CVE-2026-80585

Summary

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 Software

VendorProductVersion RangeStatus
LinuxLinux36b122baf6a8bd46b4a591f12f4ed17b22257408 < f75f174edc865738522e514d042cf5627f084859affected
LinuxLinux36b122baf6a8bd46b4a591f12f4ed17b22257408 < fca7e444c04689fe4cc6b56f2725f804a4bb1ef9affected
LinuxLinux36b122baf6a8bd46b4a591f12f4ed17b22257408 < 75e564b2ced1cc3d9a8904c7d2d2bb448fffb8b5affected
LinuxLinux36b122baf6a8bd46b4a591f12f4ed17b22257408 < 72b4a0c51a4b550d40301d60a366b429b8c8e78daffected
LinuxLinux36b122baf6a8bd46b4a591f12f4ed17b22257408 < e00b63056fb4f261455b3e5df5268a1f8ce47a87affected
LinuxLinux6.2affected
LinuxLinux0 < 6.2unaffected
LinuxLinux6.6.153 <= 6.6.*unaffected
LinuxLinux6.12.105 <= 6.12.*unaffected
LinuxLinux6.18.46 <= 6.18.*unaffected
LinuxLinux7.1.10 <= 7.1.*unaffected
LinuxLinux7.2 <= *unaffected

Weaknesses

References