How severe is CVE-2026-80585?

CVE-2026-80585 has a CVSS score of 9.4/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2026-80585?

CVE-2026-80585 affects the following packages: Kernel (Linux). Ecosystems affected: Linux.

How do I fix CVE-2026-80585?

Update Kernel to 6.6.153 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-80585 is resolved across your whole dependency graph.

How do I detect CVE-2026-80585 in my Linux dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for Kernel. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

How do I mitigate CVE-2026-80585 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

How does O3 Security protect against CVE-2026-80585?

O3 pinpoints whether CVE-2026-80585 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Is CVE-2026-80585 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-80585 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

When was CVE-2026-80585 published, and has it been updated?

CVE-2026-80585 was published on August 26, 2026 and was last updated on August 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐧

🐧 Linux

Not in CISA KEV

CRITICAL severity

CVE-2026-80585

CRITICAL

CVE-2026-80585 is a critical-severity (CVSS 9.4) vulnerability in Kernel. O3 Security confirms whether CVE-2026-80585 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.

mptcp: fastopen: only mark MPTFO subflows with SYN data

Published

Aug 26, 2026

Updated

Aug 28, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Aug 28, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Real-World Exposure

1 pkg affected

🐧Kernel

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Linux packages — download data is not available via public APIs for these ecosystems.

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".

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐧Linux	`Kernel`	≥ 6.2.0&&< 6.6.153	6.6.153

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for Kernel. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update Kernel to 6.6.153 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-80585 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-80585 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to CVE-2026-80585. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

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 a

O3 Security · Impact-Aware SCA

Is CVE-2026-80585 in your dependencies?

O3 detects CVE-2026-80585 across Linux dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

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