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CVE-2026-43215
cifs: Fix locking usage for tcon fields
Published 2026-05-06 (5 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-2026-43215 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 | 6.12.85-1 | Update the kernel and reboot. Fixed in 6.12.85-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 6.19.6-1 | Update the kernel and reboot. Fixed in 6.19.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 6.19.6-1 | Update the kernel and reboot. Fixed in 6.19.6-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
We used to use the cifs_tcp_ses_lock to protect a lot of objects that are not just the server, ses or tcon lists. We later introduced srv_lock, ses_lock and tc_lock to protect fields within the corresponding structs. This was done to provide a more granular protection and avoid unnecessary serialization. There were still a couple of uses of cifs_tcp_ses_lock to provide tcon fields. In this patch, I've replaced them with tc_lock.
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/cached_dir.c
fs/smb/client/smb2misc.c
fs/smb/client/smb2ops.c
fs/smb/client/smb2pdu.c
fs/smb/client/trace.h
- 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
- 2026-05-06 (5 months 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
- 0.41%, higher than 33.6% 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 |
|---|---|---|
| 6.0 | 6.6.128 | 953953abb66e |
| 6.0 | 6.12.75 | 601dd3b79769 |
| 6.0 | 6.18.16 | 3969db6b22e3 |
| 6.0 | 6.19.6 | 8c59eeeeffa1 |
| 6.0 | 7.0 | 96c4af418586 |
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 CIFS/SMB client code is exercised through SMB protocol activity with a remote server, including DFS referral handling, reconnects, cancelled closes, and cached directory lease cleanup. A malicious or compromised SMB server can influence these network-driven paths after a client connects.
- AC:LAttack complexity, low
- The issue is a locking/refcount race, but the remote SMB peer can repeatedly trigger reconnects, delayed/cancelled responses, DFS behavior, and lease/cached-directory activity to exercise the affected paths. There is no rare configuration or uncontrollable one-shot condition required beyond a CIFS mount being in use.
- PR:NPrivileges required, none
- The attacker does not need privileges on the vulnerable client system; they can act as the SMB server or network peer providing crafted behavior to the CIFS client. Although mounting CIFS locally is privileged, that is victim-side setup/user interaction rather than attacker privileges on the impacted host.
- UI:RUser interaction, required
- Exploitation requires the client system to mount or otherwise use an SMB/CIFS share, which is a victim/user or administrator action. Once connected, the malicious server can drive the vulnerable network paths.
- S:UScope, unchanged
- The impact is within the same Linux kernel security authority running the CIFS client. This is standard kernel memory corruption/panic potential, not a VM escape or cross-authority boundary bypass.
- C:HConfidentiality impact, high
- The inconsistent locking can corrupt the tcon reference count and allow use-after-free of `struct cifs_tcon`. Kernel UAF conditions are reasonably capable of being shaped into memory disclosure primitives.
- I:HIntegrity impact, high
- The same premature free and later dereference/use of the tcon object creates kernel heap use-after-free memory corruption. Under the required higher-severity rule, this is treated as potentially exploitable for arbitrary write or control-flow compromise.
- A:HAvailability impact, high
- At minimum, refcount corruption and use-after-free of a live CIFS tcon can lead to kernel oops, panic, or hangs in reconnect/close/DFS workqueue paths. The condition can be triggered repeatedly by malicious SMB server behavior while the share is mounted.