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".

Metrics

No CVSS v4.0

No CVSS v3.1

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

No EPSS score available.

Key SSVC decision points have not yet been added.

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Fixes

Solution

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Workaround

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References
Link Providers
https://git.kernel.org/stable/c/72b4a0c51a4b550d40301d60a366b429b8c8e78d cve-icon cve-icon
https://git.kernel.org/stable/c/75e564b2ced1cc3d9a8904c7d2d2bb448fffb8b5 cve-icon cve-icon
https://git.kernel.org/stable/c/e00b63056fb4f261455b3e5df5268a1f8ce47a87 cve-icon cve-icon
https://git.kernel.org/stable/c/f75f174edc865738522e514d042cf5627f084859 cve-icon cve-icon
https://git.kernel.org/stable/c/fca7e444c04689fe4cc6b56f2725f804a4bb1ef9 cve-icon cve-icon
History

Wed, 26 Aug 2026 17:15:00 +0000

Type Values Removed Values Added
Weaknesses CWE-482
CWE-665

Wed, 26 Aug 2026 14:45:00 +0000

Type Values Removed Values Added
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".
Title mptcp: fastopen: only mark MPTFO subflows with SYN data
First Time appeared Linux
Linux linux Kernel
CPEs cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
Vendors & Products Linux
Linux linux Kernel
References

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cve-icon MITRE

Status: PUBLISHED

Assigner: Linux

Published:

Updated: 2026-08-26T14:37:40.901Z

Reserved: 2026-08-26T14:34:25.769Z

Link: CVE-2026-80585

cve-icon Vulnrichment

No data.

cve-icon NVD

Status : Received

Published: 2026-08-26T15:17:14.750

Modified: 2026-08-26T15:17:14.750

Link: CVE-2026-80585

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-08-26T17:00:04Z

Weaknesses