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CVE-2026-89659
NFSD: Prevent client use-after-free during delegation revoke
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-2026-89659 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 | vulnerable | 6.1.190-1 | No fix published anywhere yet, neither in Debian nor upstream for the series bookworm tracks. Watch the Debian tracker page for this CVE. Advisories for bookworm have historically landed a median of 23 days after publication. |
| 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
A delegation stateid holds only a bare pointer to its owning nfs4_client and does not keep it alive. The client survives its stateids only because __destroy_client() drains cl_delegations and cl_revoked before free_client() runs. nfs4_laundromat() breaks that invariant: it unhashes an expired delegation from cl_delegations, drops deleg_lock, then revoke_delegation() relinks it onto cl_revoked under cl_lock. In that window the delegation is on neither list, so client_has_state() can report no remaining state. Every teardown path first requires cl_rpc_users to be zero, but the laundromat holds no such reference. A client whose recalled delegation has just timed out can therefore reach free_client() while revoke_delegation() is still about to dereference cl_lock, a use-after-free. Pin the client with cl_rpc_users across the revoke so teardown blocks until it completes, then reap the delegation from cl_revoked. A client already expiring reaps its own, so skip it and leave the delegation on del_recall_lru.
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/netns.h
fs/nfsd/nfs4state.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
- 9.8 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.44%, higher than 37.2% 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 |
|---|---|---|
| 3.10 | 6.12.111 | 0dd276b1324a |
| 3.10 | 6.18.51 | 3c0a53ee0b44 |
| 3.10 | 7.2.4 | 2a9d637c2a8f |
| 3.10 | 7.3 | 4683ca76b3b7 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| 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 nfs4_laundromat()/revoke_delegation() in fs/nfsd/nfs4state.c. A remote NFSv4 client reaches it over TCP/2049 by OPEN to obtain a delegation, a conflicting OPEN to force CB_RECALL, then DESTROY_CLIENTID or CREATE_SESSION replacement while the laundromat revokes.
- AC:LAttack complexity, low
- The attacker controls both sides: it creates the recalled delegation, learns FATTR4_LEASE_TIME, and issues DESTROY_CLIENTID or same-name CREATE_SESSION/SETCLIENTID_CONFIRM while laundromat unhashes. Many recalled delegations widen the unhash-to-revoke window, and the sequence is retryable.
- PR:NPrivileges required, none
- Default AUTH_SYS nfsd exports do no cryptographic authentication; any host allowed by the export ACL can EXCHANGE_ID/SETCLIENTID, OPEN to get a delegation, and DESTROY_CLIENTID. No local account or capability on the NFS server is required.
- UI:NUser interaction, none
- The attacking NFS client performs the full sequence itself (delegation, recall, teardown). No local user or administrator action on the NFS server is required at attack time.
- S:UScope, unchanged
- The use-after-free corrupts the nfs4_client object in the host kernel's nfsd state tables. Impact stays in that kernel security authority and does not cross a VM, IOMMU, or sandbox boundary.
- C:HConfidentiality impact, high
- revoke_delegation() and nfs4_put_stid() dereference sc_client (cl_lock, cl_revoked, cl_stateids) after free_client() kmem_cache_free's the mergeable client_slab object (KMEM_CACHE flags 0), a UAF the attacker can reallocate via EXCHANGE_ID to disclose kernel memory.
- I:HIntegrity impact, high
- The UAF writes include spin_lock/unlock of freed cl_lock, list_add onto cl_revoked, and refcount_dec_and_lock in nfs4_put_stid on a reclaimable nfs4_client, enabling heap corruption and control-flow hijack.
- A:HAvailability impact, high
- Use-after-free of nfs4_client in an nfsd/laundromat thread oopses or panics the NFS server even without full exploitation, and the attacker can repeat the RPC sequence to deny service.