CVE-2023-38427: Linux Kernel Buffer Overflow Vulnerability

CVE-2023-38427 Overview

CVE-2023-38427 is a critical vulnerability discovered in the Linux kernel affecting the ksmbd (in-kernel SMB server) component. The flaw exists in the deassemble_neg_contexts function within fs/smb/server/smb2pdu.c, where an integer underflow condition leads to an out-of-bounds read. This vulnerability affects Linux kernel versions prior to 6.3.8 and can be exploited remotely over the network without requiring authentication or user interaction.

Critical Impact

Remote attackers can exploit this integer underflow vulnerability to read sensitive kernel memory, potentially leading to information disclosure, denial of service, or further exploitation for arbitrary code execution in kernel context.

Affected Products

• Linux Kernel (versions prior to 6.3.8)
• NetApp H300S
• NetApp H410S
• NetApp H500S
• NetApp H700S

Discovery Timeline

• 2023-07-18 - CVE-2023-38427 published to NVD
• 2025-05-05 - Last updated in NVD database

Technical Details for CVE-2023-38427

Vulnerability Analysis

The vulnerability resides in the ksmbd module, which provides in-kernel SMB3 server functionality for the Linux kernel. The deassemble_neg_contexts function in fs/smb/server/smb2pdu.c is responsible for parsing SMB2 negotiation contexts during the SMB connection establishment phase. Due to improper arithmetic operations when calculating buffer boundaries, an integer underflow can occur when processing maliciously crafted SMB2 negotiation requests.

When the underflow occurs, the resulting value wraps around to a large positive number, causing subsequent memory read operations to access memory beyond the intended buffer boundaries. This out-of-bounds read (CWE-125) can expose sensitive kernel memory contents to attackers and potentially crash the system.

Root Cause

The root cause of this vulnerability is insufficient validation of length fields during SMB2 negotiation context parsing. The deassemble_neg_contexts function performs arithmetic operations on user-controlled length values without properly checking for integer underflow conditions. When a malicious SMB client sends crafted negotiation packets with specially calculated length values, the subtraction operation results in an underflow, bypassing intended boundary checks and enabling out-of-bounds memory access.

Attack Vector

This vulnerability is exploitable over the network by any unauthenticated attacker who can establish a TCP connection to a system running the ksmbd service. The attack does not require any user interaction or prior privileges on the target system. An attacker can craft malicious SMB2 negotiation requests that trigger the integer underflow condition during the initial connection handshake phase.

The attack flow involves:

1. Establishing a TCP connection to the target SMB service (typically port 445)
2. Sending a crafted SMB2 NEGOTIATE request with manipulated context length fields
3. Triggering the integer underflow in deassemble_neg_contexts
4. Causing an out-of-bounds read that may leak kernel memory or crash the system

The vulnerability manifests during SMB2 negotiation context parsing in the deassemble_neg_contexts function. The flaw involves improper handling of length calculations that can result in integer underflow when processing maliciously crafted SMB packets. For technical details, refer to the Linux Kernel Commit f1a411873c.

Detection Methods for CVE-2023-38427

Indicators of Compromise

• Unusual SMB traffic patterns with malformed negotiation contexts on port 445
• Kernel crash dumps or oops messages referencing deassemble_neg_contexts or ksmbd functions
• Unexpected kernel memory access violations in ksmbd module
• System instability or unexpected reboots on systems running ksmbd service

Detection Strategies

• Monitor kernel logs for ksmbd-related errors, crashes, or out-of-bounds access warnings
• Implement network intrusion detection rules to identify malformed SMB2 NEGOTIATE packets
• Deploy endpoint detection solutions capable of monitoring kernel module behavior
• Use memory sanitizers (KASAN) in development/testing environments to detect out-of-bounds reads

Monitoring Recommendations

• Enable ksmbd debug logging to capture detailed SMB protocol parsing information
• Configure kernel crash dump collection for forensic analysis of potential exploitation attempts
• Monitor network traffic for anomalous SMB2 negotiation patterns from untrusted sources
• Implement alerting for kernel oops or panic events involving the ksmbd module

How to Mitigate CVE-2023-38427

Immediate Actions Required

• Update Linux kernel to version 6.3.8 or later immediately
• If immediate patching is not possible, disable the ksmbd kernel module using modprobe -r ksmbd
• Restrict network access to SMB services using firewall rules to trusted networks only
• Consider using userspace Samba instead of ksmbd if the in-kernel SMB server is not essential

Patch Information

The vulnerability has been addressed in Linux kernel version 6.3.8. The fix is documented in the Linux Kernel ChangeLog 6.3.8 and the specific commit can be found at Linux Kernel Commit f1a411873c. NetApp users should consult the NetApp Security Advisory NTAP-20230824-0011 for firmware updates.

Workarounds

• Disable ksmbd module if not actively required: modprobe -r ksmbd && echo "blacklist ksmbd" >> /etc/modprobe.d/blacklist.conf
• Implement network segmentation to limit SMB service exposure to trusted networks only
• Use iptables/nftables to restrict port 445 access to authorized clients
• Switch to userspace Samba server as an alternative to the in-kernel ksmbd implementation

# Disable ksmbd module and prevent auto-loading
modprobe -r ksmbd
echo "blacklist ksmbd" >> /etc/modprobe.d/blacklist.conf

# Restrict SMB access to trusted network (example)
iptables -A INPUT -p tcp --dport 445 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 445 -j DROP