# CVE-2026-80585: mptcp: fastopen: only mark MPTFO subflows with SYN data

> A logic error in Linux kernel MPTCP handling of data-less TCP Fast Open SYNs can cause kernel warnings and potential state confusion, leading to denial of service.

- **Severity:** Critical 9.4 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/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:** 26 Aug 2026
- **Updated:** 16 Oct 2026
- **Also known as:** AZL-97842, BELL-CVE-2026-80585, DEBIAN-CVE-2026-80585, GHSA-xhvw-vpqw-3xgx, UBUNTU-CVE-2026-80585

## What is CVE-2026-80585?

CVE-2026-80585 is a critical vulnerability in Linux, published on 26 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:

mptcp: fastopen: only mark MPTFO subflows with SYN data

Passive TCP Fast Open accepts a valid-cookie SYN even when it carries
no data. In that case the child socket's receive queue is intentionally
left empty.

mptcp_fastopen_subflow_synack_set_params() set is_mptfo before checking
for queued SYN data. That made data-less TFO SYNs hit a WARN and, if
the warning was non-fatal, left stale MPTFO state behind. The stale
flag could later trigger a state-confusion bug in
check_fully_established().

Only mark the subflow as MPTFO after confirming that an SKB was queued.
Return quietly when the receive queue is empty.

Note that mptcp_subflow_context's is_mptfo field is now not just about
subflows where the TFO was present, but about MPTFO subflow that
consumed SYN data. Only having a valid cookie but not carrying data is
not really "doing TFO".

## Which versions are affected by CVE-2026-80585?


## How do I fix CVE-2026-80585?

Apply the vendor's patch.

## Is CVE-2026-80585 being exploited?

There is no known exploitation of CVE-2026-80585 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-80585 and fix it if it is affected.

What it is: CVE-2026-80585 is a critical vulnerability in Linux, published on 26 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-2026-80585 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-2026-80585/ (Markdown: https://insights.sbomify.com/CVE-2026-80585/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-80585
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-80585
- OSV: https://osv.dev/vulnerability/CVE-2026-80585

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-80585/
Summaries are generated and may contain mistakes; the official description above is the record.