CriticalCVE-2021-28799

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

    Published In CISA KEV since Sources: SingCERT, CVE.org

    What is affected, what to do

    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.

    The vulnerability is being actively exploited; treat it as a priority. CISA lists it as used in ransomware campaigns. Update QNAP Systems HBS 3 to version v16.0.0415, QNAP Systems HBS 3 to version v3.0.210412, QNAP Systems HBS 3 to version v3.0.210411. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • QNAP Systems HBS 3 lt *..v16.0.0415
    • QNAP Systems HBS 3 lt *..v3.0.210412
    • QNAP Systems HBS 3 lt *..v3.0.210411
    • QNAP Systems HBS 3 lt *..v16.0.0419
    Fixed in
    • QNAP Systems HBS 3 v16.0.0415
    • QNAP Systems HBS 3 v3.0.210412
    • QNAP Systems HBS 3 v3.0.210411
    • QNAP Systems HBS 3 v16.0.0419

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    10.0

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: Vendor (CNA), CVSS v3

    10.0

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: SingCERT, CVE.org

    78.3 %

    EPSS score

    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

    Source: FIRST EPSS, percentile 99.58

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    254.33 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    actively exploitedSource: CSA/SingCERT How we classify
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    10.0 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    10.0
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    78.3 %
    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    99.58
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    CISA assessment (SSVC)
    Exploitation: active · automatable · Technical impact: total
    Source: CISA Vulnrichment, as of 4 February 2025
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-285

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This flaw is network-reachable without authentication or user interaction and hands the attacker a direct login to the storage system — the shortest path to both primary data and backups. CISA flags it for known ransomware use; publicly documented mass campaigns scanned internet-exposed QNAP devices automatically and locked data into password-protected archives, hitting SMEs, branch offices and organisations using NAS as a backup target hardest. Exploitation likelihood is very high, ranking among the CVEs most likely to be exploited. Treat any internet-reachable NAS running vulnerable HBS 3 as a critical emergency; internal-only devices should be patched within days, because after an initial network compromise they are a prime target for destroying recoverability (check NIS2 reporting obligations if availability is lost).

    1. Runbook step 1

      Immediate response (0-24 h)

      • Update HBS 3 now (App Center → Hybrid Backup Sync): at least v16.0.0415 on QTS 4.5.2, v3.0.210412 on QTS 4.3.6, v3.0.210411 on QTS 4.3.4 / 4.3.3, v16.0.0419 on QuTS hero h4.5.1 and QuTScloud c4.5.1~c4.5.4. Then bring QTS/QuTS firmware itself to current. HBS 2 and HBS 1.3 are not affected but should still be inventoried.
      • Kill internet exposure: remove perimeter port forwards for the NAS admin UI (default 8080/TCP, 443/TCP), RTRR (8899/TCP) and Rsync (873/TCP). Disable UPnP forwarding in QTS ("myQNAPcloud → Auto Router Configuration") and on the router.
      • Check for active encryption: via SSH run ps | grep -i 7z and look for newly created password-protected .7z archives and ransom notes (*READ_ME*.txt) on shares. If encryption is running, do not reboot — disconnect from the network and capture volatile data first (process list with command lines, /proc/<pid>/cmdline); the command line may contain the archive password.
      • Rotate credentials: all local NAS accounts (especially admin and any admin-group accounts) plus credentials for backup destinations (cloud storage, remote NAS, Rsync targets) stored in HBS 3 jobs.
      • Integrity check: update and run QNAP "Malware Remover"; review /etc/config/crontab, autorun.sh and the user list for unknown entries.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: move NAS devices into a dedicated storage VLAN; allow management ports (8080, 443, 22) only from an admin network or via VPN. Restrict NAS egress with an allowlist of required backup targets and update servers.
      • Remote access: provide external access only through a fronting VPN with MFA; disable myQNAPcloud relay and similar cloud-connect services unless strictly required.
      • Configuration hardening: disable the default admin account, enforce 2-step verification for administrators, enable "IP Access Protection" (lockout after failed attempts), and switch off unused services (SSH, Telnet, Rsync server, RTRR server, Web Server).
      • Recoverability: enable snapshots with a retention policy and prevent snapshot deletion by non-admins; keep at least one offline or immutable backup copy outside the NAS administrative domain — HBS 3 targets alone are not sufficient, because compromised credentials reach them too.
      • IPS/perimeter: drop and log all inbound internet traffic to known QNAP ports; geo/reputation filtering is a supplement, not a substitute for closing exposure.

    3. Runbook step 3

      Detection rules

      • QNAP system logs (QuLog Center, forwarded via syslog): successful logins with unknown or non-inventoried usernames, and logins from external IPs to HBS/admin services. SPL sketch: index=nas sourcetype=qnap:syslog "login" status=success NOT src_ip IN (10.0.0.0/8,172.16.0.0/12,192.168.0.0/16).
      • NAS process telemetry: 7z/7za launched with -p (password) and -mx across many share paths; Sigma shape: process.name|endswith: '7z' AND process.command_line|contains: '-p' on NAS syslog/auditd forwarding.
      • File-system/SMB telemetry: bulk creation of .7z files with deletion of originals, plus *READ_ME*.txt dropped in multiple directories; visible via NAS SMB auditing or Zeek smb_files.log (high rename/delete rate from one source).
      • Network telemetry (Zeek/Suricata/firewall): inbound internet connections to 8080/443/8899/873 on NAS addresses; outbound Tor or unknown connections from the NAS. KQL sketch: CommonSecurityLog | where DestinationIP in (nas_ips) and DestinationPort in (8080,443,8899,873) and not(ipv4_is_private(SourceIP)).
      • Configuration changes: new user accounts, modified crontab, deleted snapshots or disabled snapshot schedules should fire as high-severity alerts.

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    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 .

    Source: CSA/SingCERT, NVD (NIST), cvelistv5, CISA KEV Catalog, CVE.org (MITRE)

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    QNAP Systems Inc.HBS 3
    • < v3.0.210411
    • < v16.0.0419
    • < v3.0.210412
    • < v16.0.0415
    • v3.0.210411
    • v16.0.0419
    • v3.0.210412
    • v16.0.0415
    qnaphybrid_backup_sync
    • < 3.0.210412
    • < 16.0.0419
    • < 3.0.210411
    • < 16.0.0415
    • 3.0.210412
    • 16.0.0419
    • 3.0.210411
    • 16.0.0415

    Vendors and products

    Weakness classes (CWE)

    • CWE-285Improper Authorization

      Class

      The product does not perform or incorrectly performs an authorization check when an actor attempts to access a resource or perform an action.

      Entry at MITRE CWE-285

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited
    CSA/SingCERT[Update] Active Exploitation of QNAP Network Attached Storage (NAS) by Ransomware (opens in a new tab)CSA/SingCERTactively exploited

    References & sources

    • qnap.comen/security-advisory/QSA-21-13x_refsource_MISC, Vendor advisory
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-28799government-resource, US government resource
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