CVE-2009-2908 in Linux
Summary
by MITRE
The d_delete function in fs/ecryptfs/inode.c in eCryptfs in the Linux kernel 2.6.31 allows local users to cause a denial of service (kernel OOPS) and possibly execute arbitrary code via unspecified vectors that cause a "negative dentry" and trigger a NULL pointer dereference, as demonstrated via a Mutt temporary directory in an eCryptfs mount.
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Analysis
by VulDB Data Team • 08/23/2021
The vulnerability described in CVE-2009-2908 represents a critical flaw in the eCryptfs filesystem implementation within the Linux kernel version 2.6.31. This vulnerability resides in the d_delete function located in fs/ecryptfs/inode.c, which is responsible for handling directory entry deletion operations within the encrypted filesystem. The flaw manifests when specific conditions cause a negative dentry to be processed, leading to a NULL pointer dereference that can result in kernel crashes or potentially arbitrary code execution. The vulnerability was demonstrated through attacks targeting Mutt temporary directories within eCryptfs mounted filesystems, highlighting the practical attack surface of this flaw.
The technical implementation of this vulnerability stems from improper handling of directory entry states within the eCryptfs subsystem. When the d_delete function processes a negative dentry - which represents a directory entry that has been deleted but not yet fully removed from the system - the function fails to properly validate the dentry's state before attempting to access its associated data structures. This oversight creates a condition where a NULL pointer dereference occurs, causing the kernel to crash with an OOPS message and potentially allowing privilege escalation. The flaw operates at the kernel level, making it particularly dangerous as it can be exploited by local users with minimal privileges to either crash the system or potentially execute malicious code with kernel-level privileges.
The operational impact of this vulnerability extends beyond simple denial of service, as it represents a potential path to privilege escalation within Linux systems utilizing eCryptfs. Local attackers can exploit this vulnerability to cause system instability through kernel crashes, but more concerning is the potential for arbitrary code execution that could allow attackers to gain elevated privileges. The vulnerability affects systems running Linux kernel 2.6.31 and earlier versions where eCryptfs is enabled and used, making it particularly relevant to enterprise environments that rely on encrypted filesystems for data protection. The attack vector through Mutt temporary directories demonstrates how seemingly innocuous applications can serve as exploitation vectors for kernel-level vulnerabilities.
Mitigation strategies for CVE-2009-2908 should focus on immediate kernel updates to versions that contain the patched implementation of the d_delete function. System administrators should prioritize patching affected systems, as the vulnerability can be exploited without requiring special privileges or network access. Additionally, monitoring for unusual kernel OOPS messages and system crashes can help detect exploitation attempts. The vulnerability aligns with CWE-476 which describes NULL pointer dereference issues, and represents a classic example of improper null pointer validation in kernel code. From an ATT&CK perspective, this vulnerability maps to privilege escalation techniques and can be categorized under T1068 which covers "Exploitation for Privilege Escalation" and T1499 which covers "Endpoint Denial of Service" through kernel-level attacks. Organizations should implement comprehensive patch management policies and consider disabling eCryptfs if it is not essential to their operations, while also ensuring that all system components are regularly updated to address known vulnerabilities in kernel subsystems.