CVE-2026-80585
CriticalCVSS 9.4Summary
In the Linux kernel's MPTCP module, mptcp_fastopen_subflow_synack_set_params() set the is_mptfo flag before checking for queued SYN data. This caused data-less TFO SYNs to trigger a WARN and leave stale MPTFO state, potentially leading to a state-confusion bug in check_fully_established(). The fix marks the subflow as MPTFO only after confirming that an SKB was queued.
Risk Assessment
The vulnerability could lead to system warnings or state errors that might be exploited to destabilize MPTCP connections.
Recommendation
Apply the Linux kernel update containing the fix for CVE-2026-80585.
Original NVD description (English source)
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".