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

lowCVSS 3.7CVE-2026-80585

CSIRTS triage

vendor: Linux product: Linux kernel Denial of service

What: MPTCP fastopen incorrectly marks MPTFO subflows without SYN data, causing protocol violation.
Who is affected: Systems using MPTCP fastopen with multiple subflows.
Urgency: Low priority; protocol violation can cause connection issues but impact is limited to multipath TCP users.
Action: Patch MPTCP fastopen to only mark subflows when SYN data is actually present.

AI-assisted analysis generated from the source advisory — verify against the original.

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Details

Source: Microsoft Security Response Center (INTL · vendor-psirt · site)
Severity: low — CVSS 3.7
Published: 2026-08-11
Exploitation: Not in CISA KEV at last sync

Original advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-80585

Exploitation outlook

EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.

Low exploitation riskCVE-2026-805850.32% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 24% of all EPSS-scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-80585	coverage & exploitation status	NVD · CVE.org

Same CVEs, other sources

How other CERTs, PSIRTs and databases cover the vulnerabilities in this advisory.

criticalCVE-2026-80585: In the Linux kernel, the following vulnerability has been resolved: mptcp: fastopen: only mark…nvd

Recent advisories for mptcp

A cluster of recent advisories against the same product widens the attack surface — attackers routinely chain freshly published CVEs on one product, so review these together.

unknownCVE-2026-89554: In the Linux kernel, the following vulnerability has been resolved: mptcp: fix uninitialized l…nvd · 2026-09-11
lowCVE-2026-80784: mptcp: pm: fix memory leak from alloc-during-teardown racemsrc · 2026-09-08
unknownCVE-2026-80784: In the Linux kernel, the following vulnerability has been resolved: mptcp: pm: fix memory leak…nvd · 2026-09-04
highCVE-2026-80588: In the Linux kernel, the following vulnerability has been resolved: mptcp: reclaim forward-all…nvd · 2026-08-26
criticalCVE-2026-80587: In the Linux kernel, the following vulnerability has been resolved: mptcp: avoid combining som…nvd · 2026-08-26
criticalCVE-2026-80586: In the Linux kernel, the following vulnerability has been resolved: mptcp: options: reset DSS …nvd · 2026-08-26