CVE-2026-64137 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
smb: client: require net admin for CIFS SWN netlink
CIFS_GENL_CMD_SWN_NOTIFY is the userspace witness-notify command. The intended sender is the cifs.witness helper, but the generic-netlink operation currently has no capability flag, so any local process can send RESOURCE_CHANGE or CLIENT_MOVE notifications to the in-kernel witness handler.
The same family exposes CIFS_GENL_MCGRP_SWN without multicast-group capability flags. Register messages sent to that group include the witness registration id and, for NTLM-authenticated mounts, the username, domain, and password attributes copied from the CIFS session. An unprivileged local process should not be able to join that group and receive those messages.
Require CAP_NET_ADMIN for incoming SWN_NOTIFY commands with GENL_ADMIN_PERM, and require CAP_NET_ADMIN over the network namespace for joining the SWN multicast group with GENL_MCAST_CAP_NET_ADMIN. The cifs.witness service runs with the privileges needed for both operations.
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Analysis
by VulDB Data Team • 07/19/2026
This vulnerability resides within the Linux kernel's CIFS (Common Internet File System) implementation, specifically addressing improper privilege escalation in the SMB (Server Message Block) client subsystem. The issue stems from insufficient access controls in the generic netlink interface used by the CIFS witness mechanism, which is designed to handle network availability notifications and client mobility events. The vulnerability allows local unprivileged processes to manipulate critical network witness operations that should be restricted to authorized system components.
The technical flaw manifests through two distinct pathways within the CIFS generic netlink family implementation. First, the CIFS_GENL_CMD_SWN_NOTIFY command lacks proper capability flags, enabling any local process to send RESOURCE_CHANGE or CLIENT_MOVE notifications directly to the in-kernel witness handler. This represents a direct violation of principle of least privilege and allows arbitrary modification of network witness state information. Second, the CIFS_GENL_MCGRP_SWN multicast group exposes sensitive authentication data including witness registration IDs, usernames, domains, and passwords from NTLM-authenticated CIFS sessions without proper capability verification. The absence of multicast group capability flags creates a pathway for unauthorized local processes to subscribe to and receive confidential network credentials.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it enables potential credential theft and network disruption attacks. An unprivileged local attacker could manipulate witness notifications to trigger false network availability events or client migration scenarios, potentially causing legitimate network services to malfunction or redirect traffic incorrectly. The exposure of authentication credentials through the multicast group presents a significant risk for credential harvesting, particularly in environments where CIFS mounts use NTLM authentication. This vulnerability directly relates to CWE-276, Access Control, and CWE-310, Cryptographic Issues, as it combines improper privilege handling with sensitive data exposure.
The mitigation strategy implemented requires explicit capability checks for both operations within the kernel. The solution mandates CAP_NET_ADMIN capability for incoming SWN_NOTIFY commands through GENL_ADMIN_PERM flag enforcement, ensuring that only processes with appropriate network administration privileges can send witness notifications. Additionally, joining the SWN multicast group now requires CAP_NET_ADMIN over the network namespace via GENL_MCAST_CAP_NET_ADMIN flag, preventing unauthorized subscription to credential-bearing network messages. This approach aligns with ATT&CK technique T1068, "Exploitation for Privilege Escalation" and T1566, "Phishing", as it addresses both local privilege escalation vectors and credential exposure threats.
The fix directly addresses the root cause by implementing proper capability-based access controls that align with Linux kernel security principles. The cifs.witness service, which legitimately requires these elevated privileges, continues to function normally while unauthorized processes are properly restricted. This solution prevents unauthorized local processes from leveraging the witness notification mechanism to disrupt network operations or extract sensitive authentication information. The implementation follows established kernel security patterns for generic netlink interface protection and maintains backward compatibility for legitimate system operations while closing the privilege escalation vector that existed before the fix.
The vulnerability represents a classic case of insufficient capability checking in kernel subsystems, where the assumption that only trusted processes would interact with network witness mechanisms proved incorrect. This flaw demonstrates how seemingly isolated kernel components can create security boundaries that allow lateral movement and information disclosure when proper access controls are not enforced. The solution reinforces the importance of capability-based security models in kernel space and highlights the necessity of protecting sensitive data flows even within privileged subsystems where traditional user-space protections may not apply.