# CVE-2026-90237: netfilter: nft_ct: move custom expectation support to helper

> A use-after-free vulnerability in the Linux kernel netfilter subsystem allows attackers to cause a denial of service or potentially execute code via crafted network packets.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 17 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-102491, BELL-CVE-2026-90237, DEBIAN-CVE-2026-90237, GHSA-9xwh-87xc-mh2g, UBUNTU-CVE-2026-90237

## What is CVE-2026-90237?

CVE-2026-90237 is a high-severity use after free vulnerability in Linux, published on 17 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

netfilter: nft_ct: move custom expectation support to helper

Originally, the ct expectation support called nf_ct_helper_ext_add() for
confirmed conntracks, which is invalid, triggering a splat. This was
fixed by commit 1710eb913bdc ("netfilter: nft_ct: skip expectations for
confirmed conntrack") which restricted it to unconfirmed conntracks.

However, early insertion of expectations into the expectations list when
the conntrack is unconfirmed leads to stale entries pointing to the
wrong hlist_head through .pprev due to ct extension reallocation.

Commit 7c9664351980 ("netfilter: move nat hlist_head to nf_conn") moved
the nat hlist_head to nf_conn for this reason:

     1. ...
     2. When reallocation of extension area occurs we need to fixup the
        bysource hash head via hlist_replace_rcu.

I'd rather not increase the size of the struct nf_conn for this feature
has very limited scope: only one expectation can be created at a time
given expect_clash() will make nf_ct_expect_related() reports EBUSY.
For this reason, relax nf_ct_expect_related() not to drop packets in
case expectation creation fails, therefore, expectation creation becomes
best effort.

To address this issue, add an internal ct helper and attach it to the
conntrack entry to streamline the custom ct expectation support with
existing ct helpers.

Expose a new nf_conntrack_helper_release() function to release the
internal helper that is allocated and attached to the conntrack entry to
create the custom expectations. The nft_ct module removal always waits
for rcu grace period, then the NULL helper callback is observed after
this.

This patch also restricts the creation of expectations to different
helpers other than this custom helper that is created for this type of
expectations.

## Which versions are affected by CVE-2026-90237?


## How do I fix CVE-2026-90237?

Apply the vendor's patch.

## Is CVE-2026-90237 being exploited?

There is no known exploitation of CVE-2026-90237 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-90237 and fix it if it is affected.

What it is: CVE-2026-90237 is a high-severity use after free vulnerability in Linux, published on 17 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-90237 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-2026-90237/ (Markdown: https://insights.sbomify.com/CVE-2026-90237/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-90237
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-90237
- OSV: https://osv.dev/vulnerability/CVE-2026-90237

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-90237/
Summaries are generated and may contain mistakes; the official description above is the record.