CVE-2007-3740 in Linux
Summary
by MITRE
The CIFS filesystem in the Linux kernel before 2.6.22, when Unix extension support is enabled, does not honor the umask of a process, which allows local users to gain privileges.
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Analysis
by VulDB Data Team • 07/26/2019
The vulnerability described in CVE-2007-3740 represents a critical access control flaw within the Linux kernel's CIFS filesystem implementation that persisted through versions prior to 2.6.22. This issue specifically affects systems utilizing the Common Internet File System protocol with Unix extension support enabled, creating a scenario where the kernel fails to respect the umask settings that are fundamental to process-level file permission management. The flaw exists at the intersection of network filesystem handling and local privilege escalation mechanisms, making it particularly dangerous in multi-user environments where CIFS shares are commonly accessed.
The technical root cause of this vulnerability lies in the improper implementation of file permission handling within the CIFS filesystem driver when Unix extensions are enabled. Normally, the umask value acts as a filter that determines which permission bits are stripped from newly created files and directories, ensuring that processes cannot inadvertently create overly permissive files. However, the CIFS implementation fails to properly apply the umask value when creating files or directories through the network filesystem, effectively bypassing the standard permission control mechanisms. This behavior creates a situation where files created via CIFS shares inherit default permissions rather than respecting the process's umask settings, potentially allowing local users to create files with unintended access rights.
The operational impact of this vulnerability extends beyond simple permission mismanagement and creates significant local privilege escalation opportunities. When a local user can manipulate file creation permissions through CIFS shares, they may be able to create files with world-writable permissions or other insecure configurations that could be exploited to gain elevated privileges. This issue particularly affects systems where users have access to CIFS shares and where Unix extensions are enabled, which is common in enterprise environments that integrate Linux systems with Windows file servers. The vulnerability essentially undermines the principle of least privilege by allowing unauthorized modification of file access controls, potentially enabling attackers to establish persistent access or escalate their privileges within the system.
The flaw aligns with CWE-276, which describes improper file permissions, and can be mapped to ATT&CK technique T1068, which involves local privilege escalation through improper file permissions. Organizations utilizing Linux systems with CIFS support must consider this vulnerability as part of their broader security posture assessment, particularly in environments where multiple users access shared resources through network filesystems. The vulnerability demonstrates how seemingly minor implementation details in kernel subsystems can have cascading security implications, affecting not just file access but potentially system integrity and user isolation mechanisms.
Mitigation strategies for CVE-2007-3740 focus primarily on upgrading to kernel versions 2.6.22 or later where the vulnerability has been addressed through proper umask implementation. System administrators should also consider disabling Unix extension support in CIFS mounts when it is not required, as this removes the attack surface entirely. Additionally, implementing proper access controls and monitoring for unauthorized file creation in shared directories can help detect exploitation attempts. Organizations should conduct comprehensive vulnerability assessments to identify systems running affected kernel versions and ensure that all CIFS implementations properly respect umask settings to prevent unauthorized privilege escalation through file permission manipulation.