# CVE-2026-72256: netfilter: xt_cluster: reject template conntracks in hash match

> A flaw in the Linux kernel netfilter module allows remote attackers to cause a denial of service by triggering a null dereference in the cluster match hash function.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Aug 2026
- **Updated:** 17 Aug 2026
- **Also known as:** BELL-CVE-2026-72256, DEBIAN-CVE-2026-72256, GHSA-hh4g-5q85-w496, UBUNTU-CVE-2026-72256

## What is CVE-2026-72256?

CVE-2026-72256 is a vulnerability in Linux, published on 15 Aug 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:

netfilter: xt_cluster: reject template conntracks in hash match

xt_cluster_mt() treats any non-NULL nf_ct_get() result as a fully
initialized conntrack and passes it to xt_cluster_hash().

This causes a state confusion bug when the raw table CT target attaches
a template conntrack to skb->_nfct before normal conntrack processing.
Templates carry IPS_TEMPLATE status but do not have a valid tuple for
hashing yet, so xt_cluster_hash() can hit its WARN_ON() path on the
zeroed l3num field.

Reject template conntracks before hashing them. This matches existing
netfilter handling for template objects and avoids hashing incomplete
conntrack state.

## Which versions are affected by CVE-2026-72256?


## How do I fix CVE-2026-72256?

Apply the vendor's patch.

## Is CVE-2026-72256 being exploited?

There is no known exploitation of CVE-2026-72256 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-72256 and fix it if it is affected.

What it is: CVE-2026-72256 is a vulnerability in Linux, published on 15 Aug 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-2026-72256 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-2026-72256/ (Markdown: https://insights.sbomify.com/CVE-2026-72256/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-72256
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-72256
- OSV: https://osv.dev/vulnerability/CVE-2026-72256

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-72256/
Summaries are generated and may contain mistakes; the official description above is the record.