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CVE-2026-64400

ksmbd: prevent path traversal bypass by restricting caseless retry

8.6 EPSS 0.6% 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-2026-64400 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

ksmbd_vfs_path_lookup() enforces LOOKUP_BENEATH to restrict path resolution within the share root. When a crafted path attempts to escape the share boundary using parent-directory components ('..'), vfs_path_parent_lookup() detects this and immediately fails, returning -EXDEV. However, a bug exists in __ksmbd_vfs_kern_path() under caseless mode. The function fails to intercept the -EXDEV error and erroneously falls through to the caseless retry logic, which is intended only for genuinely missing files. During this retry process, the path is reconstructed, leading to an unintended LOOKUP_BENEATH bypass that allows write-capable users to create zero-length files or directories outside the exported share. Fix this by ensuring that the execution only proceeds to the caseless lookup retry when the error is specifically -ENOENT. Any other errors, such as -EXDEV from a path traversal attempt, must be returned immediately.

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/vfs.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, sidm

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: 8.6 high
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:Hsource recompute it
Attack vector: network (AV:N)source
EPSS: 0.60%, higher than 47.8% 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
6.4	6.18.57	cfb5a56c2188
6.4	7.1.4	8c9a4f1327eb
6.4	7.2	54bab9ba5a9f
5.15.145	0	0
6.1.71	0	0
6.3.10	0	0

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: The vulnerable lookup is reached remotely through SMB2 CREATE requests handled by ksmbd over TCP/445.
AC:LAttack complexity, low: The attacker controls the crafted parent-directory path and can issue repeated or concurrent requests; any required lookup/create race is attacker-driven.
PR:NPrivileges required, none: Although a valid SMB session and writable tree connection are required, ksmbd supports anonymous guest sessions on writable shares, requiring no prior privileges in a reasonable deployment.
UI:NUser interaction, none: Exploitation requires only attacker-generated SMB requests and no action by a victim.
S:UScope, unchanged: The bypass affects filesystems managed by the same host kernel and does not cross a virtualization or comparable security authority boundary.
C:LConfidentiality impact, low: File contents are not exposed, but the out-of-share caseless directory walk and distinguishable errors can disclose limited pathname-existence metadata.
I:LIntegrity impact, low: The attacker can create chosen zero-length files or directories outside the export, but cannot supply file contents or overwrite arbitrary existing data through this flaw.
A:HAvailability impact, high: Repeated creation in an escaped world-writable location can exhaust root-filesystem inodes, storage, or tmpfs memory and cause system-wide service unavailability.