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CVE-2026-64388
smb/client: fix chown/chgrp with SMB3 POSIX Extensions
Published 2026-07-25 (3 months ago) by the Linux kernel CNA. Subsystem fs/smb. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-64388 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | vulnerable | 6.1.190-1 | No fix published anywhere yet, neither in Debian nor upstream for the series bookworm tracks. Watch the Debian tracker page for this CVE. Advisories for bookworm have historically landed a median of 23 days after publication. |
| Debian 13trixie | vulnerable | 6.12.113-1 | No fix published anywhere yet, neither in Debian nor upstream for the series trixie tracks. Watch the Debian tracker page for this CVE. Advisories for trixie have historically landed a median of 11 days after publication. |
| Debian 14forky | fixed | 7.1.4-1 | Update the kernel and reboot. Fixed in 7.1.4-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.4-1 | Update the kernel and reboot. Fixed in 7.1.4-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
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.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
fs/smb/client/inode.c
- What this is
- COMMON INTERNET FILE SYSTEM CLIENT (CIFS and SMB3)MAINTAINERS section matched on fs/smb/client/ · linux-cifs@vger.kernel.org · Supported
- Kernel module
- cifs
Check it on the machine
grep -wE 'CONFIG_CIFS|CONFIG_SMBFS' /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
lsmod | grep -w cifsPrints a line if that module is loaded right now, and nothing if it is not. One that is not loaded can still be loaded later by anything that needs it.
Show whether Debian builds this, per release
Debian builds this as a loadable module. Every release below compiles it as a module on every flavour, so whether the code is in the running kernel depends on whether that module is loaded.
- bookworm, trixie, forky, sidm
- m
- a loadable module on every flavour - check lsmod, can be blacklisted
- Build switches
- CONFIG_CIFS, CONFIG_SMBFS
- Reads as
- all on means the file is compiled; any one off means it is not.
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-07-25 (3 months ago)source
- CVSS
- 7.8 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.13%, higher than 2.1% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- fs/smb
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 4.18 | 6.18.57 | 3eaec47518e1 |
| 4.18 | 7.1.4 | 550cfb8a8118 |
| 4.18 | 7.2 | 760ef2c579c2 |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:LAttack vector, local
- The vulnerable branch is reached through local chown/chgrp syscalls on an already mounted SMB3 POSIX share; unsolicited SMB traffic cannot invoke it.
- AC:LAttack complexity, low
- With the affected mount configuration, the client deterministically skips the ownership update and returns success; no race, heap layout, or uncontrollable timing is required.
- PR:LPrivileges required, low
- A basic local user can exercise the path on accessible share objects, and noperm deliberately defers authorization to the server whose ownership check is bypassed because no request is sent. The attacker need not create the mount.
- UI:NUser interaction, none
- The attacker can invoke the syscall directly and exploit continued rights or falsely cached ownership through automated services without human action.
- S:UScope, unchanged
- The defect affects resources governed through the vulnerable kernel and mounted share without escaping a VM, sandbox, or other distinct security authority.
- C:HConfidentiality impact, high
- Because the intended ownership change never reaches the server, the prior owner or group can retain read access to an entire data tree believed to have been transferred or revoked.
- I:HIntegrity impact, high
- The prior owner can retain arbitrary modification rights, while falsely cached privileged ownership can cause privileged consumers to trust or execute attacker-controlled content.
- A:HAvailability impact, high
- Retained ownership of writable files or directories can permit truncation or deletion of an entire service data tree, and privileged execution through false ownership can cause a complete outage.