CVE-2026-80585 mptcp: fastopen: only mark MPTFO subflows with SYN data
A logic error in Linux kernel MPTCP handling of data-less TCP Fast Open SYNs can cause kernel warnings and potential state confusion, leading to denial of service.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-80585?
CVE-2026-80585 is a critical vulnerability in Linux, published on 26 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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".
Which versions are affected by CVE-2026-80585?
How do I fix CVE-2026-80585?
Apply the vendor's patch.
Is CVE-2026-80585 being exploited?
There is no known exploitation of CVE-2026-80585 in the wild and no public exploit code.
Sources
Also known as AZL-97842, BELL-CVE-2026-80585, DEBIAN-CVE-2026-80585, GHSA-xhvw-vpqw-3xgx, UBUNTU-CVE-2026-80585.