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Linux KernelOperating system · Linux

CVE-2021-3656

HIGH · 8.8 CVSS v3.1 Published 2022-03-04
Fix available
A fix is available. Upgrade to 4.14.245 / 4.19.205 or later.
See remediation →
90/100
Remediation priority · Urgent
Zero-click Patch available

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
A flaw was found in the KVM's AMD code for supporting SVM nested virtualization. The flaw occurs when processing the VMCB (virtual machine control block) provided by the L1 guest to spawn/handle a nested guest (L2). Due to improper validation of the "virt_ext" field, this issue could allow a malicious L1 to disable both VMLOAD/VMSAVE intercepts and VLS (Virtual VMLOAD/VMSAVE) for the L2 guest. As a result, the L2 guest would be allowed to read/write physical pages of the host, resulting in a crash of the entire system, leak of sensitive data or potential guest-to-host escape.

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
How this class of weakness works · CWE-862

The application checks who you are but not whether you're allowed to perform a given action, so any authenticated user can reach things meant for others. This is the classic “change the ID in the URL” bug. The fix is an authorization check on every request, evaluated against the acting user's permissions.

General guidance for the missing authorization class — the official description and references above are authoritative for this specific CVE. Want a bespoke review and a reviewed fix? Ask our team →

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Linux KernelOperating system
Affected:>= 4.13, < 4.14.245>= 4.15, < 4.19.205>= 4.20, < 5.4.142>= 5.5, < 5.10.60>= 5.11, < 5.13.12= 5.14
FedoraOperating system
Affected:= 33= 34
Software CollectionsApplication
Affected:all versions
OpenstackApplication
Affected:= 13
Enterprise LinuxOperating system
Affected:= 8.0
Enterprise Linux DesktopOperating system
Affected:= 7.0
Enterprise Linux EusOperating system
Affected:= 8.1= 8.2= 8.4
Enterprise Linux For Ibm Z SystemsOperating system
Affected:= 7.0= 8.0

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

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.

dbcve · scoped
Upgrade available Upgrade to 4.14.245 / 4.19.205 / 5.4.142 or later
Fixed in 4.14.2454.19.2055.4.142
Vendor patch git.kernel.org →
Recommended fix High confidence

Linux Kernel 4.14.245+ / 4.19.205+ / 5.4.142+ / 5.10.60+; Fedora 33/34 updated kernel; RHEL 8.0+, 8.1 EUS, 8.2 EUS, 8.4 EUS updated kernel

  1. Identify the currently running kernel version using 'uname -r'
  2. Check if the system is using an affected kernel version (4.13.x through 5.10.x branches as listed in the affected versions)
  3. For RHEL/CentOS systems: run 'yum update kernel' or 'dnf update kernel' to install the latest patched kernel
  4. For Fedora systems: run 'dnf update kernel' to get the patched kernel (Fedora 33 and 34 are affected)
  5. Reboot the system to load the patched kernel using 'systemctl reboot'
  6. After reboot, verify the kernel version is >= 4.14.245, >= 4.19.205, >= 5.4.142, or >= 5.10.60 using 'uname -r'
  7. For systems using KVM/QEMU: ensure nested virtualization (SVM) is only enabled for trusted guests, as an additional defense-in-depth measure
Caveat Kernel upgrades may require system reboot and could introduce compatibility issues with custom kernel modules or specific hardware drivers; ensure backups and test in staging if possible

Generated from the published advisory — verify against the referenced sources before acting.

We can perform the upgrade in your staging environment and verify nothing breaks — typical engagement from $1,950. Get the upgrade done

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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-2021-3656 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
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