CVE-2023-38429: Linux Kernel ksmbd Memory Vulnerability

CVE-2023-38429 Overview

CVE-2023-38429 is an off-by-one error in the Linux kernel ksmbd SMB server before version 6.3.4. The flaw resides in fs/ksmbd/connection.c and stems from incorrect bounds enforcement within ksmbd_smb2_check_message. The miscalculation produces an out-of-bounds memory access when the kernel processes attacker-controlled SMB2 traffic. Because ksmbd runs in kernel space and is reachable over the network, exploitation can compromise confidentiality, integrity, and availability of the host. Linux distributions and storage appliances that embed ksmbd, including NetApp products referenced in advisory NTAP-20250103-0009, inherit the issue until patched to kernel 6.3.4 or later.

Critical Impact
A remote, unauthenticated attacker can trigger out-of-bounds access in the Linux kernel ksmbd module, potentially leading to denial of service or kernel memory corruption.

Affected Products

Linux kernel versions before 6.3.4 with ksmbd enabled
NetApp products bundling affected Linux kernel builds (see NTAP-20250103-0009)
Distributions and appliances exposing ksmbd SMB shares prior to upstream patch 443d61d1fa9f

Discovery Timeline

2023-07-18 - CVE-2023-38429 published to NVD
2025-01-03 - Last updated in NVD database following NetApp advisory

Technical Details for CVE-2023-38429

Vulnerability Analysis

The defect is an off-by-one error [CWE-193] in the SMB2 message validation path of the in-kernel ksmbd server. The function ksmbd_smb2_check_message undersizes a length check by one byte before memory allocation in fs/ksmbd/connection.c. Subsequent processing reads or writes one byte beyond the allocated buffer. Because the buffer holds attacker-supplied SMB2 payload data, the out-of-bounds access occurs against an adjacent kernel slab object. The defect is reachable over the network without authentication when ksmbd is exposed and accepting connections.

Root Cause

The root cause is an arithmetic mistake in length validation that propagates into the allocation size used for SMB2 message handling. The validator approves a payload whose true footprint exceeds the computed allocation by a single byte. The kernel then performs structured access on the resulting buffer using the attacker-controlled length, stepping outside its bounds. The patch in commit 443d61d1fa9f corrects the boundary check inside ksmbd_smb2_check_message.

Attack Vector

Attack vector is network-based and pre-authentication. An attacker sends a crafted SMB2 request to a listening ksmbd service on TCP port 445. The malformed message satisfies the flawed ksmbd_smb2_check_message validation, triggering the off-by-one allocation and the subsequent out-of-bounds access during message parsing. Impact ranges from kernel panic and denial of service to potential heap corruption that can be shaped into adjacent-object overwrite, depending on slab layout and allocator state.

No public proof-of-concept exploit is referenced in the available data. Technical details of the fix are available in the upstream commit and the Linux Kernel ChangeLog 6.3.4.

Detection Methods for CVE-2023-38429

Indicators of Compromise

Unexpected ksmbd kernel oops, BUG:, or KASAN: slab-out-of-bounds entries in dmesg or /var/log/kern.log
Crashes or restarts of the ksmbd.mountd user-space helper tied to inbound SMB2 traffic
Anomalous inbound SMB2 negotiate or session-setup messages to TCP/445 from untrusted networks

Detection Strategies

Inventory hosts running kernels below 6.3.4 with the ksmbd module loaded using lsmod | grep ksmbd and uname -r
Inspect SMB2 traffic for malformed message lengths in PDU headers using packet capture or IDS signatures targeting ksmbd_smb2_check_message
Correlate kernel-mode crashes with preceding SMB2 sessions in host telemetry to identify exploitation attempts

Monitoring Recommendations

Forward kernel logs and ksmbd audit events to a centralized log platform for retention and search
Alert on repeated SMB2 protocol errors or session terminations from a single source address
Monitor inbound TCP/445 exposure on Linux servers and flag deviations from baseline

How to Mitigate CVE-2023-38429

Immediate Actions Required

Upgrade the Linux kernel to version 6.3.4 or later containing commit 443d61d1fa9f
Apply vendor updates from distribution maintainers and from NetApp per advisory NTAP-20250103-0009
Restrict inbound TCP/445 to trusted network segments and authenticated peers

Patch Information

The upstream fix is included in Linux kernel 6.3.4 and is recorded in the Linux Commit fs/ksmbd Update. Distribution-specific backports are available through standard package repositories. NetApp customers should follow guidance in the NetApp Security Advisory NTAP-20250103-0009 to obtain fixed firmware and software releases.

Workarounds

Unload the ksmbd kernel module on systems that do not require an in-kernel SMB server using modprobe -r ksmbd
Stop and disable the ksmbd.service unit and remove the ksmbd-tools package where not needed
Block TCP/445 at perimeter and host firewalls until patches are deployed

bash

# Configuration example
# Disable and unload in-kernel ksmbd until patched
systemctl stop ksmbd.service
systemctl disable ksmbd.service
modprobe -r ksmbd
echo 'blacklist ksmbd' | sudo tee /etc/modprobe.d/blacklist-ksmbd.conf

# Restrict SMB exposure with nftables
nft add rule inet filter input tcp dport 445 ip saddr != 10.0.0.0/8 drop