CVE-2026-31476: 
CBL Mariner vulnerability analysis and mitigation

CVE-2026-31476 is a denial-of-service vulnerability in the Linux kernel's ksmbd (in-kernel SMB server) component affecting multichannel session binding. When a session binding request fails (e.g., due to an incorrect password), the error path unconditionally sets sess->state = SMB2_SESSION_EXPIRED on a session that belongs to another connection's user, allowing a remote unauthenticated attacker to invalidate any active SMB session. The vulnerability was published on April 22, 2026, and affects Linux kernel versions from 5.15.1 through multiple stable branches up to 7.0-rc7. It carries a CVSS v3.1 base score of 8.2 (High) (GitHub Advisory, Red Hat Bugzilla).

The root cause is classified as CWE-274 (Improper Handling of Insufficient Privileges). During an SMB2 multichannel session binding operation, ksmbd_session_lookup_slowpath() retrieves a session object belonging to a different connection's user. When the binding fails for any reason (e.g., wrong password), the error path unconditionally marks that foreign session as SMB2_SESSION_EXPIRED, rather than limiting state changes to sessions owned by the current connection. An unauthenticated remote attacker can exploit this over the network (TCP port 445) with low complexity and no user interaction by simply sending a crafted SMB2 session binding request with incorrect credentials targeting a known session ID. No public proof-of-concept exploit code has been identified at this time (GitHub Advisory, Red Hat Bugzilla).

Successful exploitation allows a remote, unauthenticated attacker to invalidate any active SMB session on the affected system, disrupting file sharing and print services for legitimate users. The primary impact is high availability loss — active SMB connections are forcibly terminated — with a minor integrity impact (CVSS integrity: Low) due to the unauthorized state manipulation of session objects. Confidentiality is not directly impacted, and there is no evidence of lateral movement capability or data exfiltration risk from this vulnerability alone (GitHub Advisory, Feedly).

There is no known public proof-of-concept exploit and no evidence of in-the-wild exploitation as of the time of publication (Feedly). The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. The EPSS score is approximately 0.0023 (0.23%), indicating a low probability of exploitation in the near term. The attack requires only network access to TCP port 445 and no authentication, making it trivially reachable on exposed systems, but the lack of weaponized tooling reduces immediate risk (GitHub Advisory).

1. Reconnaissance: Identify Linux systems running ksmbd (in-kernel SMB server) with TCP port 445 open using network scanners such as Nmap (nmap -p 445 --script smb-security-mode <target>) or Shodan queries for SMB services on Linux hosts.
2. Enumerate active sessions: Use SMB client tools (e.g., smbclient, Impacket's smbclient.py) to probe the target and identify active SMB sessions or session IDs that can be targeted.
3. Craft malicious binding request: Construct an SMB2 SESSION_SETUP request with the SMB2_SESSION_FLAG_BINDING flag set, referencing the session ID of an active legitimate session, but supplying incorrect credentials (wrong password or NTLM hash).
4. Send binding request: Transmit the crafted packet to the target on TCP port 445. The ksmbd error path will unconditionally mark the referenced session as SMB2_SESSION_EXPIRED.
5. Session invalidation achieved: The legitimate user's active SMB session is terminated, disrupting their file sharing or print service access. Repeat as needed to maintain a denial-of-service condition against any active sessions (GitHub Advisory, Feedly).

• Network: Repeated SMB2 SESSION_SETUP requests to TCP port 445 with the SMB2_SESSION_FLAG_BINDING flag set and authentication failures from unexpected or external source IPs; high volume of failed binding attempts from a single source.
• Logs: Kernel logs (dmesg, /var/log/kern.log) showing repeated ksmbd session binding failures; SMB audit logs recording frequent STATUS_LOGON_FAILURE or session expiration events for sessions not initiated by the affected user.
• Process/Service: Unexpected disconnections of active SMB clients reported in system logs; ksmbd service logs showing sessions being marked expired without corresponding client-initiated logoff events.

Apply the patched Linux kernel versions as soon as possible: 6.1.168, 6.6.131, 6.12.80, 6.18.21, 6.19.11, or 7.0 (stable). Patches are available via kernel.org stable repositories. As a workaround, restrict network access to TCP port 445 using firewall rules to limit exposure of the ksmbd service to trusted networks only. Organizations not requiring ksmbd should disable the service entirely (GitHub Advisory, Red Hat Bugzilla, Feedly).