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CVE-2026-64138
ksmbd: validate SID in parent security descriptor during ACL inheritance
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-2026-64138 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 | 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
Introduce smb_validate_ntsd_sid() helper to safely validate Owner SID and Group SID inside the NT Security Descriptor (smb_ntsd) retrieved from the parent directory.
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 ksmbdPrints 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
Signals
- Published
- 2026-07-19 (3 months ago)source
- CVSS
- 8.8 high
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.41%, higher than 34.3% 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 | 6.12.92 | f0e5c9c663ba |
| 5.15 | 6.18.34 | 18d8db24b0a5 |
| 5.15 | 7.0.11 | 1c9d0646a995 |
| 5.15 | 7.1 | 69f030cf9548 |
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 code runs in ksmbd's in-kernel SMB server, reachable by remote attackers sending SMB2 CREATE requests over TCP port 445 after session establishment.
- AC:LAttack complexity, low
- An authenticated attacker with share write access can plant a malformed parent security.NTACL xattr and reliably trigger the bug by creating a child file; they control both the crafted on-disk descriptor and the concurrent SMB request that exercises inheritance.
- PR:LPrivileges required, low
- Exploitation requires an authenticated SMB2 session with write permission on the parent directory to create files; this is not a pre-authentication code path but does not require host root or special kernel capabilities.
- UI:NUser interaction, none
- Exploitation is fully automated over the SMB protocol and does not require any action from a separate victim user beyond normal server operation.
- S:UScope, unchanged
- The vulnerability is confined to the kernel's ksmbd security context and does not cross a VM, container, or IOMMU security boundary.
- C:HConfidentiality impact, high
- The unchecked SID size computation and memcpy perform an out-of-bounds kernel heap read; stolen memory can be embedded in the newly created file's security descriptor and retrieved by the attacker.
- I:HIntegrity impact, high
- Unvalidated out-of-bounds memory access is a memory corruption primitive in kernel context; per kernel CVSS guidance, such bugs are scored at High integrity impact even when the immediate fault is a read.
- A:HAvailability impact, high
- Reading from attacker-controlled out-of-bounds offsets can dereference unmapped or invalid kernel memory, causing an oops or panic and denying SMB service.