Bug 2492708 (CVE-2026-53183)

Summary: CVE-2026-53183 kernel: mptcp: allow subflow rcv wnd to shrink
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
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Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: rhel-process-autobot, watson-tool-maintainers
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in the Linux kernel's Multipath TCP (MPTCP) implementation. This vulnerability occurs because the TCP stack independently manages the TCP-level receive window, which can lead to an artificial inflation of the MPTCP receive window. A remote attacker could exploit this by sending specific traffic, causing the incoming data to exceed the receiver's buffer size. This can result in a Denial of Service (DoS) condition, making the system unresponsive to legitimate traffic.
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Description OSIDB Bzimport 2026-06-25 10:01:37 UTC 
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.
Comment 1 Mauro Matteo Cascella 2026-06-26 10:44:33 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026062555-CVE-2026-53183-a7a3@gregkh/T