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CVE-2026-89703
nfsd: set SC_STATUS_FREED in nfsd4_drop_revoked_stid for delegations
Published 2026-09-11 (37 days ago) by the Linux kernel CNA. Subsystem fs/nfsd. Debian urgency: unimportant.
What to do, per Debian release
Debian's own status for CVE-2026-89703 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 | not affected | 6.1.190-1 | Nothing to do. bookworm never shipped the vulnerable code. |
| 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_drop_revoked_stid() handles FREE_STATEID for admin-revoked delegations but does not set SC_STATUS_FREED before releasing cl_lock. revoke_delegation() uses this flag to detect whether FREE_STATEID has already processed the delegation -- without it, the freed delegation is added to cl_revoked via list_add(), producing a use-after-free when cl_revoked is later traversed in __destroy_client(). The SC_STATUS_REVOKED path in nfsd4_free_stateid() (line 7983) already sets SC_STATUS_FREED correctly. Apply the same pattern to the SC_STATUS_ADMIN_REVOKED path in nfsd4_drop_revoked_stid().
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/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
- 0.62%, higher than 48.5% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- unimportantsource
- 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 |
|---|---|---|
| 6.12 | 6.12.109 | d832a0587528 |
| 6.12 | 6.18.50 | 1e4795766719 |
| 6.12 | 7.2.4 | a6d89032e5c6 |
| 6.12 | 7.3 | 650d370cfbc6 |
| 6.11.6 | 0 | 0 |
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 UAF is in nfsd nfsd4_drop_revoked_stid()/revoke_delegation() in fs/nfsd/nfs4state.c. A remote NFSv4 peer reaches it over TCP/2049 via svc_process to nfsd_dispatch to nfsd4_proc_compound: SEQUENCE then FREE_STATEID (ALLOWED_WITHOUT_FH), or a v4.0 op that hits nfsd40_drop_revoked_stid on an admin-revoked delegation.
- AC:LAttack complexity, low
- The attacker holds a delegation and retries FREE_STATEID (or v4.0 use of that stateid) while nfsd4_revoke_states() unhashes with ADMIN_REVOKED and revoke_delegation() runs, then DESTROY_CLIENTID/expire so __destroy_client() walks cl_revoked. They control the NFS side of the race; unexport is a routine nfsd operational condition, not an uninfluenced layout.
- PR:NPrivileges required, none
- Typical nfsd deployments use AUTH_SYS/AUTH_NULL with no cryptographic RPC authentication. Any host allowed by the export can EXCHANGE_ID/CREATE_SESSION or SETCLIENTID, OPEN to receive a delegation, and issue FREE_STATEID; no local Linux account or capability on the NFS server is required.
- UI:NUser interaction, none
- The attacker sends the NFSv4 compounds that obtain the delegation, FREE_STATEID, and client teardown. Administrative unlock_filesystem/unexport is a server operational condition (exportfs -u or volume detach), not a required victim user action such as mounting or opening a file.
- S:UScope, unchanged
- The use-after-free corrupts a kmalloc'd nfs4_delegation (deleg_slab) in the NFS server kernel. Impact stays in that host kernel security authority and does not cross a VM, IOMMU, or sandbox boundary.
- C:HConfidentiality impact, high
- Without SC_STATUS_FREED, revoke_delegation() list_add()s a kmem_cache_free'd nfs4_delegation onto cl_revoked (KMEM_CACHE flags 0, not SLAB_TYPESAFE_BY_RCU). __destroy_client() then walks that freed object; slab reuse yields an arbitrary kernel-read primitive. Per guidance a UAF is C:H.
- I:HIntegrity impact, high
- list_add on the freed dl_recall_lru and later list_del_init/nfs4_put_stid in __destroy_client() write through reclaimed deleg_slab, including sc_free. That is a heap-spray write and control-flow hijack primitive. Per guidance a UAF is I:H.
- A:HAvailability impact, high
- Use-after-free of nfs4_delegation when __destroy_client() traverses cl_revoked oopses or panics the nfsd thread even without full exploitation, fully denying service of the NFS server host.