CVE-2024-26828: Linux Kernel Buffer Overflow Vulnerability

CVE-2024-26828 Overview

CVE-2024-26828 is an integer underflow vulnerability in the Linux kernel's CIFS (Common Internet File System) implementation. The vulnerability exists in the parse_server_interfaces() function where a type mismatch between ssize_t (signed) and size_t (unsigned) variables leads to improper bounds checking. When the bytes_left variable becomes negative, the comparison with sizeof() is performed as an unsigned comparison due to type promotion, causing the loop to continue processing when it should terminate.

Critical Impact

An attacker on an adjacent network with low privileges could exploit this integer underflow to cause high impact to system integrity and availability through memory corruption or denial of service conditions.

Affected Products

• Linux Kernel versions prior to patched releases
• Linux Kernel 6.8 RC1 through RC4
• Systems using CIFS/SMB file sharing with affected kernel versions

Discovery Timeline

• April 17, 2024 - CVE-2024-26828 published to NVD
• April 8, 2025 - Last updated in NVD database

Technical Details for CVE-2024-26828

Vulnerability Analysis

This vulnerability is classified as CWE-191 (Integer Underflow). The flaw resides in the CIFS subsystem's server interface parsing code, specifically within the parse_server_interfaces() function. The root cause is a signed/unsigned type mismatch that leads to incorrect loop termination behavior.

During normal operation, the function iterates through a buffer containing server interface information. After processing each item, the code checks if bytes_left (of type ssize_t, a signed integer) is greater than the minimum required size. However, when this value is compared against sizeof() (which returns size_t, an unsigned type), C's type promotion rules convert the signed value to unsigned. If bytes_left becomes negative (which should signal the end of valid data), the unsigned conversion results in a large positive number, causing the comparison to succeed and the loop to continue inappropriately.

This can lead to out-of-bounds memory access, processing of invalid data, potential memory corruption, and system instability or denial of service conditions.

Root Cause

The vulnerability stems from a type mismatch in the boundary checking logic. The bytes_left variable is declared as ssize_t (signed), while the size comparison uses sizeof() which returns size_t (unsigned). When performing the comparison bytes_left > sizeof(minimum_structure), the C language's implicit type promotion converts the signed bytes_left to unsigned. A negative value like -1 becomes a very large unsigned value (e.g., 0xFFFFFFFFFFFFFFFF on 64-bit systems), which is always greater than any reasonable minimum size, causing the loop to continue when it should exit.

Attack Vector

The attack requires adjacent network access (AV:A), meaning the attacker must be on the same network segment or have compromised a system that can reach the target. The attacker would need to manipulate CIFS/SMB server interface responses to trigger the underflow condition. This could be achieved by:

1. Setting up a malicious SMB server on the network
2. Crafting server interface responses with carefully calculated sizes
3. Causing the target system's CIFS client to process the malicious response
4. Triggering the integer underflow during interface parsing

The vulnerability requires low privileges and user interaction to exploit, but can result in high impact to both integrity and availability of the affected system.

Detection Methods for CVE-2024-26828

Indicators of Compromise

• Unexpected CIFS/SMB connection failures or kernel warnings related to CIFS operations
• Kernel log messages indicating memory access violations in the CIFS subsystem
• System crashes or instability when connecting to SMB file shares
• Unusual network traffic patterns involving SMB/CIFS protocols from untrusted sources

Detection Strategies

• Monitor kernel logs for CIFS-related errors, particularly in the parse_server_interfaces() function or related code paths
• Implement network monitoring to detect anomalous SMB/CIFS traffic patterns from unexpected sources
• Use kernel-level introspection tools to detect memory corruption attempts in the CIFS subsystem
• Deploy endpoint detection capabilities to identify exploitation attempts targeting kernel vulnerabilities

Monitoring Recommendations

• Enable detailed kernel logging for CIFS operations to capture potential exploitation attempts
• Monitor network connections to SMB servers, especially from untrusted or newly appeared network hosts
• Implement alerting for kernel crashes or unexpected CIFS subsystem behavior
• Review system logs regularly for signs of memory corruption or bounds checking failures

How to Mitigate CVE-2024-26828

Immediate Actions Required

• Update the Linux kernel to a patched version that addresses CVE-2024-26828
• Restrict CIFS/SMB connections to trusted servers only through firewall rules
• Monitor systems for signs of exploitation while patches are being deployed
• Consider temporarily disabling CIFS functionality if not critical to operations

Patch Information

Multiple patches have been released to address this vulnerability. The fix involves correcting the type comparison to properly handle the signed/unsigned boundary check. Official patches are available through the kernel.org git repository:

• Kernel Git Commit 7190353
• Kernel Git Commit cffe487
• Kernel Git Commit df2af9f
• Kernel Git Commit f7ff1c8

Organizations should apply the appropriate patch for their kernel version through their Linux distribution's package management system.

Workarounds

• Implement network segmentation to limit exposure of systems using CIFS to only trusted network segments
• Configure firewall rules to restrict SMB/CIFS traffic (ports 445, 139) to known and trusted servers only
• If CIFS functionality is not required, consider disabling the cifs.ko kernel module
• Use VPN or other secure tunneling for remote CIFS access to limit adjacent network attack vectors

# Temporarily disable CIFS module if not needed
sudo modprobe -r cifs
# Block CIFS/SMB from untrusted networks (example for iptables)
sudo iptables -A INPUT -p tcp --dport 445 -s ! trusted_network/24 -j DROP