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CVE-2026-64578
ksmbd: validate compound request size before reading StructureSize2
Published 2026-08-05 (2 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-2026-64578 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-4777-1 · 2026-09-08 · text | 6.1.187-1 | Update the kernel and reboot. DLA-4777-1 shipped the fix on 2026-09-08, in 6.1.187-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixedDSA-6415-1 · 2026-08-06 · text | 6.12.101-1 | Update the kernel and reboot. DSA-6415-1 shipped the fix on 2026-08-06, in 6.12.101-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.6-1 | Update the kernel and reboot. Fixed in 7.1.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.6-1 | Update the kernel and reboot. Fixed in 7.1.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4745-1 · 2026-08-18 · text | 6.12.101-1~deb12u1 | Update the kernel and reboot. DLA-4745-1 shipped the fix on 2026-08-18, in 6.12.101-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
When ksmbd validates a compound (chained) SMB2 request, ksmbd_smb2_check_message() reads pdu->StructureSize2 without first checking that the compound element is large enough to contain it. StructureSize2 is a 2-byte field at offset 64 (__SMB2_HEADER_STRUCTURE_SIZE) from the start of each element. The compound-walking logic only guarantees that a full 64-byte SMB2 header is present for the trailing element: when NextCommand is 0, len is reduced to the number of bytes remaining after next_smb2_rcv_hdr_off. A remote client can craft a compound request whose last element has exactly 64 bytes, so the 2-byte StructureSize2 read at offset 64 extends one byte past the receive buffer, producing a slab-out-of-bounds read. BUG: KASAN: slab-out-of-bounds in ksmbd_smb2_check_message (fs/smb/server/smb2misc.c:402) Read of size 2 at addr ffff888012ae31ac by task kworker/0:1/14 The buggy address is located 172 bytes inside of allocated 173-byte region Workqueue: ksmbd-io handle_ksmbd_work Call Trace: ... kasan_report (mm/kasan/report.c:595) ksmbd_smb2_check_message (fs/smb/server/smb2misc.c:402) handle_ksmbd_work (fs/smb/server/server.c:119) process_one_work (kernel/workqueue.c:3314) worker_thread (kernel/workqueue.c:3397) kthread (kernel/kthread.c:436) ret_from_fork (arch/x86/kernel/process.c:158) ret_from_fork_asm (arch/x86/entry/entry_64.S:245) Reject any compound element that is too small to hold StructureSize2 before dereferencing it.
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/smb2misc.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, 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
Signals
- Published
- 2026-08-05 (2 months ago)source
- CVSS
- 8.2 high
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.34%, higher than 25.4% 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 | 5.15.216 | 415d0fff0451 |
| 5.15 | 6.1.183 | 1b6740525f5a |
| 5.15 | 6.6.148 | 2c307126ed8e |
| 5.15 | 6.12.101 | f7550a91ab21 |
| 5.15 | 6.18.42 | f0e337e7db67 |
| 5.15 | 7.1.6 | ea128f06d2fb |
| 5.15 | 7.2 | 15b38176fd15 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4777-1 | 2026-09-08 | bookworm 6.1.187-1 | tracker |
| DLA-4745-1 | 2026-08-18 | bookworm 6.12.101-1~deb12u1 | tracker |
| DSA-6415-1 | 2026-08-06 | trixie 6.12.101-1 | 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
- ksmbd is an in-kernel SMB server reached over TCP/445; a remote client triggers the bug by sending a crafted compound SMB2 request that is processed in handle_ksmbd_work() → ksmbd_verify_smb_message() → ksmbd_smb2_check_message().
- AC:LAttack complexity, low
- The attacker fully controls compound layout and can set the trailing element's remaining length to exactly 64 bytes so StructureSize2 is read one byte past the receive buffer; no race or attacker-uncontrollable condition is required.
- PR:NPrivileges required, none
- On affected kernels before the compound SESSION_VALID check, a compound NtLmNegotiate SESSION_SETUP (pre-auth, no credentials) sets work->sess and the trailing element still reaches ksmbd_smb2_check_message(); guest/anonymous SESSION_SETUP likewise needs no host privileges.
- UI:NUser interaction, none
- Exploitation requires only attacker-sent SMB2 protocol traffic to a listening ksmbd service; no victim user action is needed.
- S:UScope, unchanged
- The out-of-bounds read and any resulting kernel impact occur in the host ksmbd/workqueue context and do not cross a VM, IOMMU, or other security-authority boundary.
- C:LConfidentiality impact, low
- The flaw is a strictly bounded slab out-of-bounds read of one byte past the receive buffer (2-byte StructureSize2 at offset 64 when only 64 bytes remain); it may disclose adjacent slab contents but not an arbitrary read primitive.
- I:NIntegrity impact, none
- The bug only reads past the buffer for a StructureSize2 comparison; for a 64-byte trailing element every valid command size is then rejected, and there is no out-of-bounds write or other modification primitive.
- A:HAvailability impact, high
- KASAN reports a slab-out-of-bounds BUG in ksmbd_smb2_check_message(), and the same access can oops/panic hardened or redzone-enabled kernels; a remote peer can retrigger it with further crafted compound requests.