CVE-2026-31612: Linux Kernel Information Disclosure Flaw

CVE-2026-31612 Overview

A memory information disclosure vulnerability exists in the Linux kernel's ksmbd (SMB3 server) component. The smb2_get_ea() function reads ea_req->EaNameLength from client requests and passes it directly to strncmp() as the comparison length without proper validation. This allows remote attackers to potentially leak uninitialized heap memory values by sending crafted SMB requests with manipulated EaNameLength values.

Critical Impact

Remote unauthenticated attackers can exploit this vulnerability over the network to leak sensitive heap memory contents from affected Linux kernel systems running ksmbd, potentially exposing sensitive data or memory layout information useful for further attacks.

Affected Products

• Linux Kernel (multiple versions with ksmbd support)
• Systems running ksmbd SMB3 file server implementation
• Linux-based file servers with SMB3 sharing enabled

Discovery Timeline

• 2026-04-24 - CVE-2026-31612 published to NVD
• 2026-04-29 - Last updated in NVD database

Technical Details for CVE-2026-31612

Vulnerability Analysis

This vulnerability resides in the kernel's in-kernel SMB3 file server (ksmbd) implementation, specifically within the smb2_get_ea() function responsible for handling Extended Attributes (EA) requests from SMB clients. The function reads the EaNameLength field from incoming client requests and uses this value directly in subsequent strncmp() calls without validating that the provided length matches the actual size of the input buffer.

When a client sends a malicious request with an EaNameLength value that exceeds the actual buffer size, the strncmp() function reads beyond the legitimate buffer boundaries into uninitialized heap memory. The comparison results can then be inferred from server responses, allowing attackers to gradually extract heap memory contents byte-by-byte through a series of crafted requests.

Root Cause

The root cause is improper input validation in the smb2_get_ea() function. The code trusts the client-supplied EaNameLength value without verifying it against the actual size of the received request buffer. This missing bounds check violates the security principle of never trusting client-provided data, particularly length fields that control memory access operations.

The fix introduces proper validation by checking the size of the name based on the received value against the overall size of the request, ensuring the EaNameLength cannot exceed the legitimate buffer boundaries before being used in memory comparison operations.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by:

1. Establishing an SMB connection to a vulnerable ksmbd server
2. Crafting SMB2 GET_EA requests with manipulated EaNameLength values exceeding the actual buffer size
3. Analyzing server responses to infer byte values from uninitialized heap memory
4. Repeating the process to systematically leak heap contents

The vulnerability allows for information disclosure of heap memory, which could reveal sensitive data such as memory addresses (defeating ASLR), cryptographic keys, credentials, or other confidential information residing in kernel heap memory.

Detection Methods for CVE-2026-31612

Indicators of Compromise

• Unusual SMB2 traffic patterns with malformed GET_EA requests
• SMB connections exhibiting repeated requests with varying EaNameLength parameters
• Anomalous ksmbd log entries indicating parsing errors or unexpected request sizes
• Network traffic analysis showing SMB2 requests with length field inconsistencies

Detection Strategies

• Monitor SMB traffic for requests with EaNameLength values that don't match actual payload sizes
• Deploy network intrusion detection rules targeting malformed SMB2 EA requests
• Enable kernel auditing for ksmbd operations and monitor for suspicious patterns
• Implement deep packet inspection for SMB3 protocol anomalies

Monitoring Recommendations

• Enable detailed ksmbd logging and forward logs to SIEM for analysis
• Configure network monitoring to alert on high volumes of SMB2 GET_EA requests from single sources
• Monitor kernel messages for memory-related warnings in ksmbd contexts
• Implement baseline analysis for normal SMB traffic patterns to detect anomalous behavior

How to Mitigate CVE-2026-31612

Immediate Actions Required

• Apply the latest kernel security patches addressing this vulnerability immediately
• If patching is not immediately possible, consider disabling ksmbd if SMB3 file sharing is not required
• Restrict network access to SMB services using firewall rules to trusted networks only
• Monitor for exploitation attempts while patches are being deployed
• Review and audit systems that have had ksmbd exposed to untrusted networks

Patch Information

Multiple patches have been released for different kernel branches to address this vulnerability. The fix properly validates the EaNameLength field against the actual request buffer size before using it in strncmp() operations.

Available kernel patches:

• Kernel Patch 243b206
• Kernel Patch 3363a77
• Kernel Patch 4b73376
• Kernel Patch 551dfb1
• Kernel Patch 6675184
• Kernel Patch dfc6878d

Workarounds

• Disable ksmbd kernel module if SMB3 file sharing functionality is not required: modprobe -r ksmbd
• Implement network-level access controls to restrict SMB access to trusted IP ranges only
• Use alternative SMB implementations like Samba userspace daemon until patches can be applied
• Deploy network segmentation to isolate file servers from untrusted network segments
• Configure firewall rules to block SMB traffic (ports 445, 139) from untrusted sources

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

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