# CVE-2026-98255: tcp: exclude old ACKs from tcp fast path

> Attackers can send old TCP ACKs to bypass validation, potentially corrupting connection state or causing denial of service in the Linux kernel.

- **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:** 6 Oct 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-106353, BELL-CVE-2026-98255, DEBIAN-CVE-2026-98255, GHSA-vm2w-hjrq-h79w, UBUNTU-CVE-2026-98255

## What is CVE-2026-98255?

CVE-2026-98255 is a vulnerability in Linux, published on 6 Oct 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:

tcp: exclude old ACKs from tcp fast path

Exclude old ACKs before SND.UNA from the tcp fast path
as well as ACKs after SND.NXT.

Such ACKs will fall through to the slow path, where tcp_ack()
performs the appropriate validation and challenge ACK handling
according to RFC5961 and Commit 3d501dd326fb1c7 ("tcp: do not
accept ACK of bytes we never sent").

This prevents old ACKs from being accepted
or modifying connection state as part of the fast path before
appropriate ACK validation is applied.
In particular, this prevents payload carried by a segment with
an excessively old ACK from advancing RCV.NXT before the ACK
is rejected.

## Which versions are affected by CVE-2026-98255?


## How do I fix CVE-2026-98255?

Apply the vendor's patch.

## Is CVE-2026-98255 being exploited?

There is no known exploitation of CVE-2026-98255 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98255 and fix it if it is affected.

What it is: CVE-2026-98255 is a vulnerability in Linux, published on 6 Oct 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-2026-98255 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-2026-98255/ (Markdown: https://insights.sbomify.com/CVE-2026-98255/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98255
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98255
- OSV: https://osv.dev/vulnerability/CVE-2026-98255

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98255/
Summaries are generated and may contain mistakes; the official description above is the record.