Time-of-check Time-of-use (TOCTOU) Race Condition in Linux kernel - CVE-2026-63867

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Main Vulnerability Database Vulnerabilities #VU138789

Time-of-check Time-of-use (TOCTOU) Race Condition in Linux kernel - CVE-2026-63867

Published: July 20, 2026


Vulnerability identifier: #VU138789
CSH Severity: Medium
CVSS v4: 6.9 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N]
CVE-ID: CVE-2026-63867
CWE-ID: CWE-367
Exploitation vector: Remote access
Exploit availability: No public exploit available

Vulnerability details

The vulnerability allows a remote attacker to cause inconsistent MPTCP receiver window signaling.

The vulnerability exists due to a time-of-check time-of-use race in the MPTCP DSS option handling in net/mptcp/options.c when processing and emitting MPTCP acknowledgments on outgoing packets. A remote attacker can trigger packet processing that causes inconsistent ack_seq values to be used to confuse the peer.

The issue arises because the MPTCP-level ack_seq was accessed locklessly multiple times, allowing different values to be used for the DSS data acknowledgment and the announced receive window within the same packet.


Affected software

Linux kernel
Ubuntu
linux (Ubuntu package)
linux-kvm (Ubuntu package)
linux-aws (Ubuntu package)
linux-gcp (Ubuntu package)
linux-azure (Ubuntu package)
linux-raspi (Ubuntu package)
linux-oracle (Ubuntu package)
linux-lowlatency (Ubuntu package)
linux-gke (Ubuntu package)
linux-aws-5.15 (Ubuntu package)
linux-lowlatency-hwe-5.15 (Ubuntu package)
linux-hwe-5.15 (Ubuntu package)
linux-fips (Ubuntu package)
linux-azure-5.15 (Ubuntu package)
linux-ibm-5.15 (Ubuntu package)
linux-realtime (Ubuntu package)
linux-azure-fde-5.15 (Ubuntu package)
linux-nvidia-tegra-igx (Ubuntu package)
linux-gkeop (Ubuntu package)
linux-nvidia-tegra (Ubuntu package)
linux-nvidia (Ubuntu package)
linux-intel-iotg (Ubuntu package)
linux-ibm (Ubuntu package)
linux-intel-iotg-5.15 (Ubuntu package)
linux-oracle-5.15 (Ubuntu package)
linux-gcp-5.15 (Ubuntu package)
linux-nvidia-tegra-5.15 (Ubuntu package)
linux-xilinx-zynqmp (Ubuntu package)
linux-intel-iot-realtime (Ubuntu package)
linux-gcp-fips (Ubuntu package)
linux-aws-fips (Ubuntu package)
linux-azure-fips (Ubuntu package)
linux-azure-fde (Ubuntu package)
linux-hwe-6.8 (Ubuntu package)
linux-lowlatency-hwe-6.8 (Ubuntu package)
linux-azure-fde-6.8 (Ubuntu package)
linux-oracle-6.8 (Ubuntu package)
linux-azure-6.8 (Ubuntu package)
linux-aws-6.8 (Ubuntu package)
linux-nvidia-lowlatency (Ubuntu package)
linux-nvidia-6.8 (Ubuntu package)
linux-ibm-6.8 (Ubuntu package)
linux-gcp-6.8 (Ubuntu package)
linux-raspi-realtime (Ubuntu package)
linux-realtime-6.8 (Ubuntu package)
linux-nvidia-bos (Ubuntu package)
linux-hwe-7.0 (Ubuntu package)
linux-gcp-7.0 (Ubuntu package)
linux-oracle-7.0 (Ubuntu package)
linux-azure-7.0 (Ubuntu package)
linux-oem-7.0 (Ubuntu package)
linux-aws-7.0 (Ubuntu package)
linux-nvidia-7.0 (Ubuntu package)

How to mitigate CVE-2026-63867

Install security update from vendor's repository.

Linux kernel - update to 7.0 rc3
linux (Ubuntu package) - addressed in versions 5.15.0-198.208, 6.8.0-146.146, 7.0.0-31.31
linux-kvm (Ubuntu package) - update to 5.15.0-1109.114
linux-aws (Ubuntu package) - addressed in versions 5.15.0-1117.124, 6.8.0-1066.69, 7.0.0-1012.12
linux-gcp (Ubuntu package) - addressed in versions 5.15.0-1118.128, 6.8.0-1070.78, 7.0.0-1011.11
linux-azure (Ubuntu package) - addressed in versions 5.15.0-1123.132, 6.8.0-1070.78, 7.0.0-1014.14
linux-raspi (Ubuntu package) - addressed in versions 5.15.0-1113.116, 6.8.0-1068.72, 7.0.0-1019.19
linux-oracle (Ubuntu package) - addressed in versions 5.15.0-1114.120, 6.8.0-1063.66, 7.0.0-1011.11
linux-lowlatency (Ubuntu package) - addressed in versions 5.15.0-195.205, 6.8.0-146.146.1
linux-gke (Ubuntu package) - addressed in versions 5.15.0-1113.119, 6.8.0-1065.73, 7.0.0-1006.7
linux-aws-5.15 (Ubuntu package) - update to 5.15.0-1117.124~20.04.1
linux-lowlatency-hwe-5.15 (Ubuntu package) - update to 5.15.0-195.205~20.04.1
linux-hwe-5.15 (Ubuntu package) - update to 5.15.0-198.208~20.04.1
linux-fips (Ubuntu package) - addressed in versions 5.15.0-198.208+fips1, 6.8.0-146.146+fips1
linux-azure-5.15 (Ubuntu package) - update to 5.15.0-1123.132~20.04.1
linux-ibm-5.15 (Ubuntu package) - update to 5.15.0-1111.115~20.04.1
linux-realtime (Ubuntu package) - addressed in versions 5.15.0-1116.125, 6.8.1-1061.62, 7.0.0-31.31.1
linux-azure-fde-5.15 (Ubuntu package) - update to 5.15.0-1122.131~20.04.1
linux-nvidia-tegra-igx (Ubuntu package) - update to 5.15.0-1059.59
linux-gkeop (Ubuntu package) - addressed in versions 5.15.0-1100.108, 6.8.0-1051.55
linux-nvidia-tegra (Ubuntu package) - addressed in versions 5.15.0-1070.72, 6.8.0-1036.39
linux-nvidia (Ubuntu package) - addressed in versions 5.15.0-1112.113, 6.8.0-1064.67, 7.0.0-1018.18
linux-intel-iotg (Ubuntu package) - update to 5.15.0-1112.118
linux-ibm (Ubuntu package) - addressed in versions 5.15.0-1111.115, 6.8.0-1066.67, 7.0.0-1013.13
linux-intel-iotg-5.15 (Ubuntu package) - update to 5.15.0-1112.118~20.04.1
linux-oracle-5.15 (Ubuntu package) - update to 5.15.0-1114.120~20.04.1
linux-gcp-5.15 (Ubuntu package) - update to 5.15.0-1118.128~20.04.1
linux-nvidia-tegra-5.15 (Ubuntu package) - update to 5.15.0-1070.72~20.04.1
linux-xilinx-zynqmp (Ubuntu package) - update to 5.15.0-1080.84
linux-intel-iot-realtime (Ubuntu package) - update to 5.15.0-1109.111
linux-gcp-fips (Ubuntu package) - addressed in versions 5.15.0-1118.128+fips1, 6.8.0-1070.78+fips1
linux-aws-fips (Ubuntu package) - addressed in versions 5.15.0-1117.124+fips1, 6.8.0-1066.69+fips1
linux-azure-fips (Ubuntu package) - addressed in versions 5.15.0-1123.132+fips1, 6.8.0-1069.77+fips1
linux-azure-fde (Ubuntu package) - update to 6.8.0-1068.75
linux-hwe-6.8 (Ubuntu package) - update to 6.8.0-149.149~22.04.1
linux-lowlatency-hwe-6.8 (Ubuntu package) - update to 6.8.0-146.146.1~22.04.1
linux-azure-fde-6.8 (Ubuntu package) - update to 6.8.0-1068.75~22.04.1
linux-oracle-6.8 (Ubuntu package) - update to 6.8.0-1067.71~22.04.1
linux-azure-6.8 (Ubuntu package) - update to 6.8.0-1070.78~22.04.1
linux-aws-6.8 (Ubuntu package) - update to 6.8.0-1066.69~22.04.1
linux-nvidia-lowlatency (Ubuntu package) - update to 6.8.0-1064.67.1
linux-nvidia-6.8 (Ubuntu package) - update to 6.8.0-1064.67~22.04.1
linux-ibm-6.8 (Ubuntu package) - update to 6.8.0-1066.67~22.04.1
linux-gcp-6.8 (Ubuntu package) - update to 6.8.0-1070.78~22.04.1
linux-raspi-realtime (Ubuntu package) - update to 6.8.0-2056.58
linux-realtime-6.8 (Ubuntu package) - update to 6.8.1-1061.62~22.04.1
linux-nvidia-bos (Ubuntu package) - update to 7.0.0-2018.18
linux-hwe-7.0 (Ubuntu package) - update to 7.0.0-31.31~24.04.1
linux-gcp-7.0 (Ubuntu package) - update to 7.0.0-1011.11~24.04.1
linux-oracle-7.0 (Ubuntu package) - update to 7.0.0-1011.11~24.04.1
linux-azure-7.0 (Ubuntu package) - update to 7.0.0-1014.14~24.04.1
linux-oem-7.0 (Ubuntu package) - update to 7.0.0-1013.13
linux-aws-7.0 (Ubuntu package) - update to 7.0.0-1012.12~24.04.1
linux-nvidia-7.0 (Ubuntu package) - update to 7.0.0-1018.18~24.04.1

External References

Related Security Bulletins