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CVE-2026-89696
nfsd: block non-SAVEFH ops after FOREIGN PUTFH to prevent NULL deref
Published 2026-09-11 (37 days ago) by the Linux kernel CNA. Subsystem fs/nfsd. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-89696 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 | 7.2.6-1 | 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. |
| 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
When CONFIG_NFSD_V4_2_INTER_SSC is enabled, nfsd4_putfh() can return success with fh_dentry and fh_export both NULL if fh_verify() returns nfserr_stale and putfh->no_verify is true. The NFSD4_FH_FOREIGN flag is set, but the compound dispatch loop only uses this flag to bypass the nfserr_nofilehandle check -- it does not prevent subsequent ops from running with a NULL fh_dentry. A remote client can exploit this by crafting a COMPOUND that includes an inter-SSC COPY (which causes check_if_stalefh_allowed() to set no_verify=true on the saved PUTFH) with an additional op inserted between the source PUTFH and SAVEFH. For example, SETATTR calls fh_want_write() which dereferences fh_export->ex_path.mnt without calling fh_verify() first, causing a NULL pointer dereference in the nfsd kthread. Fix this by gating the dispatch loop: when NFSD4_FH_FOREIGN is set and fh_dentry is NULL, only OP_SAVEFH (needed for the inter-SSC flow) and ops with ALLOWED_WITHOUT_FH (which don't need a resolved filehandle) may proceed. All other ops receive nfserr_stale, per RFC 7862 Section 15.2.3 which specifies that foreign filehandle validation is deferred to the consuming operation and NFS4ERR_STALE returned at that point.
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 nfsdPrints 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_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
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
- 0.75%, higher than 54.2% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- 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 |
|---|---|---|
| 5.6 | 5.10.270 | ffdc844e673d |
| 5.6 | 5.15.221 | 00c84f4eec71 |
| 5.6 | 6.1.188 | 2d9846fd1c76 |
| 5.6 | 6.6.157 | 977e6f006a7a |
| 5.6 | 6.12.109 | 35f248bd40b4 |
| 5.6 | 6.18.50 | 311f7d926630 |
| 5.6 | 7.2.4 | bf4d338dc862 |
| 5.6 | 7.3 | c59738a00aa5 |
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 nfsd NFSv4 COMPOUND dispatch and is triggered by a crafted NFSv4.2 COMPOUND over TCP/2049 (PUTFH of a stale fh, then SETATTR, with an inter-SSC COPY in the same compound). No local access is required.
- AC:LAttack complexity, low
- After EXCHANGE_ID/CREATE_SESSION, the attacker fully controls the compound that marks the source PUTFH no_verify and inserts SETATTR before SAVEFH, so fh_want_write hits NULL fh_export deterministically with no race or layout dependency.
- PR:NPrivileges required, none
- AUTH_SYS (the usual nfsd configuration) verifies no secret; a client just asserts a UID to create a session. SETATTR then runs on the FOREIGN fh without fh_verify, so no export access check or other credential gate is reached.
- UI:NUser interaction, none
- The attacker sends the COMPOUND directly to nfsd; no administrator or victim user action is required at exploitation time.
- S:UScope, unchanged
- The NULL dereference oopses the nfsd kthread in the host kernel's own authority and does not cross a VM, IOMMU, or sandbox boundary.
- C:NConfidentiality impact, none
- After the stale FOREIGN PUTFH, fh_export and fh_dentry are NULL rather than dangling or attacker-controlled, so the fault is on the first load of fh_export->ex_path.mnt with no disclosure.
- I:NIntegrity impact, none
- This is a pure NULL pointer dereference in fh_want_write with no write to attacker-influenced memory and no control-flow hijack primitive.
- A:HAvailability impact, high
- The NULL dereference oopses the nfsd kthread and can be replayed to exhaust nfsd threads, denying NFS service; with panic_on_oops it panics the host.