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CVE-2026-89711
NFSD: remove flawed WARN_ON_ONCE from nfsd_mode_check
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-2026-89711 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
The header for commit e75b23f9e323 ("nfsd: check d_can_lookup in fh_verify of directories") details the assumption that justified adding the WARN_ON_ONCE to nfsd_mode_check(), that assumption is invalid (in the case of NFS reexport). When NFSD exports an NFS filesystem it is very possible for nfsd_mode_check() to encounter a @dentry that doesn't have i_op->lookup (see nfs_fhget()'s NFS_ATTR_FATTR_MOUNTPOINT and NFS_ATTR_FATTR_V4_REFERRAL handling, and d_flags_for_inode()). So remove nfsd_mode_check()'s WARN_ON_ONCE(). The nfserr_notdir return on that branch must stay. It guards the subsequent lookup_one_unlocked() -> __lookup_slow() path, which calls inode->i_op->lookup() with no NULL check, so returning nfserr_notdir is what keeps a client LOOKUP into such a @dentry from dereferencing a NULL method pointer.
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/nfsfh.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 -w CONFIG_NFSD /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 switch
- CONFIG_NFSD
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
- 8.2 high
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.44%, higher than 37.1% 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 |
|---|---|---|
| 4.8 | 5.10.270 | ae251937c6f0 |
| 4.8 | 5.15.221 | e145e8d67a5d |
| 4.8 | 6.1.188 | 9f4434893a27 |
| 4.8 | 6.6.157 | 2ef131323999 |
| 4.8 | 6.12.109 | b55b4d880bb0 |
| 4.8 | 6.18.50 | 7ef182a8fe9c |
| 4.8 | 7.2.4 | a275de3bac56 |
| 4.8 | 7.3 | aa0cf48a448c |
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
- nfsd is the in-kernel NFS server. A remote client reaches nfsd_mode_check() via directory ops (LOOKUP, CREATE, OPEN) over TCP/UDP 2049: svc_recv to nfsd_dispatch to fh_verify to __fh_verify.
- AC:LAttack complexity, low
- On an nfsd host that reexports NFS, mountpoint and NFSv4 referral dentries lack i_op->lookup so d_can_lookup() is false. A client LOOKUP or other S_IFDIR fh_verify on that filehandle hits WARN_ON_ONCE deterministically with no race.
- PR:NPrivileges required, none
- nfsd_mode_check() runs in __fh_verify() after filehandle decode but before check_security_flavor() and nfsd_permission(). Typical nfsd deployments accept AUTH_SYS/AUTH_NULL, so an unauthenticated peer that can reach the export triggers it with self-asserted credentials.
- UI:NUser interaction, none
- The attacker sends the NFS RPCs (NFSv4 PUTFH plus LOOKUP, or NFSv3 LOOKUP) themselves. No victim user must mount a filesystem, open a file, or otherwise interact.
- S:UScope, unchanged
- The WARN_ON_ONCE splat and any panic_on_warn crash remain inside the NFS server kernel. This is not a VM escape, IOMMU bypass, or sandbox boundary crossing.
- C:LConfidentiality impact, low
- WARN_ON_ONCE calls __warn()/dump_stack(), disclosing kernel text addresses in dmesg. There is no use-after-free or out-of-bounds read; the path returns nfserr_notdir without copying kernel memory to the client.
- I:NIntegrity impact, none
- The fix only removes the warning. The existing nfserr_notdir return already blocks lookup_one_unlocked() from calling a NULL i_op->lookup, so there is no memory corruption, write primitive, or control-flow hijack.
- A:HAvailability impact, high
- WARN_ON_ONCE taints the kernel and panics the host when panic_on_warn is enabled or warn_limit is exceeded, so a remote NFS client can deny service by operating on a reexported mountpoint or referral directory.