CVE-2021-28799Known exploitationNetwork attack vector

Unauthenticated device login

Published May 13, 2021 · Updated Oct 21, 2025

Improper authorization in QNAP HBS 3 allows remote attackers to log in to affected NAS devices without prior authentication. QNAP has not disclosed the affected request handler or the authorization-check error that accepts an unauthorized login. Successful login grants device access; QNAP links exploitation to Qlocker, which archived stored files and removed snapshots.

CVSS severity9.8
Critical
EPSS probability78.25%
Next 30 days · Nov 1, 2026
Known exploitationReported
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper authorization in QNAP HBS 3 allows remote attackers to log in to affected NAS devices without prior authentication. QNAP has not disclosed the affected request handler or the authorization-check error that accepts an unauthorized login. Successful login grants device access; QNAP links exploitation to Qlocker, which archived stored files and removed snapshots.

The record

CVE
CVE-2021-28799
Published
May 13, 2021
Updated
Oct 21, 2025
Vendor
Linn Products Limited
Product
HBS 3 Hybrid Backup Sync
Classifications
CWE-285, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 19, 2021Exploitation reportedQlocker ransomware campaign
  2. May 13, 2021CVE publishedPublication date reported by the CVE source.
  3. Mar 31, 2022Exploitation reportedCISA Known Exploited Vulnerabilities listing
  4. Oct 21, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=unspecified <v16.0.0415
  2. Affected versionversion=unspecified <v16.0.0419
  3. Affected versionversion=unspecified <v3.0.210411
  4. Affected versionversion=unspecified <v3.0.210412

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Linn Products Limited · HBS 3 Hybrid Backup Syncversion=unspecified <v16.0.0415; version=unspecified <v16.0.0419; version=unspecified <v3.0.210411; version=unspecified <v3.0.210412

Published CVSS scores

10.0QNAP Systems, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
7.5NIST NVDAV:N/AC:L/Au:N/C:P/I:P/A:P

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Activity data is delayed. The latest report is Sep 28, 2026.

Daily unique IPsShadowserver observations through Sep 28, 2026. Gaps indicate unavailable data.
Sep 22, 2026Sep 28, 2026
View daily counts
Reporting dayUnique IPs
Sep 22, 20268
Sep 23, 20261
Sep 24, 20265
Sep 25, 202612
Sep 26, 20264
Sep 27, 202616
Sep 28, 20266
Latest reporting daySep 28, 2026
Latest daily unique IPs6
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationReported
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-285Improper Authorization
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Reported exploitation

Technologies

Your stack

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Your stack

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