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Full vulnerability report · 2021
CVE-2021-3493High confidence

Linux Kernel Privilege Escalation Vulnerability

Linux · Kernel

8.8HighCVSS 3.1
Recommended action
Patch only the product branches with a verified fix

CISA confirms exploitation in the wild and lists 2022-11-10 as the remediation due date. Verified remediation exists for at least one product or source, but 1 structured product or package state remain unresolved. Apply remediation only to the exact product branch confirmed by its source.

Fix availability varies by product
R
Operational reassessment

Published severity in operational context

Open reassessment dashboard →
Published severityHighOperational priority:Critical, raised one band.upgradedsince 20 Oct 2022

Evidence used

  • CISA confirms exploitation in the wild.
  • A structured source references public exploit or proof-of-concept material.
  • Exploitation requires an existing local or physical foothold with privileges.
  • EPSS is 49.17% for the current model date.

Compensating controls

  • Validate the affected product branch and deploy the verified fixed release.
  • Restrict local access and enforce least privilege on affected hosts.
  • Increase monitoring for the attack path and post-exploitation behaviour described in the report.

Verification

  1. Confirm that the asset runs Linux Kernel and falls inside the recorded affected range.
  2. Verify the installed build against the product-specific fixed version after deployment.
  3. Validate exposure, authentication requirements and compensating controls in the actual environment.
  4. Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
Mitigation target: Within 3 daysRemediation target: Within 90 days

This automated reassessment organises public evidence. It does not know asset exposure, business impact or control effectiveness and does not replace CVSS or a human risk decision.

Cross-source reconciliation

Remediation availability differs by product scope

Verified remediation exists for at least one product or source, but 1 structured product or package state remain unresolved. Apply remediation only to the exact product branch confirmed by its source.

Distribution package intelligence

Release-specific package status

Debian, ubuntu findings are scoped to the named distribution, release and source package. An absent finding does not mean a package is unaffected.

5 package states
Package result overrides the generic status

BlackTree has verified remediation for at least one product or source, but the relevant distribution still reports no fixed package for 1 affected package state shown here. Treat those rows as affected with no fix until that distribution publishes a fixed version.

Repository candidate not checked

A published vendor fix does not prove that a matching update is enabled and installable on a particular asset. Confirm the local package candidate before scheduling remediation.

Distribution releaseSource packageVendor stateFixed versionEvidence
Debian trixietrixie · sourcelinuxVendor fix publishedDebian records a fixed source-package version for this release.5.10.38-1Debian Security Tracker ↗Source updated 5 Oct 2026
Debian bookwormbookworm · sourcelinuxVendor fix publishedDebian records a fixed source-package version for this release.5.10.38-1Debian Security Tracker ↗Source updated 5 Oct 2026
Debian forkyforky · sourcelinuxVendor fix publishedDebian records a fixed source-package version for this release.5.10.38-1Debian Security Tracker ↗Source updated 5 Oct 2026
Debian sidsid · sourcelinuxVendor fix publishedDebian records a fixed source-package version for this release.5.10.38-1Debian Security Tracker ↗Source updated 5 Oct 2026
Ubuntu 24.04 LTSnoble · standard archivelinux-raspi-realtimeAffected, no fix publishedCanonical OVAL identifies this running kernel flavour as affected and does not publish a fixed package version in this definition.Not published in this feedCanonical Ubuntu Security ↗Source updated 5 Oct 2026
Optional official sources

National CERT insights
?CERT means Computer Emergency Response Team; CSIRT is the closely related term Computer Security Incident Response Team.

Choose official national sources for this report. Each advisory shows its original language. Your selection is remembered on this device and included in shared links.

Official European source

ENISA European Vulnerability Database

Official EUVD identifiers, advisory evidence and known-exploited context. Missing fields are not treated as evidence of low risk.

1 current
ENISA EUVD identifier

EUVD-2021-26815

CISA KEV mirrored by ENISA

The overlayfs implementation in the linux kernel did not properly validate with respect to user namespaces the setting of file capabilities on files in an underlying file system. Due to the combination of unprivileged user namespaces along with a patch carried in the Ubuntu kernel to allow unprivileged overlay mounts, an attacker could use this to gain elevated privileges.

EUVD state
Present in the current official mapping
Known exploitation
Recorded by ENISA since 20 Oct 2022. Evidence sources: cisa_kev.
ENISA score
8.8 · CVSS 3.1
Advisory evidence
No linked advisory details stored yet
Recommended actionPatch only the product branches with a verified fix

CISA confirms exploitation in the wild and lists 2022-11-10 as the remediation due date. Verified remediation exists for at least one product or source, but 1 structured product or package state remain unresolved. Apply remediation only to the exact product branch confirmed by its source.

Fix availability varies by product
01

What, why and how

The overlayfs stacking file system in Linux kernel does not properly validate the application of file capabilities against user namespaces, which could lead to privilege escalation.

What

The overlayfs stacking file system in Linux kernel does not properly validate the application of file capabilities against user namespaces, which could lead to privilege escalation.

Why

The product does not properly manage privileges while it is switching between different contexts that have different privileges or spheres of control.

How

An attacker operating through local access may attempt exploitation with low privileges. If successful, the issue may gain additional privileges.

What

The overlayfs stacking file system in Linux kernel does not properly validate the application of file capabilities against user namespaces, which could lead to privilege escalation.

Why

The product does not properly manage privileges while it is switching between different contexts that have different privileges or spheres of control.

How

An attacker operating through local access may attempt exploitation with low privileges. If successful, the issue may gain additional privileges.

02

Exploit reality and attack path

CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.

Observed exploitation
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
Confirmed in the wild

CISA added this CVE to its Known Exploited Vulnerabilities catalogue on 2022-10-20.

Public PoC / exploit material
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
Reference recorded

A structured CVE source labels at least one public reference as exploit material. BlackTree has not independently validated that it is safe, reliable or weaponised.

Likely attack path
local access → Privilege Context Switching Error → gain additional privileges
Attack surface
Local
Privileges required
Low: a basic authenticated account is required
User interaction
None
Attack complexity
Low: no specialised conditions are recorded
Security boundary
Changed: exploitation can affect a different security authority
Weakness
?CWE means Common Weakness Enumeration: a standard category for the underlying weakness.
CWE-270 ↗

CWE-270: Privilege Context Switching Error. The product does not properly manage privileges while it is switching between different contexts that have different privileges or spheres of control.

CVSS vector
?CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity.
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

Common Vulnerability Scoring System 3.1: the compact vector below is decoded into plain language.

AVLocalAttack vector: The attacker needs local access to the vulnerable system.ACLowAttack complexity: No specialised conditions are required beyond attacker-controlled input.PRLowPrivileges required: The attacker needs basic user-level privileges.UINoneUser interaction: No action by another user is required.SChangedScope: The attack can affect a component governed by a different security authority.CHighConfidentiality impact: A successful attack can cause a major loss.IHighIntegrity impact: A successful attack can cause a major loss.AHighAvailability impact: A successful attack can cause a major loss.
Post-exploitation / living off the land
The issue can support a local privilege or sandbox boundary transition; normal system utilities may then be available in the gained context.
Privilege escalationCWE-270CISA KEVPublic exploit reference
A

Official authority intelligence

Only matched European and national findings are included. Language selectors and unavailable sources are omitted.

ENISA EUVD · EUVD-2021-26815Known-exploited evidence recorded

The overlayfs implementation in the linux kernel did not properly validate with respect to user namespaces the setting of file capabilities on files in an underlying file system. Due to the combination of unprivileged user namespaces along with a patch carried in the Ubuntu kernel to allow unprivileged overlay mounts, an attacker could use this to gain elevated privileges.

Official EUVD record ↗
Cyber Security Agency of Singapore · English · CSA-SB-20210421Security Bulletin 21 Apr 2021

The Cyber Security Agency of Singapore included this CVE in its official Security Bulletin 21 Apr 2021, published on 21 April 2021. Open the linked bulletin for the product, severity and reference information published in that issue.

Official advisory ↗
CERT-FR · French · CERTFR-2021-AVI-372Multiples vulnérabilités dans le noyau Linux d'Ubuntu

CVERecord?id=CVE-2021-29649 Référence CVE CVE-2021-29650 https://www.cve.org/CVERecord?id=CVE-2021-29650 Référence CVE CVE-2021-29657 https://www.cve.org/CVERecord?id=CVE-2021-29657 Référence CVE CVE-2021-30002 https://www.cve.org/CVERecord?id=CVE-2021-30002 Référence CVE CVE-2021-31916 https://www.cve.org/CVERecord?id=CVE-2021-31916 Référence CVE CVE-2021-3483 https://www.cve.org/CVERecord?id=CVE-2021-3483 Référence CVE CVE-2021-3489 https://www.cve.org/CVERecord?id=CVE-2021-3489 Référence CVE CVE-2021-3490 https://www.cve.org/CVERecord?id=CVE-2021-3490 Référence CVE CVE-2021-3491 https://www.cve.org/CVERecord?id=CVE-2021-3491 Référence CVE CVE-2021-3493 https://www.cve.org/CVERecord?id=CVE-2021-3493 Gestion détaillée du document le 14 mai 2021 Version initiale Alertes Avis Bulletins d’actualités Mentions légales Conditions générales À propos Contact cyber.gouv.fr service-public.fr legifrance.gouv.fr info.gouv.fr france.fr info.gouv.fr/risques Premier Ministre / Secrétariat Général de la Défense et de la Sécurité Nationale / Agence nationale de la sécurité des systèmes d'information

Official advisory ↗
CERT-FR · French · CERTFR-2021-AVI-277Multiples vulnérabilités dans le noyau Linux d'Ubuntu

De multiples vulnérabilités ont été découvertes dans le noyau Linux d'Ubuntu. Elles permettent à un attaquant de provoquer une exécution de code arbitraire, un déni de service et une élévation de privilèges.

Official advisory ↗
JVN iPedia · Japanese · JVNDB-2021-005708Ubuntu Kernel における不正な認証に関する脆弱性

Ubuntu Kernel には、不正な認証に関する脆弱性が存在します。

Official advisory ↗
KISA KrCERT/CC · Korean · KNVD-5754현재 악용되고 있는 Exploit(Update. 2022-10-20)

The overlayfs stacking file system in Linux kernel does not properly validate the application of file capabilities against user namespaces, which could lead to privilege escalation.

Official advisory ↗
03

Patch and workaround

Operational remediation based on structured source evidence.

Status
?Patch availability is based on structured fixed-version fields and authoritative update references. If no fix is verified, check the vendor advisory before making a change.
Fix availability varies by product
Affected
Linux kernel: 5.4 kernel < 5.4.0-72.80, 4.4 kernel < 4.4.0-209.241, 5.8 kernel < 5.8.0-50.56, 4.15 kernel < 4.15.0-142.146
Fixed
Apply https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7c03e2cda4a584cadc398e8f6641ca9988a39d52
Action
Use the product-specific evidence above. Patch only products with a verified fixed release, and keep every affected or under-investigation state without a matching fix in the remediation queue.
Workaround
No verified workaround is recorded. Limit untrusted access and use least privilege until authoritative guidance is available.
Official vendor articles1 stored document

Linked articles are downloaded and versioned as source evidence. A CVE mention or approved update relationship does not, by itself, verify a fix for every product branch.

04

Evidence and provenance

Published 17 Apr 2021 · Last source change 21 Oct 2025, 23:25 UTC · CWE-270 · Privilege Context Switching Error

CVE recordCVE.org · 5.1
CVSS sourceCNA
EPSS source
?The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.
FIRST · tracked since 2026-08-14
European sourceENISA EUVD · EUVD-2021-26815
Product sourceCISA KEV
Remediation sourceCVE/CNA references
CWE sourceCNA
NVD statusNVD enriched

Core structured fields are present and their contributing authorities are shown above.

Material change intelligence

What changed after publication

View recent updates ↗
  1. Affected versionsThe structured affected or fixed version information changed.
    Before
    5.8 kernel < 5.8.0-50.56; 5.4 kernel < 5.4.0-72.80; 4.15 kernel < 4.15.0-142.146; 4.4 kernel < 4.4.0-209.241 · Fixed: An authoritative update reference is available, but the fixed version is not recorded in the structured CVE fields. Check the linked vendor advisory for the applicable release.
    After
    linux kernel: 5.8 kernel < 5.8.0-50.56, 5.4 kernel < 5.4.0-72.80, 4.15 kernel < 4.15.0-142.146, 4.4 kernel < 4.4.0-209.241 · Fixed: Apply https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7c03e2cda4a584cadc398e8f6641ca9988a39d52
    CNA ↗
Material fields only · duplicate refreshes suppressed · history retained for the configured operational retention period
CVE published
Added to CISA KEV
Fixed release or patch reference recorded
ENISA EUVD · EUVD-2021-26815↗CISA KEV record↗CVE authority or vendor reference↗Patch or update reference↗Public exploit reference↗Public exploit reference↗Official CVE record (CNA and ADP data)↗NVD record↗
Technical terms and abbreviations used in this report
CVE
Common Vulnerabilities and Exposures: the public identifier for one disclosed vulnerability.
CVSS
Common Vulnerability Scoring System: a technical severity framework; it is not patching priority by itself.
EPSS
Exploit Prediction Scoring System: FIRST's estimate of the probability that exploitation activity will be observed in the next 30 days; it is a forecast, not confirmation.
CWE
Common Weakness Enumeration: the standard category describing the underlying software or hardware weakness.
CNA
CVE Numbering Authority: an organisation authorised to assign and publish CVE records.
CISA ADP
Cybersecurity and Infrastructure Security Agency Authorized Data Publisher: structured enrichment added to a CVE record.
NVD
National Vulnerability Database: NIST's enrichment service for CVE records.
CERT / CSIRT
A computer security incident response team that publishes warnings or coordinates incident response.
PoC
Proof of concept: public material that demonstrates or helps reproduce exploitation.
CSAF
Common Security Advisory Framework: a machine-readable format for security advisories.
LoTL
Living off the land: abuse of legitimate tools or system functions during an attack.
Free version - for non-commercial use only.CVE-2021-3493 · cve.blacktree.nl