Linux: Time-of-check Time-of-use (TOCTOU) Race Condition
CVE-2026-63937 is a high-severity time-of-check time-of-use (TOCTOU) race condition vulnerability in Linux, with a CVSS 3.1 base score of 8.8 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 bd232801ef1d1fd985d2d4ca3cd1d888303ca86f, b1dfaa6f7a957726a6800135be3659fbe4bbf2a4, edbbe88f83b524434974e84808d3093199d67c24 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 0.14% 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 bd232801ef1d1fd985d2d4ca3cd1d888303ca86f or b1dfaa6f7a957726a6800135be3659fbe4bbf2a4 or edbbe88f83b524434974e84808d3093199d67c24 or c8cc238093ca6c99267032f6cfe78f59389f3157, whichever matches your release line, or a later version.
- Update Linux Kernel to 6.12.93 or 6.18.35 or 7.0.12, whichever matches your release line, or a later version.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Linux | Linux | 9b54e248d2644be71cb394eb85f31ad99e023a05 up to, but not including, bd232801ef1d1fd985d2d4ca3cd1d888303ca86f; 9b54e248d2644be71cb394eb85f31ad99e023a05 up to, but not including, b1dfaa6f7a957726a6800135be3659fbe4bbf2a4; 9b54e248d2644be71cb394eb85f31ad99e023a05 up to, but not including, edbbe88f83b524434974e84808d3093199d67c24; 9b54e248d2644be71cb394eb85f31ad99e023a05 up to, but not including, c8cc238093ca6c99267032f6cfe78f59389f3157; 6.11 | bd232801ef1d1fd985d2d4ca3cd1d888303ca86f, b1dfaa6f7a957726a6800135be3659fbe4bbf2a4, edbbe88f83b524434974e84808d3093199d67c24, c8cc238093ca6c99267032f6cfe78f59389f3157 |
| Linux | Linux Kernel | 6.11 up to, but not including, 6.12.93; 6.13 up to, but not including, 6.18.35; 6.19 up to, but not including, 7.0.12; 7.1 | 6.12.93, 6.18.35, 7.0.12 |
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
- ChangedThe attack can reach beyond the vulnerable component into other systems.
- 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:C/C:H/I:H/A:H. Scored by CNA.
Official description
In the Linux kernel, the following vulnerability has been resolved: KVM: SEV: Use READ_ONCE() when reading entries/indices from PSC buffer Use READ_ONCE() when reading entries/indices from the guest-accessible Page State Change buffer to defend against TOCTOU bugs. Don't bother with READ_ONCE()/WRITE_ONCE() for cases where KVM is writing (and not consuming the result!), as the guest isn't supposed to touch the buffer while it's being processed. I.e. using READ_ONCE() is all about protecting against misbehaving guests.
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
Recent Time-of-check Time-of-use (TOCTOU) Race Condition vulnerabilities
- 7.8 High severity
- 9.8 Critical severity
- 9.3 Critical severity
- 8.2 High severity
- 7.1 High severity
- 7.0 High severity
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