CVE-2021-28809Network attack vector

Unauthenticated administrator code execution via configuration changes

Published Jul 8, 2021 · Updated Sep 17, 2024

Improper access control in affected QNAP HBS 3 releases allows remote attackers to execute arbitrary code as Administrator through the RTSS server. The RTSS server listens on TCP port 8899 by default and accepts system-configuration alterations without authenticating the requester. Network access to the exposed service is sufficient; successful requests can compromise the NAS operating system.

CVSS severity9.8
Critical
EPSS probability15.80%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper access control in affected QNAP HBS 3 releases allows remote attackers to execute arbitrary code as Administrator through the RTSS server. The RTSS server listens on TCP port 8899 by default and accepts system-configuration alterations without authenticating the requester. Network access to the exposed service is sufficient; successful requests can compromise the NAS operating system.

The record

CVE
CVE-2021-28809
Published
Jul 8, 2021
Updated
Sep 17, 2024
Vendor
Linn Products Limited
Product
HBS 3 Hybrid Backup Sync
Classifications
CWE-306, CWE-284, CWE-749, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 8, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 17, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

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

  1. Affected versionversion=unspecified <v3.0.210506
  2. Affected versionversion=unspecified <v3.0.210507

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Linn Products Limited · HBS 3 Hybrid Backup Syncversion=unspecified <v3.0.210506; version=unspecified <v3.0.210507

Published CVSS scores

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

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-306Missing Authentication for Critical Function
CWE-284Improper Access Control
CWE-749Exposed Dangerous Method or Function
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.

Technologies

Your stack

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Your stack

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