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CVE-2026-63909

ksmbd: OOB read regression in smb_check_perm_dacl() ACE-walk loops

8.1 EPSS 0.6% AV:N

Published 2026-07-19 (3 months ago) by the Linux kernel CNA. Subsystem fs/smb. Debian urgency: unimportant.

What to do, per Debian release

Debian's own status for CVE-2026-63909 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	not affected	6.1.190-1	Nothing to do. bookworm never shipped the vulnerable code.
Debian 13trixie	fixed	6.12.94-1	Update the kernel and reboot. Fixed in 6.12.94-1. A running kernel keeps the old code until the machine restarts.
Debian 14forky	fixed	7.0.12-1	Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts.
Debian unstablesid	fixed	7.0.12-1	Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts.

The vulnerability

What the bug is

Commit d07b26f39246 ("ksmbd: require minimum ACE size in smb_check_perm_dacl()") introduced a transposed bounds check: if (offsetof(struct smb_ace, sid) + aces_size < CIFS_SID_BASE_SIZE) Since offsetof(..sid) is 8 and CIFS_SID_BASE_SIZE is 8, this evaluates to `aces_size < 0`. Because `aces_size` is always non-negative, this check becomes dead code and never breaks the loop. Worse, that commit removed the old 4-byte guard, meaning the loop now reads `ace->size` (offset 2) even when `aces_size` is 0-3 bytes. This re-opens a 2-byte heap out-of-bounds (OOB) read past the pntsd allocation during subsequent SMB2_CREATE operations. Fix this by properly transposing the comparison to require at least 16 bytes (8-byte offset + 8-byte SID base), matching the correct form used in smb_inherit_dacl().

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/smbacl.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: 2026-07-19 (3 months ago)source
CVSS: 8.1 high
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:Hsource recompute it
Attack vector: network (AV:N)source
EPSS: 0.63%, higher than 49.1% of all CVEssource
CISA KEV: not listedthe KEV catalogue
Debian urgency: unimportantsource
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
5.15.210	5.15.210	5500ba1d410a
6.1.176	6.1.176	f6324b4240cf
6.6.140	6.6.143	0fe08c5776a7
6.12.84	6.12.93	d333af32e445
6.18.25	6.18.35	94215d55b094
7.0.2	7.0.12	4f7c131d2bdd
7.1	7.1	0e60dafe97ec

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 bug is reachable through ksmbd's in-kernel SMB server on TCP port 445; a remote attacker sends crafted SMB2 packets that traverse the normal network receive path to smb_check_perm_dacl().
AC:LAttack complexity, low: The attacker fully controls the crafted NTSD/DACL planted via SMB and the subsequent SMB2_CREATE that triggers the walk; no race or victim-specific memory layout is required beyond standard heap behavior.
PR:LPrivileges required, low: Exploitation requires an authenticated SMB2 session (post SESSION_SETUP and TREE_CONNECT) with permission to create files and set security descriptors (WRITE_DAC) on a share with ACL xattr support; this is a low-privileged authenticated user, not an unauthenticated attacker.
UI:NUser interaction, none: No end-user interaction is required beyond the attacker operating their own SMB client against an exposed ksmbd server.
S:UScope, unchanged: The vulnerability is a kernel heap out-of-bounds read within ksmbd's own security context; it does not cross a VM, sandbox, or IOMMU security boundary.
C:HConfidentiality impact, high: The bug is an out-of-bounds heap read of ace->size past the pntsd kmalloc allocation; per kernel guidance OOB reads warrant High confidentiality impact as adjacent heap metadata or pointers may be disclosed.
I:NIntegrity impact, none: This is a read-only out-of-bounds access with no memory write, type confusion, or demonstrated ability to modify kernel data or achieve code execution through the corrupted read path.
A:HAvailability impact, high: Reading beyond the kmalloc boundary can trigger a kernel page fault, oops, or panic; the original fix commit also notes KASAN reports and possible kernel state corruption from the OOB access.