CVE-2024-56626: Linux Kernel Buffer Overflow Vulnerability

CVE-2024-56626 Overview

CVE-2024-56626 is an Out-of-Bounds Write vulnerability in the Linux kernel's ksmbd (kernel SMB daemon) component, specifically within the ksmbd_vfs_stream_write function. The vulnerability allows a client-provided offset to be interpreted as a negative value, enabling writes outside the bounds of an allocated buffer. This issue is triggered when the vfs objects = streams_xattr parameter is configured in ksmbd.conf.

Critical Impact

Local attackers with low privileges can exploit this vulnerability to achieve high impact on confidentiality, integrity, and availability through out-of-bounds memory writes, potentially leading to privilege escalation or system compromise.

Affected Products

• Linux Kernel (multiple versions)
• Linux Kernel 6.13-rc1
• Systems running ksmbd with streams_xattr VFS objects enabled

Discovery Timeline

• 2024-12-27 - CVE-2024-56626 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2024-56626

Vulnerability Analysis

This Out-of-Bounds Write vulnerability (CWE-787) exists in the ksmbd virtual filesystem layer, specifically in how the ksmbd_vfs_stream_write function handles client-supplied offset values. The function fails to properly validate that the offset parameter is non-negative before using it to calculate write positions within a buffer. When a malicious client supplies a negative offset value, the signed integer is used in pointer arithmetic, causing the write operation to target memory locations outside the intended buffer boundaries.

The vulnerability requires local access and low privileges to exploit, but once triggered, it can compromise the confidentiality, integrity, and availability of the affected system. The ksmbd daemon operates in kernel space, making any memory corruption particularly dangerous as it could lead to kernel-level code execution or privilege escalation.

Root Cause

The root cause is improper validation of the offset parameter in the ksmbd_vfs_stream_write function. The function accepts a client-provided offset value without checking if it is negative. In C, when a negative signed integer is used in array indexing or pointer arithmetic, it can result in accessing memory before the start of the buffer. This classic signed integer handling error allows the write operation to escape the bounds of the allocated buffer.

The vulnerability specifically manifests when ksmbd is configured with the vfs objects = streams_xattr parameter in ksmbd.conf, which enables alternate data stream support using extended attributes.

Attack Vector

The attack requires local access to the system with the ability to interact with the ksmbd SMB server. An attacker can craft malicious SMB requests containing negative offset values when writing to file streams. When the ksmbd daemon processes these requests, the negative offset causes the write operation to target memory outside the allocated buffer.

The exploitation process involves:

1. Establishing a connection to the ksmbd server
2. Initiating a stream write operation with a crafted negative offset value
3. The kernel writes data outside the buffer boundaries, corrupting adjacent memory structures
4. Depending on what memory is corrupted, this could lead to denial of service, privilege escalation, or arbitrary code execution in kernel context

Detection Methods for CVE-2024-56626

Indicators of Compromise

• Unexpected kernel panics or crashes related to ksmbd operations
• Anomalous memory corruption signatures in kernel logs
• System instability when SMB file streams are accessed
• Unusual ksmbd daemon behavior or unexpected process terminations

Detection Strategies

• Monitor kernel logs for ksmbd-related errors or memory corruption warnings using dmesg and journald
• Implement auditing rules to track SMB operations and identify anomalous stream write requests
• Deploy kernel-level monitoring to detect out-of-bounds memory access attempts
• Use SentinelOne's kernel-level protection to detect exploitation attempts targeting ksmbd

Monitoring Recommendations

• Enable detailed logging for ksmbd operations in /var/log/ or through systemd journal
• Monitor for unusual SMB traffic patterns that may indicate exploitation attempts
• Implement file integrity monitoring on critical system files that could be targeted post-exploitation
• Configure alerts for kernel oops or panics related to ksmbd module

How to Mitigate CVE-2024-56626

Immediate Actions Required

• Apply the official kernel patches from the Linux kernel stable branches immediately
• If patching is not immediately possible, disable ksmbd or remove the vfs objects = streams_xattr configuration
• Restrict local access to trusted users only until patches can be applied
• Monitor systems for signs of exploitation attempts

Patch Information

The Linux kernel development team has released patches across multiple stable branches. The following commits address CVE-2024-56626:

• Kernel Git Commit 164d35
• Kernel Git Commit 1aea5c
• Kernel Git Commit 313dab
• Kernel Git Commit 8cd749
• Kernel Git Commit c5797f

Debian users should refer to the Debian LTS Announcement for distribution-specific patch information.

Workarounds

• Disable ksmbd service if SMB functionality is not required: systemctl disable --now ksmbd
• Remove or comment out the vfs objects = streams_xattr parameter from ksmbd.conf
• Use alternative SMB implementations such as Samba until the kernel can be patched
• Implement network segmentation to limit access to SMB services from untrusted sources

# Disable ksmbd service as a temporary workaround
sudo systemctl stop ksmbd
sudo systemctl disable ksmbd

# Or modify ksmbd.conf to remove streams_xattr
# Comment out or remove the following line in /etc/ksmbd/ksmbd.conf:
# vfs objects = streams_xattr