CVE-2026-31432: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-31432 Overview

CVE-2026-31432 is an out-of-bounds write vulnerability in the Linux kernel's ksmbd (kernel SMB server) component. The vulnerability occurs when processing compound SMB requests where the first command consumes most of the response buffer, causing ksmbd to write beyond the allocated buffer while building a security descriptor for subsequent QUERY_INFO operations.

Critical Impact

This out-of-bounds write vulnerability in ksmbd could allow an attacker to corrupt kernel memory, potentially leading to privilege escalation, denial of service, or arbitrary code execution in kernel context.

Affected Products

• Linux kernel with ksmbd module enabled
• Systems running in-kernel SMB3 file server (ksmbd)
• Linux distributions with ksmbd configured for file sharing

Discovery Timeline

• 2026-04-22 - CVE CVE-2026-31432 published to NVD
• 2026-04-23 - Last updated in NVD database

Technical Details for CVE-2026-31432

Vulnerability Analysis

The vulnerability exists in the ksmbd module's handling of compound SMB requests, specifically when a sequence like READ + QUERY_INFO(Security) is processed. The flaw manifests when the first command in a compound request consumes a significant portion of the response buffer, leaving insufficient space for subsequent commands.

The core issue lies in the buffer size calculation mismatch between smb2_get_info_sec() and build_sec_desc(). The former function performs buffer space validation using ppntsd_size obtained from xattr (extended attributes), but this check is inadequate because build_sec_desc() synthesizes a security descriptor from POSIX ACLs that is often significantly larger than the xattr-based estimate.

When building security descriptors from POSIX ACL entries, the resulting descriptor can vary considerably in size depending on the number of ACL entries present on the target file or directory. Without proper pre-calculation of the final descriptor size, the function proceeds to write data beyond the bounds of the allocated response buffer.

Root Cause

The root cause is a buffer size miscalculation in the security descriptor generation path. The vulnerability stems from two functions using different methods to estimate required buffer space: smb2_get_info_sec() relies on xattr size for buffer checking, while build_sec_desc() dynamically constructs larger descriptors from POSIX ACLs. This discrepancy creates a condition where the actual write operation exceeds the pre-validated buffer boundaries.

Attack Vector

An attacker with network access to a ksmbd-enabled SMB server can craft malicious compound SMB requests designed to trigger this vulnerability. The attack vector involves:

1. Establishing an SMB3 connection to a vulnerable ksmbd server
2. Sending a compound request where the first command (such as READ) returns data consuming most of the response buffer
3. Following with a QUERY_INFO request for security information on a file with complex POSIX ACLs
4. The server writes the synthesized security descriptor past the buffer boundary, corrupting adjacent kernel memory

The patch addresses this by introducing smb_acl_sec_desc_scratch_len() to accurately compute the final descriptor size before allocation. This function performs proper buffer checking via smb2_calc_max_out_buf_len() and ensures exact-sized allocation with iov pinning, preventing the out-of-bounds write condition.

Detection Methods for CVE-2026-31432

Indicators of Compromise

• Unexpected kernel crashes or panics involving ksmbd module functions
• Memory corruption signatures in kernel logs related to SMB operations
• Unusual SMB compound request patterns in network traffic analysis
• System instability when serving files with complex POSIX ACL configurations

Detection Strategies

• Monitor kernel logs for ksmbd-related out-of-bounds access warnings or KASAN reports
• Deploy network intrusion detection rules to identify malformed or suspicious compound SMB requests
• Enable kernel memory debugging options (KASAN, UBSAN) in development or test environments to catch exploitation attempts
• Audit SMB server access logs for unusual query patterns targeting files with extensive ACL entries

Monitoring Recommendations

• Configure alerting on kernel oops or panic events referencing ksmbd, smb2_get_info_sec, or build_sec_desc functions
• Monitor network traffic for SMB compound requests with READ + QUERY_INFO combinations targeting security descriptors
• Track system stability metrics on servers running ksmbd for anomalies

How to Mitigate CVE-2026-31432

Immediate Actions Required

• Apply the latest kernel patches addressing this vulnerability from your distribution's package manager
• If immediate patching is not possible, consider temporarily disabling ksmbd and using userspace Samba if SMB services are required
• Review and audit POSIX ACL configurations on shared directories to understand exposure
• Monitor systems for signs of exploitation attempts

Patch Information

The Linux kernel developers have released patches to address this vulnerability. Multiple commits are available in the stable kernel tree:

• Kernel Commit 075ea208c648
• Kernel Commit 515c2daab460
• Kernel Commit d48c64fb80ad
• Kernel Commit fda9522ed6af

The fix introduces smb_acl_sec_desc_scratch_len() for accurate pre-computation of security descriptor sizes, implements proper buffer validation using smb2_calc_max_out_buf_len(), and ensures exact-sized memory allocation with iov pinning.

Workarounds

• Disable the ksmbd kernel module if in-kernel SMB server functionality is not required: modprobe -r ksmbd
• Migrate SMB file sharing services to userspace Samba (smbd) as a temporary alternative
• Restrict network access to ksmbd services using firewall rules to trusted clients only
• Reduce attack surface by simplifying POSIX ACL configurations on shared directories where feasible

# Disable ksmbd module temporarily
sudo modprobe -r ksmbd

# Prevent ksmbd from loading at boot
echo "blacklist ksmbd" | sudo tee /etc/modprobe.d/ksmbd-blacklist.conf

# Check if ksmbd is currently loaded
lsmod | grep ksmbd