CVE-2026-63867 (GCVE-0-2026-63867)

Vulnerability from cvelistv5 – Published: 2026-07-19 14:18 – Updated: 2026-08-05 12:36
VLAI
Title
mptcp: close TOCTOU race while computing rcv_wnd
Summary
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.
Severity
8.2 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H
Assigner
References
Impacted products
Vendor Product Version
Linux Linux Affected: fa3fe2b150316b294f2c662653501273ff25bba8 , < 301a33fd590c408a05c5df800e0cc1e6a8a2f8f8 (git)
Affected: fa3fe2b150316b294f2c662653501273ff25bba8 , < 68364963e5baf03f16b4420292291f75c8f66497 (git)
Affected: fa3fe2b150316b294f2c662653501273ff25bba8 , < 907ac6b1658e0277f979fcdfae2a753b495c1510 (git)
Affected: fa3fe2b150316b294f2c662653501273ff25bba8 , < c4f4cf60797974873dbc8e100144682a6f2f861f (git)
Affected: fa3fe2b150316b294f2c662653501273ff25bba8 , < 3b8cbba7c0ed31189c89f90be247b8973ffa79ef (git)
Affected: fa3fe2b150316b294f2c662653501273ff25bba8 , < 8f4f0a157e8436a05bf8c3670b24dbc258911c43 (git)
Affected: fa3fe2b150316b294f2c662653501273ff25bba8 , < 8ab24fdebc369c0dfb90f82c1650b1e66662bb45 (git)
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Linux Linux Affected: 5.11
Unaffected: 0 , < 5.11 (semver)
Unaffected: 5.15.210 , ≤ 5.15.* (semver)
Unaffected: 6.1.176 , ≤ 6.1.* (semver)
Unaffected: 6.6.143 , ≤ 6.6.* (semver)
Unaffected: 6.12.94 , ≤ 6.12.* (semver)
Unaffected: 6.18.36 , ≤ 6.18.* (semver)
Unaffected: 7.0.13 , ≤ 7.0.* (semver)
Unaffected: 7.1 , ≤ * (original_commit_for_fix)
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{
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          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nmptcp: close TOCTOU race while computing rcv_wnd\n\nThe MPTCP output path access locklessly the MPTCP-level ack_seq\nin multiple times, using possibly different values for the data_ack\nin the DSS option and to compute the announced rcv wnd for the same\npacket.\n\nRefactor the cote to avoid inconsistencies which may confuse the\npeer. Also ensure that the MPTCP level rcv wnd is updated only when\nthe egress packet actually contains a DSS ack."
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            "baseSeverity": "HIGH",
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            "version": "3.1"
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              "value": "AV:N - The flaw is in the MPTCP TCP output path (`__tcp_transmit_skb()` \u2192 `mptcp_established_options_dss()` / `mptcp_write_options()`), reachable by any remote peer that can exchange MPTCP packets over the network.\nAC:L - The attacker controls both sides of the race by timing concurrent ingress traffic (advancing `msk-\u003eack_seq` on receive) against egress transmissions on the same or another subflow; no special memory layout or victim-specific state is required.\nPR:N - Exploitation requires only the ability to complete an MPTCP/TCP connection as a remote peer; no local privileges, capabilities, or authenticated access to the victim system is needed.\nUI:N - Exploitation is fully automated over the network once an MPTCP connection is established; no victim user action is required beyond normal service connectivity.\nS:U - The impact is confined to the kernel networking stack and the affected MPTCP connection state; it does not cross a VM, sandbox, or IOMMU security boundary.\nC:N - This is a protocol-state TOCTOU inconsistency, not a memory-safety bug; it does not provide arbitrary or bounded kernel memory disclosure.\nI:L - Inconsistent DSS ACK versus receive-window tracking can corrupt MPTCP-level window state (`rcv_wnd_sent`), allowing acceptance or handling of data outside the window actually announced to the peer.\nA:H - A remote attacker can repeatedly trigger the race to cause connection disruption (reset/fallback), stall MPTCP sessions, and inflate out-of-order receive buffering via incorrect `rcv_wnd_sent` values, denying service to applications using the connection."
            }
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    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2026-63867",
    "datePublished": "2026-07-19T14:18:37.234Z",
    "dateReserved": "2026-07-19T07:54:57.017Z",
    "dateUpdated": "2026-08-05T12:36:43.517Z",
    "state": "PUBLISHED"
  },
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    "redhat_vex": {
      "aggregate_severity": "Moderate",
      "current_release_date": "2026-08-05T14:05:43+00:00",
      "cve": "CVE-2026-63867",
      "id": "CVE-2026-63867",
      "initial_release_date": "2026-07-19T00:00:00+00:00",
      "product_status:known_affected": "232",
      "product_status:known_not_affected": "43",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: mptcp: close TOCTOU race while computing rcv_wnd",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-63867.json",
      "version": "3"
    }
  }
}



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Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

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Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


Detection rules are retrieved from Rulezet.