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NVD / CVE Program· CNA redhat

CVE-2021-3656: A flaw was found in the KVM's AMD code for supporting SVM nested virtualization.

Noroxi assessment

Priority: Monitor (action score 35/100)

  • Public proof-of-concept code exists (GitHub); there is no record of exploitation in the wild.
  • Probability of exploitation in the next 30 days: 0.65% (FIRST EPSS); higher than 51.6% of all records.
  • Someone with local access to the system can trigger it. A low-privileged account is enough. No user action is required. No special conditions are required; it is repeatable.
  • Fixed version: kernel-tools-libs-devel (AlmaLinux:8) 4.18.0-305.25.1.el8_4; linux (Debian:11) 5.10.46-5; linux (Debian:12) 5.14.6-1.
  • Mentioned 1 time in Turkish national cyber security advisories (siberguvenlik.gov.tr).

Low priority for now. Upgrade to kernel-tools-libs-devel (AlmaLinux:8) 4.18.0-305.25.1.el8_4; linux (Debian:11) 5.10.46-5; linux (Debian:12) 5.14.6-1 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

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.

HighCVSS 8.8 · v3.1—Proof of concept Fix available
Published
Mar 4, 2022
Updated
Jun 17, 2026
EPSS
0.6% · 51th percentile

Report tools

JSON

Action score

35

Monitor

Low priority for now.

CVSS
35 / 40 · 8.8 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.6%

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • Proof of concept exists, no weaponized module
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2021-3656

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
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  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
linuxlinux kernel
fedoraprojectfedora
redhatsoftware collections
redhatenterprise linux server
redhatopenstack
redhatenterprise linux
redhatenterprise linux desktop
redhatenterprise linux eus
redhatenterprise linux for ibm z systems
redhatenterprise linux for ibm z systems eus
redhatenterprise linux for power big endian
redhatenterprise linux for power little endian

and 14 more

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • fedoraproject fedora33
  • fedoraproject fedora34
  • linux linux kernel4.20 and later · before 5.4.142
  • linux linux kernel5.5 and later · before 5.10.60
  • linux linux kernel5.11 and later · before 5.13.12
  • linux linux kernel4.13 and later · before 4.14.245
  • linux linux kernel4.15 and later · before 4.19.205
  • linux linux kernel5.14
  • redhat 3scale api management2.0
  • redhat codeready linux builderall versions
  • redhat enterprise linux8.0
  • redhat enterprise linux desktop7.0
  • redhat enterprise linux eus8.1
  • redhat enterprise linux eus8.2
  • redhat enterprise linux eus8.4
  • redhat enterprise linux for ibm z systems7.0
  • redhat enterprise linux for ibm z systems8.0
  • redhat enterprise linux for ibm z systems eus8.1
  • redhat enterprise linux for ibm z systems eus8.2
  • redhat enterprise linux for ibm z systems eus8.4

Versions reported by the vendor

Affected version ranges reported by the assigning authority (redhat). Independent of NVD's CPE analysis and usually ahead of it.

  • n/a KVM

    • Fixed-In v5.14-rc7 and aboveaffected

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
AlmaLinux:8kernel-tools-libs-develbefore 4.18.0-305.25.1.el8_44.18.0-305.25.1.el8_4
Debian:11linuxbefore 5.10.46-55.10.46-5
Debian:12linuxbefore 5.14.6-15.14.6-1
Debian:9linux-4.19before 4.19.208-1~deb9u14.19.208-1~deb9u1
openSUSE:Leap 15.2kernel-debugbefore 5.3.18-lp152.92.25.3.18-lp152.92.2
openSUSE:Leap 15.2kernel-defaultbefore 5.3.18-lp152.92.25.3.18-lp152.92.2
openSUSE:Leap 15.2kernel-default-basebefore 5.3.18-lp152.92.2.lp152.8.42.35.3.18-lp152.92.2.lp152.8.42.3
openSUSE:Leap 15.2kernel-docsbefore 5.3.18-lp152.92.15.3.18-lp152.92.1
openSUSE:Leap 15.2kernel-kvmsmallbefore 5.3.18-lp152.92.25.3.18-lp152.92.2
openSUSE:Leap 15.2kernel-obs-buildbefore 5.3.18-lp152.92.25.3.18-lp152.92.2
openSUSE:Leap 15.2kernel-obs-qabefore 5.3.18-lp152.92.15.3.18-lp152.92.1
openSUSE:Leap 15.2kernel-preemptbefore 5.3.18-lp152.92.25.3.18-lp152.92.2
openSUSE:Leap 15.2kernel-sourcebefore 5.3.18-lp152.92.25.3.18-lp152.92.2
openSUSE:Leap 15.2kernel-symsbefore 5.3.18-lp152.92.15.3.18-lp152.92.1
openSUSE:Leap 15.3dtb-aarch64before 5.3.18-59.24.15.3.18-59.24.1
openSUSE:Leap 15.3kernel-64kbbefore 5.3.18-59.24.15.3.18-59.24.1
openSUSE:Leap 15.3kernel-azurebefore 5.3.18-38.22.25.3.18-38.22.2
openSUSE:Leap 15.3kernel-debugbefore 5.3.18-59.24.15.3.18-59.24.1
openSUSE:Leap 15.3kernel-defaultbefore 5.3.18-59.24.15.3.18-59.24.1
openSUSE:Leap 15.3kernel-default-basebefore 5.3.18-59.24.1.18.12.15.3.18-59.24.1.18.12.1
openSUSE:Leap 15.3kernel-docsbefore 5.3.18-59.24.15.3.18-59.24.1
openSUSE:Leap 15.3kernel-kvmsmallbefore 5.3.18-59.24.15.3.18-59.24.1
openSUSE:Leap 15.3kernel-obs-buildbefore 5.3.18-59.24.15.3.18-59.24.1
openSUSE:Leap 15.3kernel-obs-qabefore 5.3.18-59.24.15.3.18-59.24.1

+96

Same product

linux: all records

Other highest-scoring records for the same primary product.

  • CVE-2026-31431crypto: algif_aead - Revert to operating out-of-placeKEV
    90Now
  • CVE-2022-0847A flaw was found in the way the "flags" member of the new pipe buffer structure was lacking proper initialization in copy_page_to_iter_pipe KEV
    89Now
  • CVE-2021-22555Heap Out-Of-Bounds Write in Netfilter IP6T_SO_SET_REPLACEKEV
    85Now

Published before and after this one in the same product

  • ← EarlierCVE-2021-3744Mar 4, 2022 · A memory leak flaw was found in the Linux kernel in the ccp_run_aes_gcm_cmd() function in drivers/crypto/ccp/ccp-ops.c, which allows attacke
    22Monitor
  • ← EarlierCVE-2021-3428Mar 4, 2022 · A flaw was found in the Linux kernel.
    22Monitor
  • Later →CVE-2022-26490Mar 6, 2022 · st21nfca_connectivity_event_received in drivers/nfc/st21nfca/se.c in the Linux kernel through 5.16.12 has EVT_TRANSACTION buffer overflows b
    31Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
almalinux:kernel-tools-libs-devel4.18.0-305.25.1.el8_4 · AlmaLinux:8Package registry (OSV)
debian:linux5.14.6-1 · Debian:12Package registry (OSV)
debian:linux-4.194.19.208-1~deb9u1 · Debian:9Package registry (OSV)
opensuse:kernel-azure5.3.18-38.22.2 · openSUSE:Leap 15.3Package registry (OSV)
opensuse:kernel-debug5.3.18-lp152.92.2 · openSUSE:Leap 15.2Package registry (OSV)
opensuse:kernel-default5.3.18-lp152.92.2 · openSUSE:Leap 15.2Package registry (OSV)
opensuse:kernel-default-base5.3.18-lp152.92.2.lp152.8.42.3 · openSUSE:Leap 15.2Package registry (OSV)
opensuse:kernel-docs5.3.18-lp152.92.1 · openSUSE:Leap 15.2Package registry (OSV)
opensuse:kernel-kvmsmall5.3.18-lp152.92.2 · openSUSE:Leap 15.2Package registry (OSV)
opensuse:kernel-obs-build5.3.18-lp152.92.2 · openSUSE:Leap 15.2Package registry (OSV)
opensuse:kernel-obs-qa5.3.18-lp152.92.1 · openSUSE:Leap 15.2Package registry (OSV)
opensuse:kernel-preempt5.3.18-lp152.92.2 · openSUSE:Leap 15.2Package registry (OSV)
opensuse:kernel-source5.3.18-lp152.92.2 · openSUSE:Leap 15.2Package registry (OSV)
opensuse:kernel-source-azure5.3.18-38.22.1 · openSUSE:Leap 15.3Package registry (OSV)
opensuse:kernel-syms5.3.18-lp152.92.1 · openSUSE:Leap 15.2Package registry (OSV)
red hat:bpftool0:4.18.0-147.56.1.el8_1 · Red Hat:rhel_eus:8.1::baseosPackage registry (OSV)
red hat:bpftool-debuginfo0:4.18.0-147.56.1.el8_1 · Red Hat:rhel_eus:8.1::baseosPackage registry (OSV)
red hat:kernel0:4.18.0-147.56.1.el8_1 · Red Hat:rhel_eus:8.1::baseosPackage registry (OSV)
red hat:kernel-abi-whitelists0:4.18.0-147.56.1.el8_1 · Red Hat:rhel_eus:8.1::baseosPackage registry (OSV)
red hat:kernel-core0:4.18.0-147.56.1.el8_1 · Red Hat:rhel_eus:8.1::baseosPackage registry (OSV)
red hat:kernel-cross-headers0:4.18.0-147.56.1.el8_1 · Red Hat:rhel_eus:8.1::baseosPackage registry (OSV)
red hat:kernel-debug0:4.18.0-147.56.1.el8_1 · Red Hat:rhel_eus:8.1::baseosPackage registry (OSV)
red hat:kernel-debug-core0:4.18.0-147.56.1.el8_1 · Red Hat:rhel_eus:8.1::baseosPackage registry (OSV)
red hat:kernel-debug-debuginfo0:4.18.0-147.56.1.el8_1 · Red Hat:rhel_eus:8.1::baseosPackage registry (OSV)
red hat:kernel-debug-devel0:4.18.0-147.56.1.el8_1 · Red Hat:rhel_eus:8.1::baseosPackage registry (OSV)
red hat:kernel-debug-modules0:4.18.0-147.56.1.el8_1 · Red Hat:rhel_eus:8.1::baseosPackage registry (OSV)
red hat:kernel-debug-modules-extra0:4.18.0-147.56.1.el8_1 · Red Hat:rhel_eus:8.1::baseosPackage registry (OSV)
rocky linux:kernel0:4.18.0-305.25.1.el8_4 · Rocky Linux:8Package registry (OSV)
rocky linux:kernel-rt0:4.18.0-305.25.1.rt7.97.el8_4 · Rocky Linux:8Package registry (OSV)
suse:kgraft-patch-SLE12-SP4_Update_1614-2.2 · SUSE:Linux Enterprise Live Patching 12 SP4Package registry (OSV)

+11

Exploit status

Proof of concept available

A public proof of concept or detection signature exists. Exploit knowledge is out there; don't delay patching.

Evidence

  • Proof-of-concept repository on GitHub ×1

Noroxi does not host or link to exploit code. Metasploit module names and Nuclei template ids are shown (names, not code); Exploit-DB entries and PoC repositories are given as counts only.

Public exploit evidence
SourceKindDate
Proof-of-concept repository on GitHubstars:22022-11-05

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

  1. Published
  2. First PoC+245 d
  3. TodayNov 6, 2026+1708 d

No history yet (it accumulates with the daily EPSS job).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

No credits in the CNA record.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

National notice (Türkiye Cybersecurity Directorate / USOM)

Official security notices citing this record; remediation advice is on the agency's page.

All national notices →

Technical details

Attack conditions

  • Someone with local access to the system can trigger it.
  • A low-privileged account is enough.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
high · data can be read
Integrity
high · data or configuration can be modified
Availability
high · the service can be disrupted

Impact can extend beyond the vulnerable component (scope changes).

Attack vector
Local
Attack complexity
Low
Privileges required
Low
User interaction
None
Scope
Changed
Confidentiality impact
High
Integrity impact
High
Availability impact
High

Weakness class (CWE)

CWE-862 · Missing Authorization

CVSS vector

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

nvd-primary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Show all references (3 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records