Linux Linux Kernel Vulnerability Permits Privilege Escalation

Status: UPDATED  |  Advisory ID: CVE-2026-89638

Key Details

Affected Products, Subsystems & Sectors

What to Know

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 Unix extensions (cifs_setattr_unix) the mode is sent

via CIFSSMBUnixSetPathInfo(). In all those cases dropping ATTR_MODE means

the cleared mode is never pushed to the server, so the setuid/setgid bit

survives the write.

This is a security issue: on local filesystems the setuid bit is stripped

when a file is written, but over these cifs.ko mounts the bit persists on

the server, potentially allowing an unexpected privilege escalation on

subsequent execution.

Fix this in two places:

1. cifs_setattr_nounix(): only take the "skip mode change" shortcut

when the mode is emulated via the DOS read-only attribute (i.e.

neither cifsacl/modefromsid nor the SMB3.1.1 POSIX extensions are

in effect), so that the cleared mode is propagated to the server

in the ACL / POSIX cases.

2. cifs_setattr_unix(): this function is only called when Unix

extensions are in effect, so the mode is always stored on the

server. Remove the shortcut entirely so that the cleared mode is

always pushed. (NVD)

What to Do

Monitor Linux's web page for any future patch releases.

References