# CVE-2026-89788: ksmbd: fix tree connection use-after-free in smb2_tree_connect()

> A race condition in the Linux kernel's ksmbd server allows a use-after-free, potentially enabling local attackers to execute code or escalate privileges.

- **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:** 16 Sep 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-101159, BELL-CVE-2026-89788, DEBIAN-CVE-2026-89788, GHSA-29wc-2986-775f, UBUNTU-CVE-2026-89788

## What is CVE-2026-89788?

CVE-2026-89788 is a critical use after free vulnerability in Linux, published on 16 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:

ksmbd: fix tree connection use-after-free in smb2_tree_connect()

ksmbd_tree_conn_connect() publishes a new tree connection in
sess->tree_conns with a single reference and returns its pointer to
smb2_tree_connect(). The handler continues to initialize the object and
build the response after publication. A concurrent session logoff can
erase the connection and drop that reference, freeing the object while
the handler still uses it.

BUG: KASAN: slab-use-after-free in smb2_tree_connect+0xe3d/0xf90
  smb2_tree_connect (fs/smb/server/smb2pdu.c:2872)
  handle_ksmbd_work
  process_one_work
  worker_thread
  kthread

After xa_store() succeeds, take a second reference before releasing
tree_conns_lock. The original reference belongs to the xarray entry and
the second belongs to the creating smb2_tree_connect() handler.

Keep the references balanced in every path:

- On normal exit or an error after publication, smb2_tree_connect()
  drops its creator reference. Error cleanup also calls
  ksmbd_tree_conn_disconnect(), which drops the xarray reference only if
  it removes the exact entry.
- SMB2 TREE_DISCONNECT uses the same helper to remove the entry and drop
  its xarray reference. The request's existing lookup reference remains
  owned by the request and is released by the existing cleanup.
- Session LOGOFF removes each entry and drops its xarray reference. If
  it wins the race, later cleanup sees that the entry is gone and does
  not drop that reference again.

To enforce this ownership, claim the disconnected state and erase the
exact entry atomically under tree_conns_lock. This guarantees one drop
for the xarray reference and one drop by each in-flight user, regardless
of which teardown path wins. If logoff removes the entry before
initialization completes, fail the connect instead of marking the
detached object TREE_CONNECTED.

## Which versions are affected by CVE-2026-89788?


## How do I fix CVE-2026-89788?

Apply the vendor's patch.

## Is CVE-2026-89788 being exploited?

There is no known exploitation of CVE-2026-89788 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89788 and fix it if it is affected.

What it is: CVE-2026-89788 is a critical use after free vulnerability in Linux, published on 16 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-2026-89788 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-2026-89788/ (Markdown: https://insights.sbomify.com/CVE-2026-89788/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89788
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89788
- OSV: https://osv.dev/vulnerability/CVE-2026-89788

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89788/
Summaries are generated and may contain mistakes; the official description above is the record.