CVE-2026-31410: Linux Kernel ksmbd UUID Vulnerability

CVE-2026-31410 Overview

CVE-2026-31410 is a vulnerability in the Linux kernel's ksmbd (in-kernel SMB3 server) component that affects how volume identifiers are handled in FS_OBJECT_ID_INFORMATION requests. The vulnerability involves improper volume identifier management where the kernel module did not properly use the filesystem's UUID (sb->s_uuid) as the primary identifier, potentially leading to information handling issues in SMB3 file sharing operations.

Critical Impact

Improper volume identification in ksmbd could affect SMB3 file sharing integrity and potentially expose filesystem metadata to connected clients.

Affected Products

• Linux Kernel (ksmbd module)
• Linux systems running in-kernel SMB3 server

Discovery Timeline

• 2026-04-06 - CVE CVE-2026-31410 published to NVD
• 2026-04-07 - Last updated in NVD database

Technical Details for CVE-2026-31410

Vulnerability Analysis

This vulnerability exists in the Linux kernel's ksmbd module, which provides an in-kernel SMB3 server implementation. The issue stems from how the module handles FS_OBJECT_ID_INFORMATION requests from SMB clients. Prior to the fix, the volume identifier was not being properly populated using the filesystem's native UUID (sb->s_uuid), which could result in inconsistent or improper volume identification responses to SMB3 clients.

The patch introduces a two-tier approach for volume identification: it primarily uses sb->s_uuid (the superblock's UUID) for filesystems that provide this value, and falls back to stfs.f_fsid obtained from vfs_statfs() for filesystems that do not supply a UUID.

Root Cause

The root cause of this vulnerability is the improper handling of volume identifiers in the ksmbd module's FS_OBJECT_ID_INFORMATION implementation. The module was not leveraging the filesystem's superblock UUID (sb->s_uuid) as the primary source for volume identification, which could lead to inconsistent behavior when responding to SMB3 client queries for filesystem object information.

Attack Vector

The attack vector for this vulnerability is through the SMB3 protocol. An authenticated client connecting to a ksmbd-hosted share could send FS_OBJECT_ID_INFORMATION requests and receive improper volume identifier responses. While the direct security impact requires further analysis, improper volume identification could affect filesystem tracking, caching behavior, or metadata handling in networked environments.

The vulnerability is addressed by ensuring that the filesystem's native UUID is used as the authoritative volume identifier, with a fallback mechanism for filesystems that don't provide UUIDs.

Detection Methods for CVE-2026-31410

Indicators of Compromise

• Unusual FS_OBJECT_ID_INFORMATION query patterns from SMB3 clients
• Inconsistent volume identifier responses logged in ksmbd debug output
• SMB3 clients reporting volume identification mismatches or errors

Detection Strategies

• Monitor ksmbd logs for abnormal filesystem information request patterns
• Review SMB3 audit logs for FS_OBJECT_ID_INFORMATION queries
• Implement kernel module version tracking to identify unpatched ksmbd instances

Monitoring Recommendations

• Enable ksmbd debugging to capture FS_OBJECT_ID_INFORMATION handling details
• Monitor SMB3 connection logs for suspicious client behavior
• Track kernel version and ksmbd module updates across Linux systems

How to Mitigate CVE-2026-31410

Immediate Actions Required

• Update the Linux kernel to a version containing the fix commits
• Review ksmbd configuration and consider disabling the service if not required
• Monitor SMB3 access logs for any unusual activity

Patch Information

The vulnerability has been addressed through multiple commits to the Linux kernel stable branches. The fixes ensure proper use of sb->s_uuid for volume identification in FS_OBJECT_ID_INFORMATION responses.

Relevant patch commits are available in the kernel Git repository:

• Kernel Git Commit 3a64125730ca
• Kernel Git Commit 3d80ebe6d1b7
• Kernel Git Commit c283a6ffe6d5
• Kernel Git Commit ce00616bc1df

Workarounds

• Disable the ksmbd kernel module if in-kernel SMB3 functionality is not required
• Use user-space Samba implementation as an alternative to ksmbd
• Implement network-level access controls to restrict SMB3 access to trusted clients
• Consider isolating systems running ksmbd to dedicated network segments

# Disable ksmbd module if not required
sudo modprobe -r ksmbd

# Prevent ksmbd from loading on boot
echo "blacklist ksmbd" | sudo tee /etc/modprobe.d/blacklist-ksmbd.conf

# Check current kernel version for patch status
uname -r