CVE-2023-6606: Linux Kernel SMB Client DoS Vulnerability

CVE-2023-6606 Overview

An out-of-bounds read vulnerability was discovered in the Linux Kernel's SMB client implementation, specifically within the smbCalcSize function located in fs/smb/client/netmisc.c. This vulnerability allows a local attacker with low privileges to read memory beyond the intended boundaries, potentially leading to system crashes (denial of service) or leakage of sensitive kernel information.

Critical Impact

Local attackers can exploit this vulnerability to crash affected systems or extract internal kernel memory contents, potentially exposing sensitive data such as cryptographic keys, credentials, or other privileged information.

Affected Products

• Linux Kernel versions prior to 6.4 and release candidates 6.4-rc4 through 6.7-rc6
• Red Hat Enterprise Linux 8.0 and 9.0
• Red Hat Enterprise Linux EUS 9.2 and 9.4
• Red Hat Enterprise Linux Server AUS 9.2 and 9.4
• Red Hat Enterprise Linux Server for Power Little Endian Update Services for SAP Solutions 9.2

Discovery Timeline

• December 8, 2023 - CVE-2023-6606 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-6606

Vulnerability Analysis

This out-of-bounds read vulnerability resides in the SMB (Server Message Block) client subsystem of the Linux Kernel. The flaw occurs within the smbCalcSize function, which is responsible for calculating the size of SMB protocol messages. When processing specially crafted SMB responses, the function fails to properly validate buffer boundaries before reading data.

The vulnerability requires local access to the system, meaning an attacker must have an authenticated session on the affected machine. However, the exploitation does not require user interaction and can be performed with low privileges. A successful exploit results in either confidential kernel memory being disclosed to the attacker or a kernel panic causing the system to crash.

The vulnerability is classified under CWE-125 (Out-of-bounds Read), indicating improper restriction of operations within the bounds of a memory buffer. This class of vulnerabilities is particularly dangerous in kernel space as it can expose sensitive kernel data structures and bypass security boundaries.

Root Cause

The root cause of CVE-2023-6606 lies in insufficient bounds checking within the smbCalcSize function when parsing SMB protocol messages. The function calculates message sizes based on fields within the SMB packet but fails to validate that the calculated offsets and lengths remain within the allocated buffer boundaries. This allows read operations to access memory locations beyond the intended buffer, leading to information disclosure or memory access violations.

Attack Vector

The attack vector is local, requiring the attacker to have valid credentials on the affected system. The attack can be initiated through:

1. Mounting a malicious SMB share that returns specially crafted responses
2. Interacting with the kernel's SMB client through privileged system calls
3. Triggering the vulnerable code path through legitimate SMB operations with manipulated data

The exploitation mechanism involves sending or receiving SMB protocol messages that cause the smbCalcSize function to read beyond buffer boundaries. When the kernel attempts to process these malformed messages, it either crashes due to accessing invalid memory or returns kernel memory contents that can be captured by the attacker.

Detection Methods for CVE-2023-6606

Indicators of Compromise

• Unexpected kernel panics or system crashes related to SMB/CIFS operations
• Kernel oops messages referencing smbCalcSize or fs/smb/client/netmisc.c
• Unusual SMB client activity from unprivileged processes
• Memory corruption warnings in kernel logs associated with CIFS/SMB subsystem

Detection Strategies

• Monitor kernel logs (dmesg) for oops or panic messages involving the SMB client subsystem
• Deploy kernel-level auditing to track SMB mount operations and client activity
• Use SentinelOne's Singularity platform to detect anomalous kernel behavior and memory access patterns
• Implement file integrity monitoring on critical kernel modules related to SMB functionality

Monitoring Recommendations

• Enable kernel auditing for SMB/CIFS-related system calls using auditd
• Configure centralized log collection to capture kernel messages from all affected systems
• Deploy endpoint detection and response (EDR) solutions capable of monitoring kernel-level operations
• Establish baseline behavior for SMB client operations to identify anomalous patterns

How to Mitigate CVE-2023-6606

Immediate Actions Required

• Update the Linux Kernel to a patched version as soon as available from your distribution
• Apply vendor-specific security patches from Red Hat, Debian, or other distributions
• Restrict local user access on affected systems to trusted users only
• Consider disabling SMB client functionality if not required for business operations

Patch Information

Security patches addressing CVE-2023-6606 have been released by major Linux distributions. Administrators should consult the following advisories for their specific distribution:

• Red Hat Security Advisory RHSA-2024:0723
• Red Hat Security Advisory RHSA-2024:0725
• Red Hat Security Advisory RHSA-2024:0881
• Red Hat Security Advisory RHSA-2024:1188
• Debian LTS Announcement January 2024

Additional technical details can be found in Kernel Bug Report #218218 and Red Hat Bug Report #2253611.

Workarounds

• Disable SMB client support by blacklisting the cifs kernel module if SMB functionality is not required
• Implement strict network segmentation to limit exposure to potentially malicious SMB servers
• Use mandatory access control systems (SELinux, AppArmor) to restrict SMB client operations
• Monitor and audit all SMB mount operations for suspicious activity

# Disable CIFS/SMB client module if not required
echo "blacklist cifs" >> /etc/modprobe.d/blacklist-cifs.conf
echo "install cifs /bin/false" >> /etc/modprobe.d/blacklist-cifs.conf

# Verify module is not loaded
lsmod | grep cifs

# If loaded, remove the module (may require unmounting SMB shares first)
modprobe -r cifs