CVE-2026-64388 in Linux
Summary
by MITRE • 07/25/2026
In the Linux kernel, the following vulnerability has been resolved:
smb/client: fix chown/chgrp with SMB3 POSIX Extensions
Ownership (chown) and group (chgrp) modifications were being ignored when mounting with SMB3 POSIX Extensions unless CIFS_MOUNT_CIFS_ACL or CIFS_MOUNT_MODE_FROM_SID were also explicitly set.
Fix this by checking for posix_extensions in cifs_setattr_nounix() when updating UID and GID, ensuring that id_mode_to_cifs_acl() is called to map and set the ownership/group information on the server.
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Analysis
by VulDB Data Team • 07/25/2026
This vulnerability resides within the Linux kernel's CIFS (Common Internet File System) client implementation, specifically affecting how file ownership and group permissions are handled during SMB3 POSIX Extensions mounting operations. The issue manifests when users attempt to modify file ownership using chown or group permissions using chgrp commands on filesystems mounted with SMB3 POSIX Extensions. The vulnerability represents a significant security gap because it allows unauthorized modification of file access controls, potentially enabling privilege escalation or data integrity violations in enterprise environments where SMB shares are extensively used.
The technical flaw occurs in the cifs_setattr_nounix() function which fails to properly check for posix_extensions when processing UID and GID updates. This function operates under the assumption that POSIX extension support is always enabled, but in reality when only SMB3 POSIX Extensions are mounted without explicit CIFS_MOUNT_CIFS_ACL or CIFS_MOUNT_MODE_FROM_SID flags, the system ignores ownership changes entirely. The fix implemented addresses this by ensuring that id_mode_to_cifs_acl() is properly invoked during ownership modification operations, which serves as the critical mapping function between local user identifiers and server-side ACL representations. This change aligns with the established pattern of how POSIX extensions should be handled in CIFS implementations, particularly when dealing with distributed file systems where access control boundaries must be strictly maintained.
The operational impact of this vulnerability extends beyond simple permission mismanagement to potentially compromise entire security domains within networked environments. Organizations relying on SMB3 POSIX Extensions for cross-platform file sharing may experience unauthorized access to files and directories if the chown/chgrp operations are silently ignored during mounting configurations that don't explicitly enable ACL support. This issue particularly affects enterprise scenarios where users need granular control over file permissions across different operating systems, as well as environments using centralized authentication systems where proper ownership tracking is critical for compliance auditing and security monitoring. The vulnerability essentially creates a silent bypass of access control mechanisms, which could be exploited by malicious actors to maintain persistence or escalate privileges within networked file systems.
Mitigation strategies should focus on applying the kernel patch immediately to all systems running affected CIFS implementations, particularly those with SMB3 POSIX Extensions enabled for file sharing operations. System administrators should also implement monitoring solutions that track chown/chgrp operations on mounted SMB shares to detect any anomalous behavior that might indicate this vulnerability being exploited. Configuration reviews should ensure that mounting options are explicitly set according to security requirements rather than relying on default behaviors. Organizations should consider implementing network segmentation and access control lists to limit exposure of SMB shares, while also conducting regular audits to verify proper ownership propagation across file systems. The fix addresses the underlying CWE-284 (Improper Access Control) vulnerability category by ensuring that proper authentication and authorization checks are performed during file attribute modifications, which aligns with ATT&CK technique T1078 (Valid Accounts) and T1566 (Phishing) through prevention of unauthorized access control manipulation.