CASCACascade Atlas

Evidence dossier

CVE-2021-28799

Improper Authorization Vulnerability in HBS 3 (Hybrid Backup Sync)

Exploited in the wild (CISA KEV since Mar 31, 2022). NVD reports CVSS 3.1 9.8. EPSS estimates 78.3% exploit likelihood as of Aug 27, 2026.

84.8–91.5Priority evidence range
Evidence current through Aug 27, 2026, 6:09 PM UTC

As of Aug 27, 2026

Normalized restatement

An improper authorization vulnerability has been reported to affect QNAP NAS running HBS 3 (Hybrid Backup Sync. ) If exploited, the vulnerability allows remote attackers to log in to a device. This issue affects: QNAP Systems Inc. HBS 3 versions prior to v16.0.0415 on QTS 4.5.2; versions prior to v3.0.210412 on QTS 4.3.6; versions prior to v3.0.210411 on QTS 4.3.4; versions prior to v3.0.210411 on QTS 4.3.3; versions prior to v16.0.0419 on QuTS hero h4.5.1; versions prior to v16.0.0419 on QuTScloud c4.5.1~c4.5.4. This issue does not affect: QNAP Systems Inc. HBS 2 . QNAP Systems Inc. HBS 1.3 .

State
PUBLISHED
Published
May 13, 2021
Updated
Oct 21, 2025
Evidence coverage
90%

Evidence chronology

What was known when

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  1. 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 ↗
  2. 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 ↗
  3. Source dateSource date omittedFirst observed by CASCA
    qnap

    Record text: Improper Authorization Vulnerability in HBS 3 (Hybrid Backup Sync)

    Inspect raw assertion
    Field
    container
    Value
    Improper Authorization Vulnerability in HBS 3 (Hybrid Backup Sync)
    Original evidence ↗
  4. Source dateFirst observed by CASCA
    CISA KEV

    Exploitation cataloged: QNAP NAS Improper Authorization Vulnerability

    Inspect raw assertion
    Field
    observed_exploitation
    Value
    QNAP NAS Improper Authorization Vulnerability
    Original evidence ↗
  5. Source dateFirst observed by CASCA
    FIRST EPSS

    EPSS score: 78.25% probability · 99.54th percentile

    Inspect raw assertion
    Field
    model_probability
    Value
    Probability 0.782500000000; percentile 0.995430000000
    Original evidence ↗
ExploitationCatalog member

CISA KEV · catalog date Mar 31, 2022 · first observed Jul 19, 2026

Exploit likelihood78.25%

FIRST EPSS · score date Aug 27, 2026 · 99.5th percentile · first observed Aug 27, 2026

SeverityCVSS 9.8

NVD · CVSS 3.1 · first observed Jul 19, 2026 · same-version scores align in this snapshot

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 assessmentEvidence supported

Eligible evidence is present for this bounded claim.

Revision
casca-direct-cvss-eligibility-v1
Cutoff
Aug 27, 2026
Resolution
None
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

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 ↗
qnapOriginal assertion
Record text

Improper Authorization Vulnerability in HBS 3 (Hybrid Backup Sync)

Inspect raw assertion
Field
container
Value
Improper Authorization Vulnerability in HBS 3 (Hybrid Backup Sync)
Source dateSource date omittedFirst observed by CASCAOriginal evidence ↗
CISA KEVOriginal assertion
Exploitation cataloged

QNAP NAS Improper Authorization Vulnerability

Inspect raw assertion
Field
observed_exploitation
Value
QNAP NAS Improper Authorization Vulnerability
Source dateFirst observed by CASCAOriginal evidence ↗
FIRST EPSSOriginal assertion
EPSS score

78.25% probability · 99.54th percentile

Inspect raw assertion
Field
model_probability
Value
Probability 0.782500000000; percentile 0.995430000000
Source dateFirst observed by CASCAOriginal evidence ↗

Applicability

Cited product scope

Trace impact →
11Underlying assertions
4Canonical products
5Target assertions
6Constraint assertions

Grouped from 10 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

12 scope groups

qnap · source assertedQNAP Systems Inc.HBS 1.3Direct source scope
Unaffected: all versions
Source-reported scope

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

Inspect raw assertion[{"status": "unaffected", "version": "all versions"}]
qnap · source assertedQNAP Systems Inc.HBS 2Direct source scope
Unaffected: all versions
Source-reported scope

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

Inspect raw assertion[{"status": "unaffected", "version": "all versions"}]
qnap · source assertedQNAP Systems Inc.HBS 3Direct source scope
Affected: unspecified to before v3.0.210411 (custom comparison)
Source-reported scope

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

Inspect raw assertion[{"status": "affected", "version": "unspecified", "lessThan": "v3.0.210411", "versionType": "custom"}]
qnap · source assertedQNAP Systems Inc.HBS 3Direct source scope
Affected: unspecified to before v3.0.210411 (custom comparison)
Source-reported scope

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

Inspect raw assertion[{"status": "affected", "version": "unspecified", "lessThan": "v3.0.210411", "versionType": "custom"}]
qnap · source assertedQNAP Systems Inc.HBS 3Direct source scope
Affected: unspecified to before v16.0.0415 (custom comparison)
Source-reported scope

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

Inspect raw assertion[{"status": "affected", "version": "unspecified", "lessThan": "v16.0.0415", "versionType": "custom"}]
qnap · source assertedQNAP Systems Inc.HBS 3Direct source scope
Affected: unspecified to before v16.0.0419 (custom comparison)
Source-reported scope

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

Inspect raw assertion[{"status": "affected", "version": "unspecified", "lessThan": "v16.0.0419", "versionType": "custom"}]
qnap · source assertedQNAP Systems Inc.HBS 3Direct source scope
Affected: unspecified to before v16.0.0419 (custom comparison)
Source-reported scope

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

Inspect raw assertion[{"status": "affected", "version": "unspecified", "lessThan": "v16.0.0419", "versionType": "custom"}]
qnap · source assertedQNAP Systems Inc.HBS 3Direct source scope
Affected: unspecified to before v3.0.210412 (custom comparison)
Source-reported scope

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

Inspect raw assertion[{"status": "affected", "version": "unspecified", "lessThan": "v3.0.210412", "versionType": "custom"}]
NVD CPE · APPLICATIONqnaphybrid_backup_syncVulnerable target · 5 assertions
Any version (unconstrained) (< 16.0.0415); Any version (unconstrained) (< 16.0.0419); Any version (unconstrained) (< 3.0.210411); Any version (unconstrained) (< 3.0.210412)Canonical identity product-aa0cb885c8e4a48ef6d9ad1bbf0eac10c521be0c4bb3395bc55ce2713d11cf25Linked exact
Scope supported
Inspect raw assertions
  1. cpe:2.3:a:qnap:hybrid_backup_sync:*:*:*:*:*:*:*:*
    Official link
    Linked exact
    Role
    Vulnerable target
    Configuration
    0 · node/0 · match 0
    Logic
    OR
    Version bounds
    through excluding 16.0.0415
    Match ID
    a18656f9-afb4-44bc-af08-e1a0a5d2ef49
  2. cpe:2.3:a:qnap:hybrid_backup_sync:*:*:*:*:*:*:*:*
    Official link
    Linked exact
    Role
    Vulnerable target
    Configuration
    2 · node/0 · match 0
    Logic
    OR
    Version bounds
    through excluding 3.0.210411
    Match ID
    78491b95-5835-411b-b188-30e496bcb10a
  3. cpe:2.3:a:qnap:hybrid_backup_sync:*:*:*:*:*:*:*:*
    Official link
    Linked exact
    Role
    Vulnerable target
    Configuration
    3 · node/0 · match 0
    Logic
    OR
    Version bounds
    through excluding 16.0.0419
    Match ID
    bb2a5649-4bb9-46e9-8a1a-b6046125bd92
  4. cpe:2.3:a:qnap:hybrid_backup_sync:*:*:*:*:*:*:*:*
    Official link
    Linked exact
    Role
    Vulnerable target
    Configuration
    4 · node/0 · match 0
    Logic
    OR
    Version bounds
    through excluding 16.0.0419
    Match ID
    bb2a5649-4bb9-46e9-8a1a-b6046125bd92
  5. cpe:2.3:a:qnap:hybrid_backup_sync:*:*:*:*:*:*:*:*
    Official link
    Linked exact
    Role
    Vulnerable target
    Configuration
    1 · node/0 · match 0
    Logic
    OR
    Version bounds
    through excluding 3.0.210412
    Match ID
    d5920ea4-eea5-4f64-b06b-eed06e824864
NVD CPE · OPERATING SYSTEMqnapqtsEnvironmental constraint · 4 assertions
Version 4.3.3; Version 4.3.4; Version 4.3.6; Version 4.5.2Canonical identity product-81578b5d517b13c57b34c6184dee501c7b2e69f5d366148b8e0d95265e548fc9Linked exact
Scope constrained
Inspect raw assertions
  1. cpe:2.3:o:qnap:qts:4.3.4:*:*:*:*:*:*:*
    Official link
    Linked exact
    Role
    Environmental constraint
    Configuration
    2 · node/1 · match 1
    Logic
    OR
    Version bounds
    No explicit bounds
    Match ID
    f0c7d2d4-769f-4297-89f4-75366ffa7618
  2. cpe:2.3:o:qnap:qts:4.3.6:*:*:*:*:*:*:*
    Official link
    Linked exact
    Role
    Environmental constraint
    Configuration
    1 · node/1 · match 0
    Logic
    OR
    Version bounds
    No explicit bounds
    Match ID
    fe9fac96-aa2a-4ca5-a170-8c0e6bd47391
  3. cpe:2.3:o:qnap:qts:4.5.2:*:*:*:*:*:*:*
    Official link
    Linked exact
    Role
    Environmental constraint
    Configuration
    0 · node/1 · match 0
    Logic
    OR
    Version bounds
    No explicit bounds
    Match ID
    1ac8bd83-ea71-4af8-aaec-e46efca99ea2
  4. cpe:2.3:o:qnap:qts:4.3.3:*:*:*:*:*:*:*
    Official link
    Linked exact
    Role
    Environmental constraint
    Configuration
    2 · node/1 · match 0
    Logic
    OR
    Version bounds
    No explicit bounds
    Match ID
    c5994c07-17fe-4784-9fa4-9675ba8b4743
NVD CPE · OPERATING SYSTEMqnapqutscloudEnvironmental constraint · 1 assertions
Any version (unconstrained) (>= c4.5.1, <= c4.5.4)Canonical identity product-c10aad70cb83f936b6928d1401d2eb8acec8be0afd0b3faf9080032603a271eeLinked exact
Scope constrained
Inspect raw assertion
  1. cpe:2.3:o:qnap:qutscloud:*:*:*:*:*:*:*:*
    Official link
    Linked exact
    Role
    Environmental constraint
    Configuration
    4 · node/1 · match 0
    Logic
    OR
    Version bounds
    from including c4.5.1; through including c4.5.4
    Match ID
    fedf704d-d5ad-45c2-ad7d-e53189c72e1a
NVD CPE · OPERATING SYSTEMqnapquts_heroEnvironmental constraint · 1 assertions
Version h4.5.1Canonical identity product-0a6983be8c95a10c38b798686f8b7e66f22caa6fe0e778c8bc34c7c367ce8dafLinked exact
Scope constrained
Inspect raw assertion
  1. cpe:2.3:o:qnap:quts_hero:h4.5.1:*:*:*:*:*:*:*
    Official link
    Linked exact
    Role
    Environmental constraint
    Configuration
    3 · node/1 · match 0
    Logic
    OR
    Version bounds
    No explicit bounds
    Match ID
    548228b3-eb2c-42c1-895b-da6dc5ca26ee

Affected-product evidence

Accepted scope and product mapping

1 canonical links · 1 source-reported links

Mapping establishedEvidence supported

vendor-37a503905b0a0167877af753ac60ba6a9fd40363b081ec9e83c43aeaec72163a · product-aa0cb885c8e4a48ef6d9ad1bbf0eac10c521be0c4bb3395bc55ce2713d11cf25

Source class
Nvd cpe vulnerable target
Assertions
5
Mapping revision
cpe23-exact-mapping-v1
Observed cutoff
Aug 27, 2026
Retained assertion IDs
1600a652-eb41-4831-9be7-d3549e0374bf2f3bb281-2146-45ef-aa76-0759689529015727c75a-e8d0-44dc-9584-a436fbdc690c8317134b-1b77-4b83-844a-64d794cc06f283cbddf5-a642-4f0b-9050-cb35c884aa05
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
8
Mapping revision
cpe23-exact-mapping-v1
Observed cutoff
Aug 27, 2026
Retained assertion IDs
126ad47b-5eb1-4494-857c-cb980e18b5a82b10ee80-5a5d-4733-b881-eda04b16bfae34248e1a-a7f5-4aed-8e78-78ab0a3b012274ca4f22-7bc2-42b0-97d4-f9cb563d214c9d21b1b9-9511-40c9-8625-181428bd0c079fd7f325-ef57-4748-ad17-aa6b9bdf4126a6c2d521-d67d-4849-ad89-7aabe0b93988d95248c6-67a4-433e-bdd4-4935c2b17348

Assessments

CVSS by origin

9.8
NVDCVSS 3.1 · role Primary · priority eligiblevalid_matchCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
7.5
NVDCVSS 2.0 · role Primary · priority eligiblevalid_matchAV:N/AC:L/Au:N/C:P/I:P/A:P
10.0
security@qnapsecurity.com.twCVSS 3.1 · role Secondary · priority eligiblevalid_matchCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
10.0
qnapCVSS 3.1 · role unknown · display onlyDirect record assessment retained outside normalized eligibilityCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

Direct CVE/CNA normalized decisions

10.0Priority eligible

qnap

CVSS 3.1 · Primary · Original assertion · rank 1

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
Validation
Valid match
Recomputed
10.0
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.