CVE-2026-53183

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.