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CVE-2026-74521
ksmbd: use memcmp() to compare ClientGUIDs
Published 2026-08-15 (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-2026-74521 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-6528-1 · 2026-09-29 · text | 6.12.111-1 | Update the kernel and reboot. DSA-6528-1 shipped the fix on 2026-09-29, in 6.12.111-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.8-1 | Update the kernel and reboot. Fixed in 7.1.8-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.8-1 | Update the kernel and reboot. Fixed in 7.1.8-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4817-1 · 2026-10-04 · text | 6.12.111-1~deb12u1 | Update the kernel and reboot. DLA-4817-1 shipped the fix on 2026-10-04, in 6.12.111-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
ClientGUID is a fixed-size binary value and can contain embedded NUL bytes. strncmp() stops comparing at the first NUL byte, so different ClientGUID values can incorrectly be treated as equal. Use memcmp() in SMB3 multichannel session binding and FSCTL_VALIDATE_NEGOTIATE_INFO to compare all SMB2_CLIENT_GUID_SIZE bytes.
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/smb2pdu.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
- 2026-08-15 (2 months ago)source
- CVSS
- 9.1 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:Nsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.41%, higher than 33.3% 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 | 6.1.189 | 7bde6f181fa3 |
| 5.15 | 6.6.158 | 5a0367bf5ce4 |
| 5.15 | 6.12.111 | ba2a5b66f1d4 |
| 5.15 | 6.18.53 | a386c8304301 |
| 5.15 | 7.1.8 | d53536329982 |
| 5.15 | 7.2 | e8bb506e6ef7 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4841-1 | 2026-10-17 | bookworm 6.1.190-1 | tracker |
| DLA-4817-1 | 2026-10-04 | bookworm 6.12.111-1~deb12u1 | tracker |
| DSA-6528-1 | 2026-09-29 | trixie 6.12.111-1 | 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 bug is in ksmbd's in-kernel SMB server on TCP/445; vulnerable ClientGUID comparisons run while handling remotely supplied SMB2 SESSION_SETUP multichannel binding and SMB2 IOCTL FSCTL_VALIDATE_NEGOTIATE_INFO requests from any network peer.
- AC:LAttack complexity, low
- The attacker controls negotiate ClientGUID bytes (including embedded NULs), can open many TCP connections, brute-force sequential session IDs, and repeatedly attempt binding until strncmp accepts a prefix-colliding GUID without conditions outside attacker control.
- PR:NPrivileges required, none
- Multichannel SESSION_SETUP binding is processed before the attacker authenticates; once a victim SMB2_SESSION_VALID session is targeted, ntlm_authenticate skips password verification on rebind, requiring only network reachability plus session ID and username guessing, not a valid server account.
- UI:NUser interaction, none
- Exploitation requires no victim interaction such as opening files, mounting shares, or clicking links; the attacker drives negotiate, binding, and IOCTL requests directly against the exposed SMB service.
- S:UScope, unchanged
- Successful exploitation grants unauthorized use of an existing SMB session and its exported share access within ksmbd's authority, not crossing into another security domain such as host kernel privilege or VM escape.
- C:HConfidentiality impact, high
- Bypassing ClientGUID verification enables hijacking another client's authenticated SMB3 multichannel session, providing read access to all files, directories, and metadata that victim session may access on server exports.
- I:HIntegrity impact, high
- A fraudulently bound channel receives the victim session's channel signing keys and full SMB operation rights, allowing arbitrary create, write, modify, and delete of files and objects permitted to that hijacked session.
- A:NAvailability impact, none
- This is a logical string-comparison flaw causing incorrect authorization decisions, not memory corruption or a crash primitive; exploitation does not inherently panic, oops, or deny kernel or ksmbd availability.