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CVE-2026-53390

ksmbd: fix out-of-bounds read in smb_check_perm_dacl()

8.1 EPSS 0.5% AV:N

Published 2026-07-19 (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-2026-53390 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	fixed	6.1.177-1	Update the kernel and reboot. Fixed in 6.1.177-1. A running kernel keeps the old code until the machine restarts.
Debian 13trixie	fixed	6.12.95-1	Update the kernel and reboot. Fixed in 6.12.95-1. A running kernel keeps the old code until the machine restarts.
Debian 14forky	fixed	7.1.3-1	Update the kernel and reboot. Fixed in 7.1.3-1. A running kernel keeps the old code until the machine restarts.
Debian unstablesid	fixed	7.1.3-1	Update the kernel and reboot. Fixed in 7.1.3-1. A running kernel keeps the old code until the machine restarts.

The vulnerability

What the bug is

The permission-check ACE walk in smb_check_perm_dacl() validates the ACE header size and caps sid.num_subauth at SID_MAX_SUB_AUTHORITIES, but it never checks that ace->size is actually large enough to contain num_subauth sub-authorities before compare_sids() dereferences them. CIFS_SID_BASE_SIZE covers the SID header up to but excluding the sub_auth[] array, and offsetof(struct smb_ace, sid) is the ACE header, so the existing guards only guarantee the 8-byte SID base, i.e. zero sub-authorities. compare_sids() then reads ace->sid.sub_auth[i] for i < min(local_sid->num_subauth, ace->sid.num_subauth). The local comparison SIDs (sid_everyone, sid_unix_NFS_mode, and the id_to_sid() result) always have at least one sub-authority, and an attacker controls the ACE revision and authority bytes (which lie within the in-bounds SID base), so they can match one of those SIDs and force the sub_auth read. A crafted ACE with size == 16 and num_subauth >= 1 placed at the tail of the security descriptor therefore causes a heap out-of-bounds read of up to SID_MAX_SUB_AUTHORITIES * sizeof(__le32) bytes past the pntsd allocation. The security descriptor is loaded by ksmbd_vfs_get_sd_xattr() into a buffer sized exactly to the on-disk data (kzalloc(sd_size) in ndr_decode_v4_ntacl()), so the read lands past the allocation. The malformed descriptor can be stored verbatim via SMB2_SET_INFO (the DACL is not normalised before being written to the security.NTACL xattr) and the read fires on a subsequent SMB2_CREATE access check, making this reachable by an authenticated client on a share that uses ACL xattrs. Add the missing num_subauth-versus-ace_size check, mirroring the identical guards already present in the sibling parsers parse_dacl() and 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: 0.47%, higher than 39.2% 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
5.15.210	5.15.212	c7488c85fd82
6.1.176	6.1.177	988c93d3bba0
6.6.140	6.6.144	36599894fa85
6.12.84	6.12.95	d5c81a095c86
6.18.25	6.18.38	7627ff8c4f99
7.1	7.1.3	e36e35660adb
7.1	7.2	1ef06004ed4b
7.0.2	0	0

Debian advisories

Advisory	Date	Shipped to	Debian
DLA-4700-1	2026-07-26	bullseye 6.1.177-1~deb11u1	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: The bug is reachable through ksmbd's in-kernel SMB2 server on TCP/445 via SMB2_SET_INFO (to store a crafted security descriptor) and SMB2_CREATE (to trigger the DACL walk), with packets handled from ksmbd_conn_handler_loop() through smb2_set_info()/smb2_open() into smb_check_perm_dacl().
AC:LAttack complexity, low: An attacker fully controls the malformed ACE fields (size, num_subauth, revision, authority bytes) and can reliably place the ACE at the tail of a security descriptor, store it verbatim, then trigger the read on the next CREATE without depending on race timing or uncontrollable memory layout.
PR:LPrivileges required, low: Exploitation requires a completed SMB2 session and share access: SMB2_SET_INFO needs FILE_WRITE_DAC/FILE_WRITE_OWNER on an open file, and the poisoned descriptor is only persisted on shares with ACL xattr support, which is authenticated low-privilege client access rather than pre-auth or host root.
UI:NUser interaction, none: No end-user action is required beyond the attacker sending crafted SMB2 protocol messages over the network; no victim must open a file, mount a share, or perform any interactive step.
S:UScope, unchanged: The vulnerability is confined to kernel heap memory reads during ksmbd DACL permission evaluation and does not cross a security boundary such as guest-to-host VM escape or sandbox escape.
C:HConfidentiality impact, high: compare_sids() performs a heap out-of-bounds read of up to SID_MAX_SUB_AUTHORITIES (15) sub-authority dwords (60 bytes) past the kzalloc()-sized security descriptor buffer, exposing adjacent kernel heap contents even though the data is not returned in the SMB response.
I:NIntegrity impact, none: The flaw is a read-only out-of-bounds access in compare_sids() with no memory write, type confusion, or demonstrated control-flow hijack primitive; integrity is not directly compromised.
A:HAvailability impact, high: Reading past the end of the kmalloc allocation can fault the kernel (oops/panic), and a malicious descriptor planted on a shared file can deny availability to any other authenticated client whose SMB2_CREATE triggers smb_check_perm_dacl().