CVE-2021-28799
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Improper Authorization Vulnerability in HBS 3 (Hybrid Backup Sync)
rank 78 = 40 + 25 + 2 + 5 + 6
rank@v1 · act now
Someone is already using it
CISA KEV catalog · public proof-of-concept · FIRST EPSS
EPSS 0.78250 · CISA KEV listed · without this subject the record ranks 38
What it costs you if it fires
NVD CVSS base score, then the CNA's own
CVSS 9.8, from NVD · without this subject the record ranks 53
It is getting worse, not staying still
FIRST EPSS series, rescorings and bad loads removed
rose on 2026-01-11 · without this subject the record ranks 76
How much of the world it touches
NVD CPE configurations
4 products in the CPE ranges · without this subject the record ranks 73
Someone else already set the date
CISA KEV remediation due date
CISA remediation date held · without this subject the record ranks 72
$3.90M = $4.99M incident cost x 1 exposure x 0.7825 likelihood
udv@v1 · unaudited · wrong for you until you say what you run
$4.99M is the cost of a breach that happened, from IBM Cost of a Data Breach Report 2026, global average. It is not the value of any asset of yours. That average already covers breaches of every severity, so scaling it by exposure understates a partial-impact record. That is an assumption, not a measurement. Your own asset value goes in the decision record.
Hindsight · the state
Foresight · where to check next
No weakness class or product neighbourhood is held for this record, so this index cannot name anywhere to check next. That is an absence in the record, not a finding about the software.
Sources disagree here.
Both shown, dated. We don't pick winners. disagreement-rules@v1
Running QNAP Systems Inc. HBS 3? Check your version
A deterministic range check against the ranges this record publishes. It tells you where a version sits in the record, not whether your deployment is exploitable. No account needed.
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Harm, if it fires from the vector and the weakness
Written for the ticket you're about to file.
CISA KEV & Federal Remediation Deadline from the record and CISA
The fix, and how long it's been waiting.
Exploit Status & Known PoCs a ladder, not a score
Current rung: Ransomware campaign use known, 2022-03-31.
Affected & Fixed Versions as published
A rung lights from a row in the index, never from an assessment. "Held sources" is the true scope of the claim: this index reads the CVE record, NVD, CISA KEV, FIRST's EPSS and public proof-of-concept repositories, and it does not observe attacks.
Attack path
What it is stated by the CNA
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 . receipt
NVD's analysis a second authority
- severity
- 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- weakness
- CWE-285
- affected versions
- qnap hybrid backup sync < 16.0.0415; qnap hybrid backup sync < 3.0.210412; qnap hybrid backup sync < 3.0.210411; and 1 more, listed in full below
- also scored by
- [email protected] 10 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
The CNA scored it 10 and NVD 9.8. Both are shown because both were published; we do not pick one.
Dated events
Movement the material steps
This score more than doubled inside 72 hours. Both observations are in the full history below.
Show all 43 observations
43 observations held. A highlighted step crossed a promotion threshold. A step marked with a scoring model moved because the instrument was replaced, not because the vulnerability changed. receipt
The full biography
Listed by CISA 322 days after it published, exploit probability more than doubled between 2025-11-18 and 2025-11-21 (0.28728 → 0.86248) and the record had been public for 5 years, 3 months when this index was last written.
No material movement in 50 days as of the last ingest this index recorded.
Siblings and hubs
Records related to this one
None. No alias set, and nothing in the corpus cites this identifier or shares its product family within 60 days.
Browse the index from here
These are index-wide views, not findings about CVE-2021-28799.
What usually comes next
Everything above needs no account. What is below is work: assembling it, scoping it to your estate, watching it, and handing it to someone else.