# CVE-2026-89689: nfsd: don't free session slots that are still in use

> A use-after-free vulnerability in the Linux kernel NFS server allows attackers to corrupt memory, potentially leading to code execution or system crashes.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 11 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-89689, DEBIAN-CVE-2026-89689, GHSA-49vg-hxcf-g87m, UBUNTU-CVE-2026-89689

## What is CVE-2026-89689?

CVE-2026-89689 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: don't free session slots that are still in use

nfsd4_sequence() can free the very slot it is currently processing.
When the session shrinker has reduced se_target_maxslots below
se_fchannel.maxreqs, the shrink path checks three conditions before
calling free_session_slots():

  1. se_target_maxslots < maxreqs  (shrink was advertised)
  2. slot->sl_generation == se_slot_gen  (slot is up-to-date)
  3. seq->maxslots <= se_target_maxslots  (client acknowledges)

However, seq->slotid is never checked against se_target_maxslots.
A client using a slot in the range [se_target_maxslots, maxreqs) can
satisfy all three conditions: its slot has the current generation
(set by a prior SEQUENCE), and it sends sa_highest_slotid <=
se_target_maxslots to acknowledge the reduction.

free_session_slots() then kfrees every slot at index >=
se_target_maxslots, including the caller's own slot. The function
continues to write sl_seqid, sl_flags, sl_generation, and stores the
dangling pointer in cstate->slot. Later, nfsd4_store_cache_entry()
copies up to maxresp_cached bytes of the compound reply into the freed
sl_data[] array, corrupting whatever slab object now occupies that
address.

Additionally, a concurrent thread processing SEQUENCE on a different
high-numbered slot can have its slot freed out from under it.
NFSD4_SLOT_INUSE is set under nn->client_lock before the lock is
released, so any concurrent thread past SEQUENCE will have its slot
marked. However, free_session_slots() does not check NFSD4_SLOT_INUSE
before freeing.

Fix both problems by:
 1. Checking that the current request's slotid is below the shrink
    boundary.
 2. Scanning slots in the to-be-freed range for NFSD4_SLOT_INUSE and
    deferring the shrink if any are active.

## Which versions are affected by CVE-2026-89689?


## How do I fix CVE-2026-89689?

Apply the vendor's patch.

## Is CVE-2026-89689 being exploited?

There is no known exploitation of CVE-2026-89689 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89689 and fix it if it is affected.

What it is: CVE-2026-89689 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.
Exploitation: There is no known exploitation of CVE-2026-89689 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-89689/ (Markdown: https://insights.sbomify.com/CVE-2026-89689/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89689
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89689
- OSV: https://osv.dev/vulnerability/CVE-2026-89689

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89689/
Summaries are generated and may contain mistakes; the official description above is the record.