CVE-2026-80585
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".
CSIRTS triage
- 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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Exploitation outlook
- Low exploitation risk0.32% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 24% of all EPSS-scored CVEs.
Advisory coverage (2)
External references
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