CVE-2025-68767: Linux Kernel Privilege Escalation Flaw

CVE-2025-68767 Overview

A vulnerability has been identified in the Linux kernel's HFS+ filesystem driver where inadequate verification of inode mode bits when loading from disk can lead to corrupted or bogus S_IFMT bits in inode->i_mode. This flaw was discovered through syzbot fuzzing and affects how the kernel handles the 16-bit "mode" field from HFS+ filesystem images.

The vulnerability stems from insufficient validation when the kernel loads inode information from an HFS+ formatted disk. When the S_IFMT bits of the mode field are corrupted on disk, the kernel fails to detect this corruption, potentially leading to incorrect file type handling and unpredictable system behavior.

Critical Impact

Corrupted inode mode bits can cause the Linux kernel to misidentify file types, potentially leading to system instability or security bypass when processing maliciously crafted HFS+ filesystem images.

Affected Products

• Linux Kernel (HFS+ filesystem driver)
• Systems mounting HFS+ formatted volumes
• Any Linux distribution with HFS+ filesystem support enabled

Discovery Timeline

• 2026-01-13 - CVE CVE-2025-68767 published to NVD
• 2026-01-13 - Last updated in NVD database

Technical Details for CVE-2025-68767

Vulnerability Analysis

The vulnerability exists in the HFS+ filesystem driver's inode loading mechanism. According to historical documentation, the permissions field in HFS+ was treated as reserved in Mac OS 8 and 9, and this reserved field was explicitly initialized with 0. The field must remain 0 as long as it is reserved.

The kernel fails to properly validate the mode field when it transitions from reserved (value 0) to an active permissions field. When the mode field contains a non-zero value, the file type must be validated against the directory flag:

• If dir == 1, the file must be S_IFDIR (directory)
• If dir == 0, the file must be one of: S_IFREG (regular file), S_IFLNK (symbolic link), S_IFCHR (character device), S_IFBLK (block device), S_IFIFO (FIFO), or S_IFSOCK (socket)

Without this validation, corrupted or maliciously crafted mode bits can propagate into the kernel's internal inode representation, leading to type confusion.

Root Cause

The root cause is the lack of validation of the S_IFMT bits in the 16-bit mode field loaded from HFS+ disk structures. The HFS+ driver trusted the on-disk mode value without verifying that the file type bits were consistent with the directory flag (dir) stored in the HFS+ catalog record. This allows corrupted filesystem images to inject bogus file type information into the kernel.

Attack Vector

An attacker could exploit this vulnerability by crafting a malicious HFS+ filesystem image with corrupted inode mode fields. When a victim mounts this filesystem image (e.g., from a USB drive, disk image, or network share), the kernel would process the corrupted mode bits without validation. This could potentially be leveraged for:

• File type confusion attacks where the kernel treats a file as a different type than intended
• Potential privilege escalation if the kernel mishandles special file types
• System instability or crashes due to unexpected inode states

The attack requires the victim to mount an attacker-controlled HFS+ filesystem, which typically requires either physical access or the ability to provide disk images to the target system.

Detection Methods for CVE-2025-68767

Indicators of Compromise

• Unexpected kernel warnings or errors related to HFS+ filesystem operations
• System instability when mounting HFS+ formatted volumes
• Kernel log messages indicating inode mode inconsistencies in hfsplus module
• Unusual file type mismatches on mounted HFS+ volumes

Detection Strategies

• Monitor kernel logs (dmesg, /var/log/kern.log) for HFS+ driver warnings or errors
• Implement filesystem mount monitoring to track HFS+ volume mount operations
• Use integrity monitoring tools to detect unexpected filesystem behavior
• Deploy endpoint detection solutions capable of monitoring kernel module activity

Monitoring Recommendations

• Enable enhanced kernel logging for filesystem operations
• Monitor for unusual mount operations involving external or untrusted media
• Track kernel module loading, particularly the hfsplus module
• Implement alerting on kernel oops or panic events related to filesystem drivers

How to Mitigate CVE-2025-68767

Immediate Actions Required

• Update the Linux kernel to a patched version that includes the inode mode verification fix
• Avoid mounting untrusted HFS+ filesystem images until patching is complete
• If HFS+ support is not required, consider disabling or blacklisting the hfsplus kernel module
• Review and restrict mount permissions for non-privileged users

Patch Information

The Linux kernel development team has released patches to address this vulnerability. The fix adds proper verification of inode mode bits when loading from disk, ensuring that the S_IFMT bits are consistent with the directory flag. The following kernel git commits contain the fix:

• Kernel Git Commit 001f449
• Kernel Git Commit 005d4b0
• Kernel Git Commit 05ec9af
• Kernel Git Commit 91f114b
• Kernel Git Commit edfb2e6

Apply the appropriate patch for your kernel version or update to a kernel release that includes these fixes.

Workarounds

• Blacklist the hfsplus kernel module by adding blacklist hfsplus to /etc/modprobe.d/blacklist.conf if HFS+ support is not required
• Restrict mount operations to trusted administrators only
• Use mandatory access control (SELinux, AppArmor) to limit filesystem mount capabilities
• Implement strict policies for removable media and external filesystem access

# Disable HFS+ module loading (if not required)
echo "blacklist hfsplus" | sudo tee /etc/modprobe.d/blacklist-hfsplus.conf
sudo update-initramfs -u

# Verify module is not loaded
lsmod | grep hfsplus