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VulnerabilityAwaiting classification

CVE-2026-63937

HIGH · 8.8 CVSS v3.1 Published 2026-07-19
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
90/100
Remediation priority · Urgent
Zero-click 5 weeks old

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

KVM SEV (AMD Secure Encrypted Virtualization) has a TOCTOU (Time-Of-Check-Time-Of-Use) vulnerability in its Page State Change (PSC) buffer handling. When KVM reads entries/indices from the guest-accessible PSC buffer, it was not using READ_ONCE() to protect against the compiler reordering or caching reads. A malicious or compromised guest could potentially manipulate buffer values between when KVM checks and uses them, leading to incorrect state transitions or memory corruption.

MitigationApply the kernel patch to add READ_ONCE() when reading entries/indices from the PSC buffer in KVM SEV. Ensure the guest cannot access the buffer while it is being processed, though the primary fix is the addition of proper memory barriers via READ_ONCE().

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Changed
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify if the system uses AMD processors with SEV support
    Check /proc/cpuinfo for 'svm' flag (AMD Virtualization) and look for SEV capability in dmesg: 'dmesg | grep -i sev'
    Affected if System lacks AMD SVM or SEV support - not vulnerable in that case
  2. Check if KVM SEV is loaded and active
    Run 'lsmod | grep kvm_amd' and check if SEV is listed in /sys/module/kvm_amd/parameters/sev
    Affected if KVM AMD module not loaded or SEV parameter not present - not vulnerable
  3. Determine the running kernel version
    Run 'uname -r' to get kernel version; check if kernel is older than the patched version (compare against upstream fix commit in kernel source)
    Affected if Running an unpatched kernel version - likely vulnerable
  4. Verify presence of vulnerable PSC buffer code
    Check kernel source for sev_es.c or sev.c: look for direct reads of psc->entries or psc->index without READ_ONCE() wrapper around lines handling guest Page State Change buffer
    Affected if Code reads PSC buffer entries/indices without READ_ONCE() - vulnerable
  5. Check if any SEV-encrypted VMs are running
    Run 'virsh list --all' and check VM XML configs for '<os><type machine="pc-q35">...</type></os><memory unit="KiB">...</memory><sev><owner>...</sev>' or query libvirt: 'virsh domcapabilities' shows SEV support
    Affected if No SEV-encrypted VMs defined or running - vulnerability not exploitable in practice

A system is affected if it runs an unpatched Linux kernel on AMD hardware with KVM SEV enabled, has SEV-encrypted virtual machines, and the vulnerable PSC buffer code lacks READ_ONCE() protections.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

From vendor data
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Apply the kernel patch to add READ_ONCE() when reading entries/indices from the PSC buffer in KVM SEV. Ensure the guest cannot access the buffer while it is being processed, though the primary fix is the addition of proper memory barriers via READ_ONCE().

Have this fixed Scoped from the published advisory
  • Consultation4.0 h
  • Implementation2.0 h
  • Testing8.0 h
  • Review / QA4.0 h
18.0 hours of engineering $3,080
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-63937 in production — separate from our analysis above.

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What this is

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  • Version or environment caveats, and links to real fixes
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