CyberSecBulletin
CVE-2026-63867NVD status: Analyzed

Linux: Time-of-check Time-of-use (TOCTOU) Race Condition

Published Updated By the CyberSecBulletin desk

CVE-2026-63867 is a high-severity time-of-check time-of-use (TOCTOU) race condition vulnerability in Linux, with a CVSS 3.1 base score of 8.2 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. The record lists versions 301a33fd590c408a05c5df800e0cc1e6a8a2f8f8, 68364963e5baf03f16b4420292291f75c8f66497, 907ac6b1658e0277f979fcdfae2a753b495c1510 as the first without the flaw, so updating Linux to one of those releases or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.27% chance of exploitation within 30 days, which is low compared with other CVEs.

Next patch cycle

High severity with no evidence of exploitation so far.

What to do

  1. Update Linux to 301a33fd590c408a05c5df800e0cc1e6a8a2f8f8 or 68364963e5baf03f16b4420292291f75c8f66497 or 907ac6b1658e0277f979fcdfae2a753b495c1510 or c4f4cf60797974873dbc8e100144682a6f2f861f, whichever matches your release line, or a later version.
  2. Update Linux Kernel to 5.15.210 or 6.1.176 or 6.6.143 or 6.12.94, whichever matches your release line, or a later version.
  3. Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.

Affected products and versions

VendorProductAffected versionsFixed in
LinuxLinuxfa3fe2b150316b294f2c662653501273ff25bba8 up to, but not including, 301a33fd590c408a05c5df800e0cc1e6a8a2f8f8; fa3fe2b150316b294f2c662653501273ff25bba8 up to, but not including, 68364963e5baf03f16b4420292291f75c8f66497; fa3fe2b150316b294f2c662653501273ff25bba8 up to, but not including, 907ac6b1658e0277f979fcdfae2a753b495c1510; fa3fe2b150316b294f2c662653501273ff25bba8 up to, but not including, c4f4cf60797974873dbc8e100144682a6f2f861f; fa3fe2b150316b294f2c662653501273ff25bba8 up to, but not including, 3b8cbba7c0ed31189c89f90be247b8973ffa79ef; fa3fe2b150316b294f2c662653501273ff25bba8 up to, but not including, 8f4f0a157e8436a05bf8c3670b24dbc258911c43; fa3fe2b150316b294f2c662653501273ff25bba8 up to, but not including, 8ab24fdebc369c0dfb90f82c1650b1e66662bb45; 5.11301a33fd590c408a05c5df800e0cc1e6a8a2f8f8, 68364963e5baf03f16b4420292291f75c8f66497, 907ac6b1658e0277f979fcdfae2a753b495c1510, c4f4cf60797974873dbc8e100144682a6f2f861f, 3b8cbba7c0ed31189c89f90be247b8973ffa79ef, 8f4f0a157e8436a05bf8c3670b24dbc258911c43
LinuxLinux Kernel5.11 up to, but not including, 5.15.210; 5.16 up to, but not including, 6.1.176; 6.2 up to, but not including, 6.6.143; 6.7 up to, but not including, 6.12.94; 6.13 up to, but not including, 6.18.36; 6.19 up to, but not including, 7.0.13; 7.15.15.210, 6.1.176, 6.6.143, 6.12.94, 6.18.36, 7.0.13

Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.

How the severity score breaks down

Attack vector
NetworkCan be exploited remotely over a network, including the internet.
Attack complexity
LowNo special conditions are needed; the attack works reliably.
Privileges required
NoneNo account or login is needed.
User interaction
NoneNo action by a victim is required.
Scope
UnchangedImpact stays within the vulnerable component.
Confidentiality
NoneNo data is disclosed.
Integrity
LowLimited modification of data is possible.
Availability
HighThe component can be taken down completely.

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H. Scored by CNA.

Official description

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.

Source: the CVE record as published by its numbering authority, via NVD and CVE.org.

The underlying weakness: Time-of-check Time-of-use (TOCTOU) Race Condition

The product checks the state of a resource before using that resource, but the resource's state can change between the check and the use in a way that invalidates the results of the check.

How developers prevent this class of flaw

  • The most basic advice for TOCTOU vulnerabilities is to not perform a check before the use. This does not resolve the underlying issue of the execution of a function on a resource whose state and identity cannot be assured, but it does help to limit the false sense of security given by the check.
  • When the file being altered is owned by the current user and group, set the effective gid and uid to that of the current user and group when executing this statement.
  • Limit the interleaving of operations on files from multiple processes.

More Time-of-check Time-of-use (TOCTOU) Race Condition vulnerabilities (CWE-367)

Timeline

  1. CVE record published.
  2. CISA published its SSVC assessment (exploitation: none, automatable: yes, technical impact: partial).
  3. Record last changed materially (score, affected products, status or references).

References

Recent Time-of-check Time-of-use (TOCTOU) Race Condition vulnerabilities

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