CVE-2021-3656

Guest-to-host escape via missing VMCB intercepts

Published Mar 4, 2022 · Updated Aug 3, 2024

Improper VMCB validation in KVM 4.13 through 5.14-rc6 allows guest OS users to read and write host physical memory. recalc_intercepts trusts the L1-supplied virt_ext field, allowing both VMLOAD/VMSAVE intercepts and virtual VMLOAD/VMSAVE to be disabled for L2. Exploitation requires an AMD host with nested virtualization enabled and control of an L1 guest; consequences include host data exposure, corruption, crash, or guest-to-host escape.

CVSS severity8.8
High
EPSS probability0.66%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper VMCB validation in KVM 4.13 through 5.14-rc6 allows guest OS users to read and write host physical memory. recalc_intercepts trusts the L1-supplied virt_ext field, allowing both VMLOAD/VMSAVE intercepts and virtual VMLOAD/VMSAVE to be disabled for L2. Exploitation requires an AMD host with nested virtualization enabled and control of an L1 guest; consequences include host data exposure, corruption, crash, or guest-to-host escape.

The record

CVE
CVE-2021-3656
Published
Mar 4, 2022
Updated
Aug 3, 2024
Vendor
The Linux Kernel Organization
Product
KVM (Kernel-based Virtual Machine)
Classifications
CWE-862, T1611
Attack vector
local
Privileges
Unavailable

Timeline

How it unfolded

  1. Mar 4, 2022CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=Fixed-In v5.14-rc7 and above

What conditions does exploitation require?

Attack vectorlocal
Required privilegesUnavailable in this record.

What is affected?

The Linux Kernel Organization · KVM (Kernel-based Virtual Machine)version=Fixed-In v5.14-rc7 and above

Published CVSS scores

8.8Red HatCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
8.8NVDCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
PrivilegesUnavailable
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-862Missing Authorization
T1611Escape to Host

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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