CVE-2026-64388

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chown/chgrp Failure in Linux Kernel SMB3 POSIX Extensions

Publication date: 2026-07-25

Last updated on: 2026-08-11

Assigner: kernel.org

Description

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.

CVSS Scores

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Meta Information

Published

2026-07-25

Last Modified

2026-08-11

Generated

2026-08-14

AI Q&A

2026-07-25

EPSS Evaluated

2026-08-13

NVD

CVE-2026-64388

EUVD

EUVD-2026-48932

Affected Vendors & Products

Vendor	Product	Version / Range
linux	linux_kernel	*

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-UNKNOWN

Attack-Flow Graph

Executive Summary

This vulnerability in the Linux kernel affects the SMB client when using SMB3 POSIX Extensions. It allows ownership and group modifications (chown/chgrp) to be ignored unless specific mount options are set. The fix ensures proper handling of UID and GID changes by checking for posix_extensions and calling id_mode_to_cifs_acl().

Detection Guidance

This vulnerability affects the Linux kernel's SMB client functionality when using SMB3 POSIX Extensions. Detection requires checking kernel versions and SMB client configurations. Examine if your system uses CIFS_MOUNT_CIFS_ACL or CIFS_MOUNT_MODE_FROM_SID mounts with POSIX extensions enabled. Check kernel logs for related errors during chown/chgrp operations on SMB shares.

Impact Analysis

If you use SMB3 POSIX Extensions for file sharing, this vulnerability could prevent proper ownership and group changes from being applied. This may lead to incorrect file permissions, access issues, or unintended data exposure if permissions are not enforced as expected.

Compliance Impact

This vulnerability could potentially impact compliance with GDPR or HIPAA if it affects systems handling sensitive data. Incorrect ownership or group permissions due to the chown/chgrp issue might lead to unauthorized access or data exposure, violating confidentiality requirements in these regulations.

Mitigation Strategies

Update the Linux kernel to the patched version where the fix for SMB3 POSIX Extensions chown/chgrp is included. Ensure mounts using SMB3 POSIX Extensions explicitly set CIFS_MOUNT_CIFS_ACL or CIFS_MOUNT_MODE_FROM_SID to enforce ownership and group modifications.

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