# CVE-2026-31681: netfilter: xt_multiport: validate range encoding in checkentry

> A missing validation in the Linux kernel netfilter module allows malformed rules to cause out-of-bounds reads, potentially leading to a denial of service.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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:** 25 Apr 2026
- **Updated:** 15 Oct 2026
- **Also known as:** AZL-84219, BELL-CVE-2026-31681, DEBIAN-CVE-2026-31681, GHSA-f822-mpgr-7cqx, ROOT-OS-DEBIAN-11-CVE-2026-31681, ROOT-OS-DEBIAN-12-CVE-2026-31681, ROOT-OS-UBUNTU-2204-CVE-2026-31681, ROOT-OS-UBUNTU-2404-CVE-2026-31681, UBUNTU-CVE-2026-31681

## What is CVE-2026-31681?

CVE-2026-31681 is a medium-severity denial of service vulnerability in Linux kernel, published on 25 Apr 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: xt_multiport: validate range encoding in checkentry

ports_match_v1() treats any non-zero pflags entry as the start of a
port range and unconditionally consumes the next ports[] element as
the range end.

The checkentry path currently validates protocol, flags and count, but
it does not validate the range encoding itself. As a result, malformed
rules can mark the last slot as a range start or place two range starts
back to back, leaving ports_match_v1() to step past the last valid
ports[] element while interpreting the rule.

Reject malformed multiport v1 rules in checkentry by validating that
each range start has a following element and that the following element
is not itself marked as another range start.

## Which versions are affected by CVE-2026-31681?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-31681?

Apply the vendor's patch.

## Is CVE-2026-31681 being exploited?

There is no known exploitation of CVE-2026-31681 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-31681 and fix it if it is affected.

What it is: CVE-2026-31681 is a medium-severity denial of service vulnerability in Linux kernel, published on 25 Apr 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-2026-31681 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2026-31681/ (Markdown: https://insights.sbomify.com/CVE-2026-31681/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31681
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31681
- OSV: https://osv.dev/vulnerability/CVE-2026-31681

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31681/
Summaries are generated and may contain mistakes; the official description above is the record.