CVE-2026-63867Network attack vector

Remote MPTCP disruption via inconsistent DSS ACK

Published Jul 19, 2026 · Updated Jul 19, 2026

Race condition in the MPTCP output path in Linux Kernel allows remote attackers to disrupt established MPTCP sessions. The path reads msk->ack_seq repeatedly without locking, allowing a packet's DSS data ACK and advertised receive window to use different sequence values. A remote attacker must be an MPTCP peer and win the ingress-versus-egress race; the inconsistent flow-control state can confuse the peer.

CVSS severity8.2
High
EPSS probability0.59%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Race condition in the MPTCP output path in Linux Kernel allows remote attackers to disrupt established MPTCP sessions. The path reads msk->ack_seq repeatedly without locking, allowing a packet's DSS data ACK and advertised receive window to use different sequence values. A remote attacker must be an MPTCP peer and win the ingress-versus-egress race; the inconsistent flow-control state can confuse the peer.

The record

CVE
CVE-2026-63867
Published
Jul 19, 2026
Updated
Jul 19, 2026
Vendor
The Linux Kernel Organization
Product
Linux Kernel
Classifications
T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 19, 2026CVE publishedPublication date reported by the CVE source.
  2. Jul 19, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=5.11
  2. Affected versionversion=fa3fe2b150316b294f2c662653501273ff25bba8 <301a33fd590c408a05c5df800e0cc1e6a8a2f8f8
  3. Affected versionversion=fa3fe2b150316b294f2c662653501273ff25bba8 <3b8cbba7c0ed31189c89f90be247b8973ffa79ef
  4. Affected versionversion=fa3fe2b150316b294f2c662653501273ff25bba8 <68364963e5baf03f16b4420292291f75c8f66497
  5. Affected versionversion=fa3fe2b150316b294f2c662653501273ff25bba8 <8ab24fdebc369c0dfb90f82c1650b1e66662bb45
  6. Affected versionversion=fa3fe2b150316b294f2c662653501273ff25bba8 <8f4f0a157e8436a05bf8c3670b24dbc258911c43
  7. Affected versionversion=fa3fe2b150316b294f2c662653501273ff25bba8 <907ac6b1658e0277f979fcdfae2a753b495c1510
  8. Affected versionversion=fa3fe2b150316b294f2c662653501273ff25bba8 <c4f4cf60797974873dbc8e100144682a6f2f861f

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The Linux Kernel Organization · Linux Kernelversion=5.11; version=fa3fe2b150316b294f2c662653501273ff25bba8 <301a33fd590c408a05c5df800e0cc1e6a8a2f8f8; version=fa3fe2b150316b294f2c662653501273ff25bba8 <3b8cbba7c0ed31189c89f90be247b8973ffa79ef; version=fa3fe2b150316b294f2c662653501273ff25bba8 <68364963e5baf03f16b4420292291f75c8f66497; version=fa3fe2b150316b294f2c662653501273ff25bba8 <8ab24fdebc369c0dfb90f82c1650b1e66662bb45; version=fa3fe2b150316b294f2c662653501273ff25bba8 <8f4f0a157e8436a05bf8c3670b24dbc258911c43; version=fa3fe2b150316b294f2c662653501273ff25bba8 <907ac6b1658e0277f979fcdfae2a753b495c1510; version=fa3fe2b150316b294f2c662653501273ff25bba8 <c4f4cf60797974873dbc8e100144682a6f2f861f

Published CVSS scores

8.2kernel.orgCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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