Look up another CVE · open CVE-2025-21955 in the interactive tracker
CVE-2025-21955
ksmbd: prevent connection release during oplock break notification
Published 2025-04-01 (19 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-2025-21955 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 | 6.12.20-1 | Update the kernel and reboot. Fixed in 6.12.20-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 6.12.20-1 | Update the kernel and reboot. Fixed in 6.12.20-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 6.12.20-1 | Update the kernel and reboot. Fixed in 6.12.20-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
ksmbd_work could be freed when after connection release. Increment r_count of ksmbd_conn to indicate that requests are not finished yet and to not release the connection.
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/connection.c
fs/smb/server/connection.h
fs/smb/server/oplock.c
fs/smb/server/server.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, sidm
- 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
- 2025-04-01 (19 months ago)source
- CVSS
- 8.8 high
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.50%, higher than 41.1% 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 | 6.6.84 | 09aeab680331 |
| 5.15 | 6.12.20 | a4261bbc33fb |
| 5.15 | 6.13.8 | f17d1c63a76b |
| 5.15 | 6.14 | 3aa660c05924 |
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 the in-kernel SMB server listening on TCP/445; the entire path (oplock grant, oplock break trigger, and the connection reset that wins the race) is driven purely by SMB2 traffic from remote clients.
- AC:LAttack complexity, low
- The attacker owns both sides of the race — connection A holds the oplock, connection B triggers the break notification, and the attacker chooses exactly when to reset A's socket — and can retry the sequence unlimited times over the network.
- PR:LPrivileges required, low
- Reaching an oplock/lease break requires an established SMB2 session and tree connect (SMB2_CREATE with an oplock or lease context), so valid low-privileged share credentials (or a guest mapping) are needed; it is not a pre-authentication code path.
- UI:NUser interaction, none
- No victim action is required — the attacker's own two connections create the oplock, trigger the break, and tear down the connection entirely on their own schedule.
- S:UScope, unchanged
- The use-after-free is confined to kernel memory within the same security authority; no VM, IOMMU, or sandbox boundary is crossed.
- C:HConfidentiality impact, high
- The freed ksmbd_conn/tcp_transport objects are dereferenced after release, and reclaiming those kmalloc-64 slots with attacker-sprayed data yields a read primitive over kernel memory and leaks server/session state.
- I:HIntegrity impact, high
- ksmbd_conn_write() performs an indirect call through conn->transport->ops->writev read out of the freed transport, giving control-flow hijack and arbitrary-write potential once the slab is reclaimed with controlled contents.
- A:HAvailability impact, high
- Even without successful grooming the stale dereference of the freed connection, transport, and released socket reliably oopses or panics the kernel, killing the SMB server host.