CVE-2023-0210: Linux Kernel ksmbd DoS Vulnerability

CVE-2023-0210 Overview

A heap-based buffer overflow vulnerability exists in the Linux kernel's ksmbd (kernel SMB daemon) NTLMv2 authentication implementation. This flaw allows unauthenticated remote attackers to trigger an immediate system crash, resulting in a denial of service condition on Linux-based systems running the affected ksmbd module.

Critical Impact

Unauthenticated remote attackers can crash Linux systems by exploiting improper memory handling in the ksmbd NTLMv2 authentication process, causing immediate denial of service without requiring any user interaction.

Affected Products

• Linux Kernel (multiple versions with ksmbd module enabled)
• Linux-based systems utilizing ksmbd for SMB file sharing
• Enterprise and embedded Linux deployments with kernel SMB services

Discovery Timeline

• 2023-01-04 - Vulnerability disclosed on OpenWall OSS Security mailing list
• 2023-03-27 - CVE-2023-0210 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-0210

Vulnerability Analysis

The vulnerability resides in the ksmbd module's handling of NTLMv2 authentication requests. When processing authentication data, the ksmbd implementation fails to properly validate the size of incoming authentication tokens before copying them into kernel memory buffers. This results in a heap-based buffer overflow (CWE-122) and out-of-bounds write (CWE-787) condition.

The attack is particularly dangerous because it can be triggered remotely over the network without any authentication credentials. An attacker simply needs network access to the SMB service port (typically TCP 445) to send a maliciously crafted NTLMv2 authentication request that overflows the allocated buffer, corrupting adjacent heap memory and causing an immediate kernel panic.

Root Cause

The root cause is insufficient bounds checking in the ksmbd authentication handling code. When processing NTLMv2 authentication blobs, the code does not verify that the size of the authentication data fits within the allocated buffer before performing memory operations. This allows attackers to supply oversized authentication data that exceeds buffer boundaries, leading to heap corruption.

The vulnerability is classified under CWE-122 (Heap-based Buffer Overflow) and CWE-787 (Out-of-bounds Write), indicating that the flaw stems from improper memory safety practices in the authentication parsing routines.

Attack Vector

The attack can be executed remotely over the network without requiring any privileges or user interaction. An attacker with network access to the ksmbd service can craft a malicious SMB connection request containing an oversized NTLMv2 authentication blob.

When the vulnerable ksmbd module processes this request, the oversized authentication data overflows the heap buffer, corrupting kernel memory structures and triggering an immediate system crash. The attack requires no authentication, making it particularly dangerous for systems exposing SMB services to untrusted networks.

The vulnerability mechanism involves sending crafted SMB/CIFS packets with malformed NTLMv2 authentication tokens that exceed expected buffer sizes. The kernel's ksmbd module attempts to process these tokens without adequate size validation, resulting in heap memory corruption. Technical details are available in the OpenWall OSS Security Discussion and the cifsd Team Commit.

Detection Methods for CVE-2023-0210

Indicators of Compromise

• Unexpected kernel panics or system crashes on systems running ksmbd
• Suspicious SMB connection attempts with malformed authentication data on TCP port 445
• Kernel log entries indicating heap corruption or buffer overflow in ksmbd-related functions
• Unusual network traffic patterns targeting SMB services from external sources

Detection Strategies

• Monitor kernel logs for ksmbd-related crash dumps and heap corruption messages
• Implement network intrusion detection rules to identify malformed SMB/CIFS authentication packets
• Deploy endpoint detection solutions capable of monitoring kernel module behavior and crash events
• Analyze SMB traffic for anomalous NTLMv2 authentication blob sizes exceeding normal parameters

Monitoring Recommendations

• Enable detailed kernel logging for the ksmbd module to capture authentication processing events
• Configure network monitoring to alert on high volumes of failed SMB authentication attempts
• Implement system stability monitoring to detect unexpected reboots or kernel panics
• Review firewall logs for connection attempts to SMB ports from untrusted network segments

How to Mitigate CVE-2023-0210

Immediate Actions Required

• Update the Linux kernel to a patched version that includes the fix for CVE-2023-0210
• If updates cannot be applied immediately, disable the ksmbd module on affected systems
• Restrict network access to SMB services using firewall rules to trusted hosts only
• Consider switching to Samba user-space implementation as an alternative to ksmbd

Patch Information

The vulnerability has been addressed in upstream Linux kernel commits. The fix adds proper bounds checking for NTLMv2 authentication data before processing, preventing the heap buffer overflow condition.

Relevant patches are available from:

• Linux Kernel Commit ID #797805d
• cifsd Team Commit

Additionally, NetApp has released a Security Advisory for affected products.

Workarounds

• Disable the ksmbd kernel module if not required: modprobe -r ksmbd
• Block external access to SMB ports (TCP 445, TCP 139) at the network perimeter
• Use iptables or nftables rules to restrict SMB access to trusted IP addresses only
• Migrate SMB file sharing functionality to Samba user-space daemon as a temporary measure

# Configuration example
# Disable ksmbd module and prevent auto-loading
sudo modprobe -r ksmbd
echo "blacklist ksmbd" | sudo tee /etc/modprobe.d/blacklist-ksmbd.conf
sudo update-initramfs -u

# Restrict SMB access to trusted networks only
sudo iptables -A INPUT -p tcp --dport 445 -s 192.168.1.0/24 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 445 -j DROP