CASCACascade Atlas

Evidence dossier

CVE-2021-3493

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.

Exploited in the wild (CISA KEV since Oct 20, 2022). NVD reports CVSS 3.1 7.8. Severity assessments differ within at least one CVSS version. EPSS estimates 49.2% exploit likelihood as of Aug 27, 2026.

76.2–80.2Priority evidence range
Evidence current through Aug 27, 2026, 6:09 PM UTC

As of Aug 27, 2026

Normalized restatement

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.

State
PUBLISHED
Published
Apr 17, 2021
Updated
Oct 21, 2025
Evidence coverage
83%

Evidence chronology

What was known when

Download this view's receipt →
  1. Source dateSource date omittedFirst observed by CASCA
    canonical

    Record text: 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.

    Inspect raw assertion
    Field
    container
    Value
    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.
    Original evidence ↗
  2. Source dateSource date omittedFirst observed by CASCA
    CISA-ADP

    Record text: CISA ADP Vulnrichment

    Inspect raw assertion
    Field
    container
    Value
    CISA ADP Vulnrichment
    Original evidence ↗
  3. Source dateSource date omittedFirst observed by CASCA
    CVE Program

    Record text: CVE Program Container

    Inspect raw assertion
    Field
    container
    Value
    CVE Program Container
    Original evidence ↗
  4. Source dateFirst observed by CASCA
    CISA KEV

    Exploitation cataloged: Linux Kernel Privilege Escalation Vulnerability

    Inspect raw assertion
    Field
    observed_exploitation
    Value
    Linux Kernel Privilege Escalation Vulnerability
    Original evidence ↗
  5. Source dateFirst observed by CASCA
    FIRST EPSS

    EPSS score: 49.17% probability · 98.79th percentile

    Inspect raw assertion
    Field
    model_probability
    Value
    Probability 0.491660000000; percentile 0.987930000000
    Original evidence ↗

Assessments differ

NVD7.8CVSS 3.1 · source date omitted
NVD7.2CVSS 2.0 · source date omitted
security@ubuntu.com8.8CVSS 3.1 · source date omitted

Values are shown separately by source and CVSS version.

ExploitationCatalog member

CISA KEV · catalog date Oct 20, 2022 · first observed Jul 19, 2026

Exploit likelihood49.17%

FIRST EPSS · score date Aug 27, 2026 · 98.8th percentile · first observed Aug 27, 2026

SeverityAssessments differ

NVD · CVSS 3.1 · first observed Jul 19, 2026 · values shown separately below

Evidence detail

Source limits and decisions

Why each evidence state has this value

casca-unknown-reasons-v1
Exploitation statusEvidence supported

Eligible evidence is present for this bounded claim.

Revision
evidence-policy-v1.1.0
Cutoff
Aug 27, 2026
Resolution
None
Exploit likelihoodEvidence supported

Eligible evidence is present for this bounded claim.

Revision
evidence-policy-v1.1.0
Cutoff
Aug 27, 2026
Resolution
None
Severity assessmentAssessments differ

Eligible assertions materially conflict and remain visible side by side.

Revision
casca-direct-cvss-eligibility-v1
Cutoff
Aug 27, 2026
Resolution
Resolve conflict
Affected productsSource-reported scope

The cited source assertion is retained while canonical product linkage remains open.

Revision
casca-factor-d-obligations-v1
Cutoff
Aug 27, 2026
Resolution
Resolve identity

Source comparison

Who said what

canonicalOriginal assertion
Record text

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.

Inspect raw assertion
Field
container
Value
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.
Source dateSource date omittedFirst observed by CASCAOriginal evidence ↗
CISA-ADPIndependent enrichment
Record text

CISA ADP Vulnrichment

Inspect raw assertion
Field
container
Value
CISA ADP Vulnrichment
Source dateSource date omittedFirst observed by CASCAOriginal evidence ↗
CVE ProgramDerivative copy
Record text

CVE Program Container

Inspect raw assertion
Field
container
Value
CVE Program Container
Source dateSource date omittedFirst observed by CASCAOriginal evidence ↗
CISA KEVOriginal assertion
Exploitation cataloged

Linux Kernel Privilege Escalation Vulnerability

Inspect raw assertion
Field
observed_exploitation
Value
Linux Kernel Privilege Escalation Vulnerability
Source dateFirst observed by CASCAOriginal evidence ↗
FIRST EPSSOriginal assertion
EPSS score

49.17% probability · 98.79th percentile

Inspect raw assertion
Field
model_probability
Value
Probability 0.491660000000; percentile 0.987930000000
Source dateFirst observed by CASCAOriginal evidence ↗

Applicability

Cited product scope

Trace impact →
3Underlying assertions
1Canonical products
3Target assertions
0Constraint assertions

Grouped from 2 configuration nodes in this exact snapshot. Visual grouping is navigational; asset exposure and root cause require cited evidence.

Identity source boundaries
  • Cpe dictionary1,775,266 records · observed through 2026-07-21T06:45:29.809Z27d65b0f-b718-4b4f-bb79-c47c68d09dfa
  • Cpe match643,502 records · observed through 2026-07-21T08:13:17.697Z955dae73-7302-438b-aee1-058d7cc5d48e

2 scope groups

canonical · source assertedUbuntulinux kernelDirect source scope
Affected: 5.8 kernel to before 5.8.0-50.56 (custom comparison)Affected: 5.4 kernel to before 5.4.0-72.80 (custom comparison)Affected: 4.15 kernel to before 4.15.0-142.146 (custom comparison)Affected: 4.4 kernel to before 4.4.0-209.241 (custom comparison)
Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion[{"status": "affected", "version": "5.8 kernel", "lessThan": "5.8.0-50.56", "versionType": "custom"}, {"status": "affected", "version": "5.4 kernel", "lessThan": "5.4.0-72.80", "versionType": "custom"}, {"status": "affected", "version": "4.15 kernel", "lessThan": "4.15.0-142.146", "versionType": "custom"}, {"status": "affected", "version": "4.4 kernel", "lessThan": "4.4.0-209.241", "versionType": "custom"}]
NVD CPE · OPERATING SYSTEMcanonicalubuntu_linuxVulnerable target · 3 assertions
Any version (unconstrained) (< 18.04); Any version (unconstrained) (>= 18.04.1, < 20.04); Any version (unconstrained) (< 20.10)Canonical identity product-a18840e4673d48e569064752e9575849e99b3f173c63d7c965c4c193bcaebef2Linked exact
Scope supported
Inspect raw assertions
  1. cpe:2.3:o:canonical:ubuntu_linux:*:*:*:*:-:*:*:*
    Official link
    Linked exact
    Role
    Vulnerable target
    Configuration
    1 · node/0 · match 0
    Logic
    OR
    Version bounds
    through excluding 20.10
    Match ID
    acab2ee0-ec21-4669-840f-94f6c3028c64
  2. cpe:2.3:o:canonical:ubuntu_linux:*:*:*:*:lts:*:*:*
    Official link
    Linked exact
    Role
    Vulnerable target
    Configuration
    0 · node/0 · match 1
    Logic
    OR
    Version bounds
    from including 18.04.1; through excluding 20.04
    Match ID
    2027773b-26da-408e-b71d-40d05690d9ea
  3. cpe:2.3:o:canonical:ubuntu_linux:*:*:*:*:lts:*:*:*
    Official link
    Linked exact
    Role
    Vulnerable target
    Configuration
    0 · node/0 · match 0
    Logic
    OR
    Version bounds
    through excluding 18.04
    Match ID
    90a80de7-3efa-464c-9d31-c46521ffbfbf

Affected-product evidence

Accepted scope and product mapping

1 canonical links · 1 source-reported links

Mapping establishedEvidence supported

vendor-c57a6167e95991f72f9616ac32b40463ac13c5f4929fcce3efc058b09a445b54 · product-a18840e4673d48e569064752e9575849e99b3f173c63d7c965c4c193bcaebef2

Source class
Nvd cpe vulnerable target
Assertions
3
Mapping revision
cpe23-exact-mapping-v1
Observed cutoff
Aug 27, 2026
Retained assertion IDs
0b328987-96d4-4b67-8022-e686bb2a39304420cb19-61ca-461d-b7ca-cc92804aaa40fa7c36a7-d734-425c-b549-83641dce92c8
Source-reported scopeSource-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Vendor specified only by source · Product specified only by source

Source class
Direct cve affected
Assertions
1
Mapping revision
cpe23-exact-mapping-v1
Observed cutoff
Aug 27, 2026
Retained assertion IDs
3decc7d9-371f-4b1f-8f31-1b08f2e96fa7

Assessments

CVSS by origin

7.8
NVDCVSS 3.1 · role Primary · priority eligiblevalid_matchCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
7.2
NVDCVSS 2.0 · role Primary · priority eligiblevalid_matchAV:L/AC:L/Au:N/C:C/I:C/A:C
8.8
security@ubuntu.comCVSS 3.1 · role Secondary · priority eligiblevalid_matchCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
8.8
canonicalCVSS 3.1 · role unknown · display onlyDirect record assessment retained outside normalized eligibilityCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

Direct CVE/CNA normalized decisions

8.8Priority eligible

canonical

CVSS 3.1 · Primary · Original assertion · rank 1

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
Validation
Valid match
Recomputed
8.8
Decision reason
Evidence supported
Policy
casca-direct-cvss-eligibility-v1

Assessments are retained side by side under closed precedence. Cascade never averages CVSS.

Evidence boundaries

  • KEV membership is authoritative for the catalog, not proof of exposure in any environment.
  • EPSS is a dated model probability and not an individual-environment prediction.
  • Affected or fixed status applies only to the exact cited product and version scope; remaining scope stays source-scoped.
  • NVD-carried upstream facts remain derivative; independent corroboration requires a separately authored source.
  • Only NVD metrics validated under the generation-bound calculator are Public Priority eligible; direct CVE record metrics remain display-only.
  • NVD CVSS source eligibility is closed: NVD-authored, exact record-source, or registered same-CVE container origin; unmapped sources remain display-only.
  • Core replay supports the active and immediately prior generation; retrospective valid-at replay is deferred.
  • OSV aggregation and OSV-converted NVD material are not independent corroboration.
  • Red Hat facts are vendor assertions for the exact supplied products.
  • Nonmembership and not-yet-observed states are not proof of safety.