CVE-2026-80585

/// Technical 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".

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References