Linux: Time-of-check Time-of-use (TOCTOU) Race Condition
CVE-2026-31678 is a high-severity time-of-check time-of-use (TOCTOU) race condition vulnerability in Linux, with a CVSS 3.1 base score of 7.8 out of 10. It can be exploited with local access to the system by an attacker with a low-privileged account, without any action from a user. The record lists versions f0ec49d3a39c1f5a4f3270ea27ca0854fad2a2a2, 9d56aced21fb9c104e8a3f3be9b21fbafe448ffc, 42f0d3d81209654c08ffdde5a34b9b92d2645896 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 0.13% chance of exploitation within 30 days, which is low compared with other CVEs.
High severity with no evidence of exploitation so far.
What to do
- Update Linux to f0ec49d3a39c1f5a4f3270ea27ca0854fad2a2a2 or 9d56aced21fb9c104e8a3f3be9b21fbafe448ffc or 42f0d3d81209654c08ffdde5a34b9b92d2645896 or bbe7bd722bfaea36aab3da6cc60fb4a05c644643, whichever matches your release line, or a later version.
- Update Linux Kernel to 6.1.168 or 6.6.131 or 6.12.80 or 6.18.21, whichever matches your release line, or a later version.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Linux | Linux | a9020fde67a6eb77f8130feff633189f99264db1 up to, but not including, f0ec49d3a39c1f5a4f3270ea27ca0854fad2a2a2; a9020fde67a6eb77f8130feff633189f99264db1 up to, but not including, 9d56aced21fb9c104e8a3f3be9b21fbafe448ffc; a9020fde67a6eb77f8130feff633189f99264db1 up to, but not including, 42f0d3d81209654c08ffdde5a34b9b92d2645896; a9020fde67a6eb77f8130feff633189f99264db1 up to, but not including, bbe7bd722bfaea36aab3da6cc60fb4a05c644643; a9020fde67a6eb77f8130feff633189f99264db1 up to, but not including, 98b726ab5e2a4811e27c28e4d041f75bba147eab; a9020fde67a6eb77f8130feff633189f99264db1 up to, but not including, b8c56a3fc5d879c0928f207a756b0f067f06c6a8; a9020fde67a6eb77f8130feff633189f99264db1 up to, but not including, 6931d21f87bc6d657f145798fad0bf077b82486c; 4.3 | f0ec49d3a39c1f5a4f3270ea27ca0854fad2a2a2, 9d56aced21fb9c104e8a3f3be9b21fbafe448ffc, 42f0d3d81209654c08ffdde5a34b9b92d2645896, bbe7bd722bfaea36aab3da6cc60fb4a05c644643, 98b726ab5e2a4811e27c28e4d041f75bba147eab, b8c56a3fc5d879c0928f207a756b0f067f06c6a8 |
| Linux | Linux Kernel | 4.3 up to, but not including, 6.1.168; 6.2 up to, but not including, 6.6.131; 6.7 up to, but not including, 6.12.80; 6.13 up to, but not including, 6.18.21; 6.19 up to, but not including, 6.19.11; 7.0 | 6.1.168, 6.6.131, 6.12.80, 6.18.21, 6.19.11 |
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
- LocalThe attacker needs local access or must trick a user into running something.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- LowA basic user account is enough.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H. Scored by CNA.
Official description
In the Linux kernel, the following vulnerability has been resolved: openvswitch: defer tunnel netdev_put to RCU release ovs_netdev_tunnel_destroy() may run after NETDEV_UNREGISTER already detached the device. Dropping the netdev reference in destroy can race with concurrent readers that still observe vport->dev. Do not release vport->dev in ovs_netdev_tunnel_destroy(). Instead, let vport_netdev_free() drop the reference from the RCU callback, matching the non-tunnel destroy path and avoiding additional synchronization under RTNL.
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
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Patches and release notes
- git.kernel.org/stable/c/42f0d3d81209654c08ffdde5a34b9b92d2645896
- git.kernel.org/stable/c/6931d21f87bc6d657f145798fad0bf077b82486c
- git.kernel.org/stable/c/98b726ab5e2a4811e27c28e4d041f75bba147eab
- git.kernel.org/stable/c/9d56aced21fb9c104e8a3f3be9b21fbafe448ffc
- git.kernel.org/stable/c/b8c56a3fc5d879c0928f207a756b0f067f06c6a8
- git.kernel.org/stable/c/bbe7bd722bfaea36aab3da6cc60fb4a05c644643
Other references
Recent Time-of-check Time-of-use (TOCTOU) Race Condition vulnerabilities
- 7.0 High severity
- 8.8 High severity
- 8.2 High severity
- 7.1 High severity
- 7.0 High severity
- 7.5 High severity
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