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CVE-2026-98261
cifs: Fix server use-after-free in cifs_chan_skip_or_disable()
Published 2026-10-06 (13 days ago) by the Linux kernel CNA. Subsystem fs/smb. Debian urgency: unimportant.
What to do, per Debian release
Debian's own status for CVE-2026-98261 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 | not affected | 6.1.190-1 | Nothing to do. bookworm never shipped the vulnerable code. |
| 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
When a secondary channel is no longer supported by the server, cifs_chan_skip_or_disable() drops the channel reference with cifs_put_tcp_session() and then continues to use the server pointer by calling cifs_signal_cifsd_for_reconnect() on it and reading its primary_server pointer. cifs_put_tcp_session() can drop the last reference of the channel and tear it down, so both the channel and the primary server (whose reference is also dropped by cifs_put_tcp_session()) can be freed before they are signaled for reconnect. Signal the channel and the primary server and capture the primary server pointer before dropping the channel reference with cifs_put_tcp_session().
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/smb2pdu.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
- 8.1 high
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.44%, higher than 36.8% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- unimportantsource
- 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 |
|---|---|---|
| 6.6.15 | 6.6.158 | 0338489960dd |
| 6.8 | 6.12.112 | edd52eae5fcf |
| 6.8 | 6.18.54 | fcc0a935bb6e |
| 6.8 | 7.2.8 | 7a1b27780b11 |
| 6.8 | 7.3 | 717e0a25036b |
| 6.7.3 | 0 | 0 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| 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 trigger is the SMB2 NEGOTIATE response a remote server sends while a secondary channel reconnects. If it omits SMB2_GLOBAL_CAP_MULTI_CHANNEL with chan_count>1, smb2_reconnect() calls smb3_update_ses_channels(disable_mchan=true) and then cifs_chan_skip_or_disable(). That function drops the channel's last reference and then uses the server.
- AC:HAttack complexity, high
- cifs_put_tcp_session() only SIGKILLs the channel's cifsd thread. clean_demultiplex_info() sleeps at least 125 ms before kfree_sensitive(server), and the freed accesses (set_need_reco, server->primary_server) run a few instructions after the put. The reconnecting thread must be descheduled across that window, which the remote attacker cannot control. The attacker must also be the server or on-path.
- PR:NPrivileges required, none
- The attacker acts as the SMB server or an on-path party and needs no account or privilege on the client. The capability check runs in smb2_reconnect() right after cifs_negotiate_protocol(), before cifs_setup_session() authenticates anything.
- UI:NUser interaction, none
- An on-path attacker can force a reconnect of an existing multichannel mount and forge the unsigned NEGOTIATE response, so the victim takes no action. Signing and the 3.1.1 preauth hash are checked only after the bug fires.
- S:UScope, unchanged
- The freed TCP_Server_Info and the code using it are both in the client kernel, so this is a kernel memory-safety issue within the same security authority.
- C:HConfidentiality impact, high
- After the freed channel TCP_Server_Info is reused, cifs_chan_skip_or_disable() reads server->primary_server out of it and dereferences that pointer. An attacker who controls the reallocated object controls that pointer.
- I:HIntegrity impact, high
- cifs_signal_cifsd_for_reconnect() spin-locks and writes tcpStatus first in the freed channel and then through the primary_server pointer read from it. With the memory reallocated, that is a lock and write at an attacker-influenced address.
- A:HAvailability impact, high
- Taking srv_lock and writing tcpStatus in a freed TCP_Server_Info, or following a stale primary_server pointer, can corrupt the slab or oops the client kernel.