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NVD / CVE Program· CNA qnap

CVE-2021-28799: Improper Authorization Vulnerability in HBS 3 (Hybrid Backup Sync)

Noroxi assessment

Priority: Now (action score 92/100)

  • CISA has confirmed exploitation in the wild (Mar 31, 2022); it is known to be used in ransomware campaigns. Remediation deadline for US federal agencies: Apr 21, 2022.
  • A ready-made attack module is public (Nuclei); exploiting it takes no special expertise.
  • Probability of exploitation in the next 30 days: 78.2% (FIRST EPSS); higher than 99.5% of all records.
  • Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
  • According to CISA the attack is automatable: targets can be found by mass scanning.
  • According to the record, no fixed version is available yet.

Patch today or shut off access. Until a fix is released, restrict access to the affected component and follow the vendor's advisory.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · CISA KEV · FIRST EPSS

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 .

CriticalCVSS 9.8 · v3.1KEVWeaponized No fix recorded
Published
May 12, 2021
Updated
Jun 16, 2026
EPSS
78.2% · 99th percentile

Report tools

JSON

Action score

92

Now

Patch today or shut off access.

CVSS
39 / 40 · 9.8 / 10
CISA KEV
30 / 30 · Listed
EPSS
23 / 30 · 78.2%

CISA SSVC decision

Exploitation
active
Automatable
yes
Technical impact
total

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
9.8
CNA · qnap
10.0
0.2 point gap

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Actively exploited in the wild

CISA added this to its Known Exploited Vulnerabilities catalog on Mar 31, 2022. That signals real attacks, not theory.

Remediation deadline for US federal agencies: Apr 21, 2022.

Known to be used in ransomware campaigns.

Get told when this record changes

  • In CISA KEV: exploitation in the wild confirmed
  • Weaponized exploit exists
  • No fixed version yet

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2021-28799

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

One daily email digest (06:00 UTC); connect a webhook for KEV listings and weaponized exploits instantly.

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
qnaphybrid backup sync
qnapqts
qnapquts hero
qnapqutscloud

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • qnap hybrid backup syncbefore 16.0.0415
  • qnap hybrid backup syncbefore 3.0.210412
  • qnap hybrid backup syncbefore 3.0.210411
  • qnap hybrid backup syncbefore 16.0.0419

Versions reported by the vendor

Affected version ranges reported by the assigning authority (qnap). Independent of NVD's CPE analysis and usually ahead of it.

  • QNAP Systems Inc. HBS 1.3

    • all versionsnot affected

  • QNAP Systems Inc. HBS 2

    • all versionsnot affected

  • QNAP Systems Inc. HBS 3

    • unspecified and later · before v16.0.0415affected
    • unspecified and later · before v3.0.210412affected
    • unspecified and later · before v3.0.210411affected
    • unspecified and later · before v3.0.210411affected
    • unspecified and later · before v16.0.0419affected
    • unspecified and later · before v16.0.0419affected

Same product

qnap: all records

Other highest-scoring records for the same primary product.

Published before and after this one in the same product

  • Later →CVE-2024-53695Mar 7, 2025 · HBS 3 Hybrid Backup Sync
    25Monitor
  • Later →CVE-2025-62842Jan 2, 2026 · HBS 3 Hybrid Backup Sync
    28Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

No fix version is recorded for this entry. Check the references for vendor advisories.

Exploit status

Weaponized

A ready exploit tool or active in-the-wild use is known for this vulnerability. This should be your top priority.

Evidence

  • Nuclei detection template ×1

Noroxi does not host or link to exploit code. Metasploit module names and Nuclei template ids are shown (names, not code); Exploit-DB entries and PoC repositories are given as counts only.

Public exploit evidence
SourceKindDate
Nuclei detection templateNuclei template CVE-2021-28799 —

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

  1. Published
  2. CISA KEV+322 d
  3. TodayNov 8, 2026+2005 d

No history yet (it accumulates with the daily EPSS job).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

No commit or PR link among the references.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • No account or password is required.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
high · data can be read
Integrity
high · data or configuration can be modified
Availability
high · the service can be disrupted
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality impact
High
Integrity impact
High
Availability impact
High

Weakness class (CWE)

CWE-285 · Improper Authorization

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

nvd-primary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

Change log

No changes recorded on tracked fields yet. Score, KEV, exploit maturity and fix status changes appear here.

References

Show all references (1 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records