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CVE-2026-89699
nfsd: validate symlink target length in NFSv4 CREATE
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-2026-89699 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
nfsd4_decode_create() accepts an unbounded cr_datalen from the wire for NF4LNK symlink targets, allowing a client to force a kmalloc of up to the maximum RPC payload size (several MiB) per COMPOUND op that persists until compound teardown. The VFS rejects oversized targets with ENAMETOOLONG, but the allocation has already occurred. Reject cr_datalen == 0 early with nfserr_inval and cr_datalen greater than NFS4_MAXPATHLEN (PATH_MAX) with nfserr_nametoolong to bound the allocation.
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/nfs4xdr.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
- 0.50%, higher than 41.6% 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 |
|---|---|---|
| 2.6.12 | 5.10.270 | 36011308b3f0 |
| 2.6.12 | 5.15.221 | 888dad4b5573 |
| 2.6.12 | 6.1.188 | 8661d3cb38fd |
| 2.6.12 | 6.6.157 | 3ef5e7a01a4d |
| 2.6.12 | 6.12.109 | b24843ea3de1 |
| 2.6.12 | 6.18.50 | 895a485cd375 |
| 2.6.12 | 7.2.4 | 45ec115cbfec |
| 2.6.12 | 7.3 | 041f57056e5f |
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 flaw is reached through the in-kernel NFS server (nfsd) when a remote client sends an NFSv4 COMPOUND containing OP_CREATE of type NF4LNK over TCP/UDP port 2049; nfsd4_decode_create() runs during XDR decode of that compound.
- AC:LAttack complexity, low
- A client fully controls cr_datalen on the wire; one COMPOUND with an NF4LNK target larger than PATH_MAX but within the RPC payload deterministically forces svcxdr_dupstr()/kmalloc of that size, with no race or attacker-uncontrollable condition.
- PR:NPrivileges required, none
- The oversized kmalloc occurs in nfsd4_decode_compound() before SEQUENCE/session checks, PUTFH, fh_verify(), or export flavor enforcement; AUTH_NULL and AUTH_UNIX are accepted at the RPC layer, so an unauthenticated peer that can reach nfsd needs no host credentials.
- UI:NUser interaction, none
- Exploitation requires only attacker-sent NFSv4 COMPOUND RPCs; no administrator or end-user action on the server is needed beyond nfsd already listening.
- S:UScope, unchanged
- Impact is confined to kernel memory consumption on the NFS server within the same security authority; there is no VM escape, IOMMU bypass, or sandbox boundary crossing.
- C:NConfidentiality impact, none
- The defect only copies attacker-supplied symlink-target bytes into a correctly sized kmalloc buffer; there is no out-of-bounds read, use-after-free, or other disclosure primitive.
- I:NIntegrity impact, none
- There is no memory corruption or attacker-controlled write; the buffer is sized to cr_datalen and is only a temporary copy that filesystems later reject with ENAMETOOLONG.
- A:HAvailability impact, high
- Each COMPOUND can force a GFP_KERNEL kmalloc of up to the maximum RPC payload (several MiB) on an nfsd thread until compound teardown; flooding concurrent requests across nfsd threads can exhaust kernel memory and OOM or stall the host.