnormal file locking and asynchronous notification patterns that could indicate exploitation attempts. The vulnerability also highlights the importance of comprehensive code review processes and memory safety testing in kernel development, as similar issues have been identified in other operating system components through systematic security analysis approaches.