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CVE-2026-64399

ksmbd: add permission checks for FSCTL_DUPLICATE_EXTENTS_TO_FILE

9.8 EPSS 0.5% AV:N

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-2026-64399 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	fixedDLA-4720-1 · 2026-08-05 · text	6.1.180-1	Update the kernel and reboot. DLA-4720-1 shipped the fix on 2026-08-05, in 6.1.180-1. A running kernel keeps the old code until the machine restarts.
Debian 13trixie	fixed	6.12.96-1	Update the kernel and reboot. Fixed in 6.12.96-1. A running kernel keeps the old code until the machine restarts.
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.
Debian 12bookworm · linux-6.12	fixedDLA-4724-1 · 2026-08-07 · text	6.12.100-1~deb12u1	Update the kernel and reboot. DLA-4724-1 shipped the fix on 2026-08-07, in 6.12.100-1~deb12u1. A running kernel keeps the old code until the machine restarts.

The vulnerability

What the bug is

The FSCTL_DUPLICATE_EXTENTS_TO_FILE arm of smb2_ioctl() overwrites the destination file's data via vfs_clone_file_range() with neither the share-level KSMBD_TREE_CONN_FLAG_WRITABLE check nor a per-handle fp->daccess check that the other write-bearing arms carry. A client can overwrite destination data on a read-only share, or from a handle opened with only FILE_WRITE_ATTRIBUTES (which still yields an FMODE_WRITE filp). FILE_WRITE_ATTRIBUTES-only destination handle overwrote the file's data via the clone. Add both checks, matching the FSCTL_SET_SPARSE permission fix; require FILE_WRITE_DATA since this writes data.

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/server/smb2pdu.c

What this is: KERNEL SMB3 SERVER (KSMBD)MAINTAINERS section matched on fs/smb/server/ · linux-cifs@vger.kernel.org · Maintained
Kernel module: ksmbd

Check it on the machine

grep -wE 'CONFIG_SMBFS|CONFIG_SMB_SERVER' /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 ksmbd

Prints 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, sid, bookworm · linux-6.12m

m: a loadable module on every flavour - check lsmod, can be blacklisted

Build switches: CONFIG_SMBFS, CONFIG_SMB_SERVER
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

Debian tracker CVE record NVD kernel CNA record

Signals

Published: 2026-07-25 (3 months ago)source
CVSS: 9.8 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:Hsource recompute it
Attack vector: network (AV:N)source
EPSS: 0.49%, higher than 40.8% 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
5.15	6.1.178	bf460ad5958d
5.15	6.6.145	620d133d4692
5.15	6.12.96	9b9cf7e65cbe
5.15	6.18.39	baae7b39673e
5.15	7.1.4	c917e4522d25
5.15	7.2	388e4139db27

Debian advisories

Advisory	Date	Shipped to	Debian
DLA-4724-1	2026-08-07	bookworm 6.12.100-1~deb12u1	tracker
DLA-4723-1	2026-08-07	bullseye 6.1.180-1~deb11u1	tracker
DLA-4720-1	2026-08-05	bookworm 6.1.180-1	tracker

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:NAttack vector, network: An attacker reaches smb2_ioctl() remotely through ksmbd’s SMB service on TCP port 445.
AC:LAttack complexity, low: The attacker directly controls both file handles, offsets, length, and request timing; no race or uncontrollable condition is required on a clone-capable filesystem.
PR:NPrivileges required, none: A passwordless guest session on a guest-enabled share can obtain the required session, tree connection, and file handles without credentials.
UI:NUser interaction, none: The attacker independently negotiates the SMB session, opens the files, and submits the malicious IOCTL without victim action.
S:UScope, unchanged: The unauthorized file modification and any resulting code execution remain within the host kernel and services governed by the same security authority.
C:HConfidentiality impact, high: In a plausible share containing service-loaded code or configuration, attacker-controlled extent replacement can yield code execution and consequent access to sensitive data.
I:HIntegrity impact, high: The attacker can overwrite arbitrary destination ranges with chosen source data despite a read-only share or a handle lacking FILE_WRITE_DATA.
A:HAvailability impact, high: Repeated or complete corruption of critical files, databases, executables, or service configuration can cause a persistent loss of the affected system or service.