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CVE-2026-98171
smb: client: fix next_buffer UAF and NextCommand bounds in compound PDUs
Published 2026-10-06 (13 days 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-2026-98171 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-4841-1 · 2026-10-17 · text | 6.1.190-1 | Update the kernel and reboot. DLA-4841-1 shipped the fix on 2026-10-17, in 6.1.190-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixedDSA-6568-1 · 2026-10-16 · text | 6.12.113-1 | Update the kernel and reboot. DSA-6568-1 shipped the fix on 2026-10-16, in 6.12.113-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.2.8-1 | Update the kernel and reboot. Fixed in 7.2.8-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.2.8-1 | Update the kernel and reboot. Fixed in 7.2.8-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
Fix several related bounds checking and pointer lifecycle issues in receive_encrypted_standard()'s handling of compound encrypted frames: - Clear next_buffer after assigning it to server->bigbuf. A stale next_buffer pointer can lead to a use-after-free on subsequent error paths. - Update pdu_length to the decrypted plaintext size (buf_size). Using the pre-decryption length allows NextCommand to point into stale ciphertext residue. - Reject next_cmd values smaller than MID_HEADER_SIZE(server). - Fix an integer overflow in the upper bound check by verifying pdu_length - next_cmd < MID_HEADER_SIZE(server), ensuring the trailing slice is large enough for a header.
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/client/smb2ops.c
- What this is
- COMMON INTERNET FILE SYSTEM CLIENT (CIFS and SMB3)MAINTAINERS section matched on fs/smb/client/ · linux-cifs@vger.kernel.org · Supported
- Kernel module
- cifs
Check it on the machine
grep -wE 'CONFIG_CIFS|CONFIG_SMBFS' /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 cifsPrints 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_CIFS, CONFIG_SMBFS
- 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
- 2026-10-06 (13 days ago)source
- CVSS
- 8.8 high
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.51%, higher than 41.9% 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 |
|---|---|---|
| 4.19 | 6.1.189 | 72eaef1f37a3 |
| 4.19 | 6.6.158 | 491e33144dee |
| 4.19 | 6.12.112 | 874994657970 |
| 4.19 | 6.18.54 | 96c436e4b010 |
| 4.19 | 7.2.8 | 858d5ac22cb8 |
| 4.19 | 7.3 | 05762c5bc1cf |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4841-1 | 2026-10-17 | bookworm 6.1.190-1 | tracker |
| DSA-6568-1 | 2026-10-16 | trixie 6.12.113-1 | 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
- The malformed input is an encrypted SMB2 compound response (TRANSFORM header, inner NextCommand chain) sent by the SMB server over TCP/445 and parsed by receive_encrypted_standard() via cifs_demultiplex_thread -> smb3_receive_transform, so the bytes that cause the bug arrive over a routable network protocol.
- AC:LAttack complexity, low
- The malicious server fully controls the sequence: a compound response whose first PDU has non-zero NextCommand, then a trailing PDU with NextCommand=0 and STATUS_PENDING or an unmatched MID with a bad header makes cifs_handle_standard() return non-zero, freeing the stale next_buffer every time; no race or external state is involved.
- PR:NPrivileges required, none
- The attacker is the SMB server endpoint. It needs no account or privilege on the victim client, because it negotiates the session and encryption keys itself and so can produce valid encrypted frames.
- UI:RUser interaction, required
- A victim user or admin has to mount (or be redirected by DFS to) a share hosted on the attacker-controlled server before the client receives any encrypted compound response, so user action is required.
- S:UScope, unchanged
- The corruption is confined to the client kernel's own cifs receive buffers and slab/mempool objects, which is a standard kernel compromise and does not cross into a separate security authority.
- C:HConfidentiality impact, high
- The error path frees server->smallbuf/bigbuf while it is still installed, which is a UAF on a kernel heap object. The object can be reallocated and its contents processed as SMB responses, a primitive usable for kernel memory disclosure.
- I:HIntegrity impact, high
- After the bad free, allocate_buffers() keeps the dangling smallbuf/bigbuf, and cifs_read_from_socket() writes the next frame's server-chosen bytes into the freed object (with a later double free), giving an attacker-controlled write into reallocated kernel memory.
- A:HAvailability impact, high
- The use-after-free and double free of the cifs receive buffers in the demultiplex thread can corrupt the slab or mempool, causing a kernel oops or panic on the client and making every mount on that host unavailable.