CVE-2026-89638

/// Technical description

In the Linux kernel, the following vulnerability has been resolved:

smb: client: clear setuid/setgid bit on write with cifsacl/modefromsid/posix extensions

When a file has the setuid or setgid bit set and is written to, the VFS

strips those bits and issues a setattr with ATTR_KILL_SUID/ATTR_KILL_SGID

together with an ATTR_MODE carrying the already-cleared mode.

Both cifs_setattr_unix() and cifs_setattr_nounix() unconditionally dropped

ATTR_MODE in that case:

/* skip mode change if it's just for clearing setuid/setgid */

if (attrs->ia_valid & (ATTR_KILL_SUID|ATTR_KILL_SGID))

attrs->ia_valid &= ~ATTR_MODE;

This is fine for the default mount, where the mode is only emulated via

the DOS read-only attribute and cannot represent the setuid/setgid bits

anyway. However, with the "cifsacl" or "modefromsid" mount options the

mode is stored on the server through an ACL (id_mode_to_cifs_acl()), with

the SMB3.1.1 POSIX extensions the mode is sent to the server directly,

and with the SMB1

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References