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CVE-2026-89665
nfsd: reject out-of-range useconds in NFSv2 SETATTR/CREATE
Published 2026-09-11 (37 days ago) by the Linux kernel CNA. Subsystem fs/nfsd. Debian urgency: not yet assigned.
Debian does not build this code. The per-release build detail is under "Where it lives" below.
What to do, per Debian release
Debian's own status for CVE-2026-89665 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 NFSv2 sattr decoder converts the wire useconds to nanoseconds in svcxdr_decode_sattr(): iap->ia_atime.tv_nsec = tmp2 * NSEC_PER_USEC; tmp2 is a u32 and NSEC_PER_USEC is 1000, so the product is computed in unsigned long. On ILP32 that is 32 bits, and an out-of-range useconds value such as 4294968 wraps to tv_nsec == 704. The corruption therefore happens during decode, before any proc function can inspect the value, and a later range check on tv_nsec would see an in-range result and accept it. Rejecting in the decoder yields an RPC GARBAGE_ARGS reply. NFSv2 defines no NFSERR_INVAL, so there is no NFS-level status to return for a malformed time argument, and the check cannot move to the proc function the way the v3/v4 nsec range checks do. Guard the raw useconds before the multiplication and reject values greater than 1000000. useconds == 1000000 is kept: it is the Sun convention for "set to the current server time", and the in-tree Linux NFSv2 client emits it in both the atime and the mtime field for a plain touch / utimes(file, NULL) (see encode_sattr() and xdr_encode_current_server_time() in fs/nfs/nfs2xdr.c). Rejecting 1000000 would turn that common operation into a hard decode failure for both SETATTR and CREATE. 1000000 * NSEC_PER_USEC is 10^9, which does not wrap on ILP32, so the Sun convention value passes through safely. Only genuinely out-of-range values (> 1000000) are rejected. The atime and mtime guards are therefore symmetric. The decoder only applied the Sun convention in the mtime block, which clears ATTR_ATIME_SET|ATTR_MTIME_SET when mtime useconds == 1000000. If a client puts 1000000 in the atime field but not in the mtime field, the atime block stored an out-of-range tv_nsec (10^9) and left ATTR_ATIME_SET set, so the bogus value reached the filesystem. Apply the convention in the atime block as well, clearing ATTR_ATIME_SET so the server uses its current time and ignores the value. Only ATTR_ATIME_SET is cleared there. The mtime block keeps its existing behavior, where 1000000 means "set both atime and mtime to now". [ cel: various tweaks, addenda, and clean-ups ]
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/nfsxdr.c
- What this is
- KERNEL NFSD, SUNRPC, AND LOCKD SERVERSMAINTAINERS section matched on fs/nfsd/ · linux-nfs@vger.kernel.org · Supported
- Kernel module
- none to look for. The file would have gone into nfsd, and Debian does not compile it into that module, so finding nfsd in lsmod would say nothing about this CVE.
Check it on the machine
grep -wE 'CONFIG_NFSD|CONFIG_NFSD_V2' /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.
Show whether Debian builds this, per release
Debian does not build this code. In 5 releases below, every kernel flavour Debian publishes has the switch off, so the vulnerable file is compiled into no kernel binary Debian ships.
- bookworm, trixie, forky, sid, bookworm · linux-6.12n
- n
- not enabled on any flavour - this release does not build the code
- Build switches
- CONFIG_NFSD, CONFIG_NFSD_V2
- 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
- 8.2 high
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:Lsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.62%, higher than 48.7% 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 | cdc3299f7072 |
| 2.6.12 | 5.15.221 | 09a79d9ab35a |
| 2.6.12 | 6.1.188 | 5296a2442d12 |
| 2.6.12 | 6.6.157 | 1db456ffc22e |
| 2.6.12 | 6.12.109 | a937dd1aa2d9 |
| 2.6.12 | 6.18.50 | ad02d095439f |
| 2.6.12 | 7.2.4 | 1195483965a5 |
| 2.6.12 | 7.3 | 26709c8ffe73 |
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's NFSv2 XDR decoder svcxdr_decode_sattr(), used by SETATTR, CREATE, and SYMLINK on the in-kernel NFS server (TCP/UDP 2049); a remote client triggers it by sending those procedures with an out-of-range sattr useconds field.
- AC:LAttack complexity, low
- The attacker fully controls the sattr atime/mtime useconds fields and can deterministically supply a value greater than 1000000, or 1000000 in atime only; the ILP32 multiply wrap and the LP64 out-of-range nsec store need no race or other condition the attacker cannot influence.
- PR:NPrivileges required, none
- Typical nfsd deployments accept AUTH_SYS/AUTH_NULL on reachable exports, so a network peer can obtain a filehandle and assert an owner UID without verified credentials; NFSv2 SETATTR/CREATE then applies the decoded timestamps under those unauthenticated credentials.
- UI:NUser interaction, none
- The attacker alone looks up a filehandle and issues SETATTR or CREATE with a crafted useconds value; no victim user action is required beyond nfsd already exporting a writable filesystem with NFSv2 enabled.
- S:UScope, unchanged
- Impact is confined to the NFS server's own kernel and filesystem inode timestamp metadata under the same OS security authority; there is no VM escape, IOMMU bypass, or other cross-boundary effect.
- C:NConfidentiality impact, none
- The decoder only converts a client useconds field into ia_atime/ia_mtime.tv_nsec and later writes it via setattr; there is no out-of-bounds read, use-after-free, or other kernel-memory disclosure primitive.
- I:HIntegrity impact, high
- Out-of-range nsec is not rejected and notify_change()/timestamp_truncate() store it verbatim on nanosecond filesystems, so ext4_encode_extra_time() overflows the extra field and clobbers epoch bits and XFS mis-stores the timestamp, persistently corrupting on-disk inode metadata; when uncertain versus Low, High is required.
- A:LAvailability impact, low
- Corrupted on-disk timestamps can cause xfs scrub to flag inodes corrupt and can break mtime-based tools on the exported file, degrading service around that inode. No oops, panic, hang, or other kernel crash was found on this path, so availability loss is partial rather than total.