CVE-2026-89662 NFSD: Prevent lock owner use-after-free during client teardown
A use-after-free in the Linux kernel NFS server allows attackers to cause a denial of service via a NULL dereference during client teardown.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-89662?
CVE-2026-89662 is a critical use after free vulnerability in Linux, published on 11 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
NFSD: Prevent lock owner use-after-free during client teardown
__destroy_client() releases a client's open owners, but a lock owner whose only reference is a blocked lock (nbl) stays on cl_ownerstr_hashtbl. client_has_state() does not count a bare owner, so DESTROY_CLIENTID can reach __destroy_client() with such owners present.
__destroy_client() then walks the table, calling remove_blocked_locks() on each owner without a reference. Freeing a blocked lock drops the owner reference held via flc_owner. The per-net laundromat reaps blocked locks from nn->blocked_locks_lru independently of client state. The two paths share blocked_locks_lock only for the list splice, not the owner's lifetime. The laundromat therefore frees the owner as __destroy_client() dereferences it, a NULL dereference in remove_blocked_locks().
nfsd4_release_lockowner() holds a reference across the same call; __destroy_client() does not. Hold cl_lock across the walk, taking a reference and unhashing each owner, then drop it before remove_blocked_locks() and nfs4_put_stateowner(), which take blocked_locks_lock and cl_lock.
Which versions are affected by CVE-2026-89662?
How do I fix CVE-2026-89662?
Apply the vendor's patch.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2026-89662 being exploited?
There is no known exploitation of CVE-2026-89662 in the wild and no public exploit code.
Sources
Also known as AZL-100374, BELL-CVE-2026-89662, DEBIAN-CVE-2026-89662, GHSA-h9v2-7c48-mpxj, UBUNTU-CVE-2026-89662.