CVE-2021-3631

Cross-VM file access via single-category MCS labels

Published Mar 2, 2022 · Updated Nov 19, 2024

Incorrect permission assignment in libvirt before 7.5.0 allows guest OS users to read or modify another VM's files through sVirt. virSecuritySELinuxMCSFind can accept two equal random category values, producing a one-category label whose set is a subset of a different VM's two-category label. Exploitation requires a compromised guest, SELinux sVirt dynamic labeling, and the chance assignment of category sets with that subset relationship.

CVSS severity6.3
Medium
EPSS probability0.49%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Incorrect permission assignment in libvirt before 7.5.0 allows guest OS users to read or modify another VM's files through sVirt. virSecuritySELinuxMCSFind can accept two equal random category values, producing a one-category label whose set is a subset of a different VM's two-category label. Exploitation requires a compromised guest, SELinux sVirt dynamic labeling, and the chance assignment of category sets with that subset relationship.

The record

CVE
CVE-2021-3631
Published
Mar 2, 2022
Updated
Nov 19, 2024
Vendor
libvirt
Product
libvirt
Classifications
CWE-732, T1565.001, T1005
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Mar 2, 2022CVE publishedPublication date reported by the CVE source.
  2. Nov 19, 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 - libvirt v7.5.0

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

libvirt · libvirtversion=Fixed-In - libvirt v7.5.0

Published CVSS scores

6.3UbuntuCVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N
6.3Oracle LinuxCVSS:3.0/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N

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.

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No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-732Incorrect Permission Assignment for Critical Resource
T1565.001Stored Data Manipulation
T1005Data from Local System

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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