CVE-2025-68764: Linux Kernel NFS Mount Flag Vulnerability

CVE-2025-68764 Overview

A vulnerability has been resolved in the Linux kernel affecting NFS (Network File System) automounted filesystems. The issue involves the failure of automounted filesystems to properly inherit critical superblock mount options such as ro (read-only), noexec, nodev, and sync flags from the parent mount. When a filesystem is automounted, it needs to preserve user-set superblock mount options, but this security constraint was not being enforced correctly.

Critical Impact

Security-sensitive mount flags may not be inherited by automounted NFS filesystems, potentially allowing execution of binaries, device access, or write operations on mounts intended to be restricted.

Affected Products

• Linux kernel (NFS subsystem)
• Systems utilizing NFS automount functionality
• Enterprise Linux distributions with NFS client configurations

Discovery Timeline

• 2026-01-05 - CVE CVE-2025-68764 published to NVD
• 2026-01-08 - Last updated in NVD database

Technical Details for CVE-2025-68764

Vulnerability Analysis

This vulnerability exists within the Linux kernel's NFS client implementation, specifically in the automount handling code path. When an NFS filesystem triggers an automount operation (such as when traversing into a submount), the kernel should preserve and apply the same security-relevant mount flags that were specified on the parent filesystem.

The affected mount flags include:

• ro - Read-only mount, preventing write operations
• noexec - Prevents execution of binaries from the mounted filesystem
• nodev - Prevents interpretation of device special files
• sync - Forces synchronous I/O operations

Without proper inheritance of these flags, automounted child filesystems could operate with more permissive settings than intended by the system administrator, creating a security gap in the expected access controls.

Root Cause

The root cause of this vulnerability lies in the NFS automount code path not properly propagating user-set superblock mount options to child mounts. When the kernel processes an automount request for an NFS filesystem, the mount flag inheritance logic failed to copy these security-critical options from the parent mount context to the new mount being created.

This oversight means that even if an administrator explicitly mounted an NFS export with restrictive options like ro,noexec,nodev, any submounts triggered by automount would not automatically inherit these restrictions.

Attack Vector

The attack vector for this vulnerability is local in nature. An attacker with access to a system with misconfigured NFS automounts could potentially exploit the missing mount flag inheritance by:

1. Navigating to trigger an automount on a submount path
2. Exploiting the lack of inherited restrictions on the newly mounted filesystem
3. If noexec was intended but not inherited, executing malicious binaries from the NFS share
4. If ro was intended but not inherited, writing malicious content to the filesystem
5. If nodev was intended but not inherited, potentially accessing device nodes inappropriately

The vulnerability requires the target system to be using NFS with automount functionality and to have security-relevant mount flags that should be inherited by submounts.

Detection Methods for CVE-2025-68764

Indicators of Compromise

• Unexpected write operations occurring on NFS mounts that were intended to be read-only
• Binary execution from NFS mounts configured with noexec flag on parent
• Device file access on NFS mounts where nodev was expected
• Mount flag discrepancies between parent and child automounted filesystems visible in /proc/mounts

Detection Strategies

• Monitor /proc/mounts or mount output for NFS automounted filesystems with inconsistent mount flags compared to their parent mounts
• Audit kernel logs for NFS automount events and correlate with expected mount flag configurations
• Implement file integrity monitoring on NFS mounts to detect unexpected modifications on intended read-only shares
• Use system auditing tools to track execution events from NFS-mounted filesystems

Monitoring Recommendations

• Enable kernel auditing for mount operations using the audit subsystem
• Configure alerts for any process execution originating from NFS mount paths that should have noexec enabled
• Periodically verify mount flags on all NFS mounts including automounted children
• Monitor for unexpected file system modifications on NFS shares intended as read-only

How to Mitigate CVE-2025-68764

Immediate Actions Required

• Apply the latest kernel security patches that address this NFS automount issue
• Review all NFS mount configurations to identify mounts using security-relevant flags (ro, noexec, nodev, sync)
• Consider disabling NFS automount functionality temporarily if security-critical mount flags are in use
• Manually verify mount flags on existing NFS mounts and remount with explicit flags if necessary

Patch Information

The Linux kernel development team has released patches to address this vulnerability. Multiple commits have been applied to the stable kernel trees:

• Linux Kernel Commit 4b296944e632
• Linux Kernel Commit 612cc98698d6
• Linux Kernel Commit 8675c69816e4
• Linux Kernel Commit df9b003a2eca

System administrators should update to a kernel version containing these fixes from their distribution's package repositories.

Workarounds

• Disable NFS automount functionality by removing or commenting out automount entries in /etc/auto.* configuration files
• Explicitly mount all NFS submounts with the required security flags rather than relying on automount inheritance
• Use autofs configuration to explicitly specify mount flags for each submount entry
• Implement network-level access controls to restrict NFS access to trusted hosts only

# Configuration example
# Verify current NFS mount flags
mount | grep nfs

# Remount NFS share with explicit security flags
mount -o remount,ro,noexec,nodev,sync /mnt/nfs_share

# Check autofs configuration for explicit flag settings
# In /etc/auto.nfs, ensure explicit flags:
# /mnt/nfs_submount  -ro,noexec,nodev,sync  nfs_server:/export/path