CVE-2025-68761: Linux Kernel HFS Use-After-Free Flaw

CVE-2025-68761 Overview

A Use After Free vulnerability has been identified in the Linux kernel's HFS filesystem implementation. The vulnerability exists in the hfs_correct_next_unused_CNID() function, where the code incorrectly dereferences a node pointer after releasing its reference count through hfs_bnode_put(node). This creates a race condition where the memory backing the node structure may be freed or reallocated before the subsequent dereference occurs.

Critical Impact
This Use After Free vulnerability in the Linux kernel's HFS filesystem could potentially lead to memory corruption, kernel crashes, or privilege escalation attacks on systems that mount HFS-formatted media.

Affected Products

Linux kernel (HFS filesystem module)
Systems mounting HFS-formatted media
Linux distributions with HFS filesystem support enabled

Discovery Timeline

2026-01-05 - CVE CVE-2025-68761 published to NVD
2026-01-08 - Last updated in NVD database

Technical Details for CVE-2025-68761

Vulnerability Analysis

The vulnerability resides in the HFS filesystem's handling of Catalog Node IDs (CNIDs). In the hfs_correct_next_unused_CNID() function, improper ordering of operations creates a Use After Free condition. The code calls hfs_bnode_put(node) which decrements the reference count on a B-tree node. If this causes the reference count to reach zero, the node may be freed. However, the code subsequently attempts to access the node on the following line, accessing memory that may no longer be valid.

This type of vulnerability is particularly dangerous in kernel context because it can lead to arbitrary memory access, kernel panic conditions, or potentially be leveraged for privilege escalation if an attacker can control the contents of the freed memory region before the dangling dereference occurs.

Root Cause

The root cause is an incorrect ordering of operations in hfs_correct_next_unused_CNID(). Reference-counted objects must not be accessed after their reference is released. The original code violated this fundamental principle by:

Calling hfs_bnode_put(node) to release the reference
Dereferencing the node pointer on the subsequent line

The fix involves reordering these operations so that all necessary accesses to the node occur while the reference is still held.

Attack Vector

Exploitation of this vulnerability requires the ability to mount an HFS-formatted filesystem on the target system. An attacker could craft a malicious HFS image designed to trigger the vulnerable code path and manipulate memory allocation timing to control the contents of the freed memory region. While direct remote exploitation is unlikely, local attackers with the ability to mount filesystems or introduce removable media could potentially leverage this vulnerability.

The fix is straightforward - the two lines of code simply needed to be reordered so that any dereference of node occurs before calling hfs_bnode_put(node). This ensures the reference is held during all accesses to the node structure.

Detection Methods for CVE-2025-68761

Indicators of Compromise

Unexpected kernel panics or oops messages referencing HFS or B-tree node operations
System crashes when mounting or accessing HFS-formatted media
Kernel log messages indicating memory corruption in HFS-related code paths
Unusual memory access patterns in kernel address space related to filesystem operations

Detection Strategies

Monitor kernel logs for HFS filesystem-related errors or warnings using dmesg filtering
Implement kernel crash dump analysis to identify exploitation attempts
Deploy kernel-level memory corruption detection tools such as KASAN
Review system audit logs for suspicious HFS mount operations

Monitoring Recommendations

Enable and monitor kernel address sanitizer (KASAN) in development/testing environments
Configure alerting on kernel panic events, particularly those involving filesystem subsystems
Implement integrity monitoring for kernel modules, especially hfs.ko
Monitor for unexpected HFS filesystem mount operations on critical systems

How to Mitigate CVE-2025-68761

Immediate Actions Required

Apply the kernel patches from the stable kernel tree immediately
Disable HFS filesystem support if not required by blacklisting the hfs module
Restrict the ability to mount filesystems to trusted users only
Audit systems for any mounted HFS filesystems and assess necessity

Patch Information

The Linux kernel maintainers have released patches to address this vulnerability. The fix reorders the operations in hfs_correct_next_unused_CNID() to ensure all node dereferences occur before releasing the reference count.

Patches are available from the stable kernel git repository:

System administrators should update to the latest stable kernel version that includes these commits.

Workarounds

Blacklist the HFS kernel module to prevent automatic loading: add blacklist hfs to /etc/modprobe.d/blacklist.conf
Remove the HFS module if already loaded using modprobe -r hfs (requires no mounted HFS filesystems)
Restrict mount capabilities using SELinux or AppArmor policies
Implement strict control over removable media that might contain HFS filesystems

bash

# Configuration example
# Disable HFS filesystem module loading
echo "blacklist hfs" >> /etc/modprobe.d/blacklist.conf
echo "install hfs /bin/false" >> /etc/modprobe.d/blacklist.conf

# Remove the module if currently loaded
modprobe -r hfs 2>/dev/null || echo "HFS module not loaded or in use"

# Verify module is not loaded
lsmod | grep hfs