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CVE-2026-89706
nfsd: Reset write verifier when async COPY writeback fails
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-2026-89706 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
Async COPY captures nn->writeverf at request time and reports it to the client via CB_OFFLOAD after the worker kthread completes. When the post-copy vfs_fsync_range() or filemap_check_wb_err() in _nfsd_copy_file_range() reports an error, the worker correctly leaves NFSD4_COPY_F_COMMITTED clear so that CB_OFFLOAD encodes wr_stable_how as NFS_UNSTABLE, but the server's write verifier is not rotated. A client that receives NFS_UNSTABLE in CB_OFFLOAD follows up with COMMIT to make the copied data durable. With the verifier unchanged, COMMIT returns the same value the client just received via CB_OFFLOAD, and the client concludes the copy is durable -- silently dropping the data whose writeback in fact failed. This violates the UNSTABLE+COMMIT durability contract (RFC 7862 section 15.1, RFC 8881 section 18.32) and matches the bug just fixed in nfsd_vfs_write() and nfsd_commit(). Rotate nn->writeverf at the writeback-failure site. The async COPY worker has no svc_rqst, so commit_reset_write_verifier() is not available here; calling nfsd_reset_write_verifier() directly mirrors the trace-less reset already used by nfsd_file_check_write_error() for the same purpose. Filter out -EAGAIN and -ESTALE, matching commit_reset_write_verifier(), since neither indicates a durable-storage failure.
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:H/A:Nsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.53%, higher than 43.3% 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 |
|---|---|---|
| 5.10.220 | 5.10.270 | cc5f7cef0659 |
| 5.14 | 5.15.221 | c6d2f6372c7e |
| 5.14 | 6.1.188 | 8a08ffe73eb2 |
| 5.14 | 6.6.157 | 6b13d26cc21b |
| 5.14 | 6.12.109 | 31d4d0a62ec4 |
| 5.14 | 6.18.50 | 591134e059e3 |
| 5.14 | 7.2.4 | bf0cd31a9abc |
| 5.14 | 7.3 | f5cb2276954c |
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 NFSv4.2 async COPY worker (_nfsd_copy_file_range), reached by sending a COMPOUND with OP_COPY (ca_synchronous=false) to the in-kernel NFS server over TCP/2049. No local access to the server is required.
- AC:LAttack complexity, low
- A client can induce the writeback-failure path by filling the export or hitting quota so vfs_fsync_range()/filemap_check_wb_err() returns ENOSPC/EDQUOT after the copy dirties pages, then issue async COPY; CB_OFFLOAD UNSTABLE plus COMMIT with an unchanged verifier is the specified client sequence and needs no race.
- PR:NPrivileges required, none
- Default nfsd deployments use AUTH_SYS/AUTH_NULL with IP-based exports and no cryptographic authentication, so any host allowed by the export can EXCHANGE_ID/CREATE_SESSION, OPEN source and destination files, and issue async COPY by asserting a uid.
- UI:NUser interaction, none
- The attacking NFS client issues COPY, receives CB_OFFLOAD, and sends COMMIT itself. No administrator or end-user action on the NFS server is required beyond nfsd already serving an NFSv4.2 export.
- S:UScope, unchanged
- Silent durability failure affects file data on the exported filesystem in the same kernel security authority as nfsd. This is not a VM escape, IOMMU bypass, or sandbox breakout.
- C:NConfidentiality impact, none
- The defect only skips rotating nn->writeverf after a failed COPY fsync; it does not read out-of-bounds, leak kernel memory, or disclose file contents beyond ordinary NFS access.
- I:HIntegrity impact, high
- vfs_fsync_range() consumes the writeback error, COMMIT then returns the same verifier, and the client drops the UNSTABLE copy as durable, silently discarding destination file data. Entire COPY ranges, including whole-file copies, can be lost while applications treat them as persisted.
- A:NAvailability impact, none
- The missing verifier rotation does not oops, panic, deadlock, or hang nfsd; the server continues serving RPCs. Impact is incorrect durability reporting and data loss, not loss of the NFS service itself.