CVE-2025-68786: Linux Kernel ksmbd Underflow Vulnerability

CVE-2025-68786 Overview

CVE-2025-68786 is an Integer Underflow vulnerability discovered in the Linux kernel's ksmbd (SMB3 kernel server) subsystem. The vulnerability exists in the lock-range checking logic where computing size - 1 can underflow when size equals zero, potentially leading to unexpected behavior in file locking operations.

Critical Impact
Integer underflow in ksmbd lock-range validation when file size is zero may cause undefined behavior in SMB3 file sharing operations.

Affected Products

Linux kernel with ksmbd (SMB3 kernel server) enabled
Systems running vulnerable kernel versions prior to patches

Discovery Timeline

January 13, 2026 - CVE-2025-68786 published to NVD
January 13, 2026 - Last updated in NVD database

Technical Details for CVE-2025-68786

Vulnerability Analysis

This vulnerability occurs in the ksmbd module, which provides in-kernel SMB3 file sharing server functionality for Linux systems. The flaw manifests when handling file lock range validation. Specifically, when processing a file where the current size (i_size) equals the requested size parameter, the code path invokes check_lock_range(filp, i_size, size - 1, WRITE).

When size equals zero, the computation of size - 1 results in an integer underflow. In unsigned integer arithmetic, subtracting 1 from 0 wraps around to the maximum value (such as 0xFFFFFFFF on 32-bit or 0xFFFFFFFFFFFFFFFF on 64-bit systems), potentially causing the lock range check to operate on an unexpectedly large range.

Root Cause

The root cause is missing boundary validation before performing arithmetic operations on the size parameter. The code did not account for the edge case where size equals zero before computing size - 1, leading to an unsigned integer underflow. The fix adds a conditional check to skip the lock-range validation entirely when the size equals the current i_size, avoiding the problematic subtraction operation.

Attack Vector

The attack vector for this vulnerability involves SMB3 file sharing operations. An attacker with access to an SMB3 share served by ksmbd could potentially trigger this condition by performing file operations on files with specific size characteristics, particularly zero-length files. The practical exploitability depends on how the underflowed value is used in subsequent lock range calculations.

The vulnerability mechanism involves the following logic flaw in the lock-range check function. When a file write operation is processed where both i_size and the requested size are zero, the call to check_lock_range() receives an underflowed end-range parameter. The fix implements an early return to skip the range check when size == i_size, preventing the arithmetic underflow from occurring. For full technical details, see the kernel patch 52fcbb92e0d3.

Detection Methods for CVE-2025-68786

Indicators of Compromise

Unusual SMB3 file locking behavior on zero-length files
Kernel log messages indicating invalid lock ranges in ksmbd operations
Unexpected ksmbd crashes or kernel panics during file operations

Detection Strategies

Monitor kernel logs for ksmbd-related error messages involving lock range operations
Audit systems running ksmbd for unpatched kernel versions
Implement network monitoring for anomalous SMB3 traffic patterns targeting file locking operations

Monitoring Recommendations

Enable kernel auditing for ksmbd module operations
Deploy file integrity monitoring on systems running ksmbd
Review SMB3 server access logs for suspicious file operation patterns involving zero-length files

How to Mitigate CVE-2025-68786

Immediate Actions Required

Apply the latest kernel patches that address CVE-2025-68786
If immediate patching is not possible, consider temporarily disabling ksmbd and using alternative SMB implementations (such as Samba in user space)
Restrict network access to SMB3 services to trusted hosts only

Patch Information

Multiple kernel patches have been released to address this vulnerability. The fix skips the lock-range check when size equals the current i_size, preventing the integer underflow condition. Available patches include:

Update your Linux kernel to a version containing one of these patches to remediate the vulnerability.

Workarounds

Disable ksmbd kernel module if SMB3 kernel server functionality is not required: modprobe -r ksmbd
Use Samba user-space implementation as an alternative SMB server
Implement network-level access controls to limit exposure of SMB services to trusted networks only

bash

# Disable ksmbd kernel module
modprobe -r ksmbd

# Prevent ksmbd from loading at boot
echo "blacklist ksmbd" >> /etc/modprobe.d/blacklist.conf

# Verify ksmbd is not loaded
lsmod | grep ksmbd