CVE-2009-1630 in Linux
Summary
by MITRE
The nfs_permission function in fs/nfs/dir.c in the NFS client implementation in the Linux kernel 2.6.29.3 and earlier, when atomic_open is available, does not check execute (aka EXEC or MAY_EXEC) permission bits, which allows local users to bypass permissions and execute files, as demonstrated by files on an NFSv4 fileserver.
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Analysis
by VulDB Data Team • 09/03/2019
The vulnerability described in CVE-2009-1630 represents a critical authorization flaw within the Linux kernel's Network File System (NFS) client implementation. This issue specifically affects versions of the Linux kernel up to and including 2.6.29.3, where the nfs_permission function in fs/nfs/dir.c fails to properly validate execute permissions during file access operations. The flaw becomes particularly dangerous when atomic_open functionality is enabled, as this creates a pathway for local attackers to circumvent normal file permission controls. The vulnerability manifests when users attempt to execute files stored on NFSv4 file servers, allowing unauthorized execution of binaries or scripts that should otherwise be restricted based on their permission bits.
The technical root cause of this vulnerability lies in the improper implementation of permission checking within the NFS client code. When the nfs_permission function processes file access requests, it neglects to verify the execute permission bit (MAY_EXEC) that should normally be checked before allowing file execution. This oversight occurs specifically in the context of atomic_open operations, which are designed to provide atomic file creation and opening in a single system call. The missing execute permission check creates a fundamental security gap where files can be executed even when their permission bits indicate they should not be executable, effectively bypassing the standard Unix permission model that relies on read, write, and execute bits for access control.
The operational impact of this vulnerability extends beyond simple unauthorized file execution, as it fundamentally undermines the security model of NFS-mounted filesystems. Local users with access to systems running vulnerable kernel versions can exploit this flaw to execute arbitrary code on NFS-mounted filesystems, potentially escalating privileges or gaining access to sensitive data. This vulnerability is particularly concerning in enterprise environments where NFS is commonly used for file sharing, as it allows attackers to bypass traditional file permission controls that are expected to protect against unauthorized execution. The attack vector is relatively straightforward since it only requires local access to the system, making it a significant concern for environments where local privilege escalation is a risk. This flaw directly relates to CWE-284, which addresses improper access control, and represents a clear violation of the principle of least privilege in system security.
The exploitation of this vulnerability aligns with several techniques documented in the ATT&CK framework, particularly those related to privilege escalation and execution. Attackers can leverage this flaw to perform local privilege escalation by executing malicious code on NFS-mounted filesystems, potentially gaining higher privileges than initially granted. The vulnerability also intersects with techniques involving process injection and code execution, as it allows for the execution of arbitrary binaries without proper permission validation. Organizations using NFSv4 file systems in their infrastructure face significant risk from this vulnerability, as it can be exploited by both malicious insiders and external attackers who gain local access to systems. The impact is further amplified in environments where NFS is used for shared directories containing sensitive applications or system binaries, as these files could be executed without proper authorization.
Mitigation of this vulnerability requires immediate kernel updates to versions that contain the fix for the nfs_permission function implementation. System administrators should prioritize upgrading to kernel versions 2.6.30 or later, where the execute permission check has been properly implemented. Additionally, organizations should consider implementing network segmentation and access controls to limit local access to systems running NFS clients, as this reduces the attack surface for exploitation. Monitoring for unauthorized file execution on NFS-mounted filesystems should be enhanced, particularly in environments where local users have access to the systems. The fix implemented in the patched kernel versions ensures that execute permission bits are properly validated during atomic_open operations, restoring the expected security model for NFS file access. Organizations should also review their NFS configuration and ensure that appropriate access controls are in place to minimize the impact of potential exploitation, including regular security audits of NFS-mounted filesystems and proper permission management practices.