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CVE-2026-89679

nfsd: fix null dereference in nfsd4_setattr for deleg timestamp attrs

7.5 EPSS 0.7% AV:N

Published 2026-09-11 (37 days ago) by the Linux kernel CNA. Subsystem fs/nfsd. Debian urgency: unimportant.

What to do, per Debian release

Debian's own status for CVE-2026-89679 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	not affected	6.12.113-1	Nothing to do. trixie never shipped the vulnerable code.
Debian 14forky	fixed	7.2.6-1	Update the kernel and reboot. Fixed in 7.2.6-1. A running kernel keeps the old code until the machine restarts.
Debian unstablesid	fixed	7.2.6-1	Update the kernel and reboot. Fixed in 7.2.6-1. A running kernel keeps the old code until the machine restarts.

The vulnerability

What the bug is

When a SETATTR request includes FATTR4_WORD2_TIME_DELEG_ACCESS or FATTR4_WORD2_TIME_DELEG_MODIFY in the attribute bitmap, nfsd4_setattr() sets deleg_attrs=true and calls nfs4_preprocess_stateid_op() to validate the stateid. If the client supplies the NFSv4 "one stateid" (all-0xFF bytes), check_special_stateids() returns nfs_ok without populating the output nfs4_stid pointer, because the special-stateid path in nfs4_preprocess_stateid_op() jumps to done: with s==NULL, and the "if (s)" block that would set *cstid is skipped. The local variable `st` remains NULL. Back in nfsd4_setattr(), the if (deleg_attrs) block then unconditionally dereferences st->sc_type (at offset 4 from NULL), causing a kernel oops. This is remotely triggerable by any NFSv4 client: send COMPOUND [PUTROOTFH, SETATTR(ONE_STATEID, {bmval2=FATTR4_WORD2_TIME_DELEG_ACCESS, ...})]. No authentication, delegation, or prior state is required. Fix by adding a NULL check before the dereference. A special stateid is not a delegation stateid, so the existing nfserr_bad_stateid return value is already correct; we only need to guard the pointer dereference itself.

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/nfsd/nfs4proc.c

What this is: KERNEL NFSD, SUNRPC, AND LOCKD SERVERSMAINTAINERS section matched on fs/nfsd/ · linux-nfs@vger.kernel.org · Supported
Kernel module: nfsd

Check it on the machine

grep -wE 'CONFIG_NFSD|CONFIG_NFSD_V4' /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 nfsd

Prints 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_NFSD, CONFIG_NFSD_V4
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

Debian tracker CVE record NVD kernel CNA record

Signals

Published: 2026-09-11 (37 days ago)source
CVSS: 7.5 high
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:Hsource recompute it
Attack vector: network (AV:N)source
EPSS: 0.72%, higher than 52.9% of all CVEssource
CISA KEV: not listedthe KEV catalogue
Debian urgency: unimportantsource
Subsystem: fs/nfsd

Upstream fixes

The versions the CNA records as introducing and fixing this, and the commit for each. source

Introduced	Fixed in	Commit
6.14	6.18.50	3c5119b799a7
6.14	7.2.4	3c896db12389
6.14	7.3	fe456c8c0931

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: nfsd is the in-kernel NFSv4 server; nfsd4_setattr() is reached from NFSPROC4_COMPOUND over TCP/2049. A remote peer triggers it with PUTROOTFH then SETATTR; no local access to the server is required.
AC:LAttack complexity, low: COMPOUND [PUTROOTFH, SETATTR(ONE_STATEID, FATTR4_WORD2_TIME_DELEG_ACCESS)] makes check_special_stateids() return nfs_ok with st still NULL, then nfsd4_setattr() unconditionally reads st->sc_type. The crash is deterministic; no race, special layout, or rare config is required.
PR:NPrivileges required, none: The NULL deref runs in nfsd4_setattr() before nfsd_setattr()/nfsd_permission and needs no NFSv4 session, delegation, or prior state (v4.0 COMPOUND). Default AUTH_SYS/AUTH_NULL credentials are self-asserted, so any host that can reach the NFS port can trigger it.
UI:NUser interaction, none: The attacker sends the crafted NFSv4 COMPOUND themselves; no victim must mount an export, open a file, or otherwise interact.
S:UScope, unchanged: The NULL dereference oopses an nfsd thread on the NFS server. Impact stays inside that kernel's security authority and does not cross a VM, IOMMU, or sandbox boundary.
C:NConfidentiality impact, none: This is a pure NULL-pointer read of st->sc_type at offset 4 from NULL. It oopses without leaking kernel memory or providing an information-disclosure primitive.
I:NIntegrity impact, none: The bug only reads through a NULL nfs4_stid pointer; it does not write memory, corrupt adjacent objects, or yield a control-flow hijack. A remote NFS client cannot map the NULL page.
A:HAvailability impact, high: The NULL dereference causes a kernel oops in the nfsd thread. Repeated requests can exhaust nfsd threads or panic the host (panic_on_oops), denying NFS service and taking down the server.