CriticalCVE-2023-27992

    Zyxel Multiple NAS Devices Command Injection Vulnerability (CVE-2023-27992)

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

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    What is affected, what to do

    The pre-authentication command injection vulnerability in the Zyxel NAS326 firmware versions prior to V5.21(AAZF.14)C0, NAS540 firmware versions prior to V5.21(AATB.11)C0, and NAS542 firmware versions prior to V5.21(ABAG.11)C0 could allow an unauthenticated attacker to execute some operating system (OS) commands remotely by sending a crafted HTTP request.

    The vulnerability is being actively exploited; treat it as a priority. Update Zyxel NAS326 firmware to version V5.21(AAZF.14)C0, Zyxel NAS540 firmware to version V5.21(AATB.11)C0, Zyxel NAS542 firmware to version V5.21(ABAG.11)C0. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Zyxel NAS326 firmware lt *..V5.21(AAZF.14)C0
    • Zyxel NAS540 firmware lt *..V5.21(AATB.11)C0
    • Zyxel NAS542 firmware lt *..V5.21(ABAG.11)C0
    Fixed in
    • Zyxel NAS326 firmware V5.21(AAZF.14)C0
    • Zyxel NAS540 firmware V5.21(AATB.11)C0
    • Zyxel NAS542 firmware V5.21(ABAG.11)C0

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

    Metrics

    9.8

    Priority

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

    Source: NVD, CVSS v3

    9.8

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: SingCERT, pwn.watch, CVE.org

    82.3 %

    EPSS score

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

    Source: FIRST EPSS, percentile 99.66

    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
    231.31 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 exploited
    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
    9.8 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
    9.8
    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
    82.3 %
    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
    99.66
    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 27 January 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-78

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This flaw is network-reachable without authentication or user interaction and yields OS command execution on the device, making it a natural fit for automated mass exploitation of the kind botnet campaigns routinely run against Zyxel NAS hardware. Exploitation likelihood is very high — it ranks among the CVEs most likely to be exploited — and while CISA lists no known ransomware use, NAS units typically hold backups and file shares whose loss or exfiltration is directly business-critical. Typical targets are SMEs, branch offices and home-office environments with the web UI exposed to the internet. Score internet-exposed devices as critical with remediation within hours; internally segmented units without egress are high priority for the next maintenance window.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Apply the firmware update now: upgrade NAS326 to V5.21(AAZF.14)C0, NAS540 to V5.21(AATB.11)C0 and NAS542 to V5.21(ABAG.11)C0 or later. Download only from the official Zyxel portal and verify checksums.
      • Remove internet exposure: delete port forwards/NAT rules for the web admin interface (typically TCP 80/443) on edge routers and firewalls. Disable UPnP on routers, since SOHO NAS devices often expose themselves. Turn off Zyxel cloud/remote-access features (e.g. myZyxelCloud) until patched.
      • Inventory: run an external attack-surface check (Shodan/Censys queries against your own ranges) and internal nmap -sV sweeps for Zyxel HTTP banners to find every NAS, including shadow IT in branch offices and home-office setups.
      • Check for compromise before returning to service: treat any device that was internet-reachable before patching as potentially compromised. Review running processes, cron jobs, startup scripts and unknown files in /tmp, /var and data volumes. If anything is found: isolate, back up data, factory reset and reinstall patched firmware.
      • Rotate credentials: change NAS admin passwords and every credential stored on the device (SMB/FTP accounts, backup targets, cloud-sync tokens, AD bindings) after patching.
      • Check lifecycle status: confirm in the Zyxel advisory whether the affected models still receive security updates; plan replacement for unsupported units.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: move NAS units into a dedicated storage VLAN. Allow the admin interface (HTTP/HTTPS) only from a management network or admin jump hosts; restrict data protocols (SMB TCP 445, NFS) to authorised client segments.
      • Egress filtering: default-deny outbound traffic from NAS devices, allowing only required destinations (NTP, Zyxel update servers, defined backup targets). This blocks payload downloads and botnet C2 even if exploitation succeeds.
      • IPS/WAF rules: enable vendor signatures for CVE-2023-27992 (check Suricata ET ruleset or your IPS vendor feed). Add generic command-injection rules for HTTP traffic to NAS segments: shell metacharacters (;, |, `, $() combined with wget, curl, busybox, chmod, sh in URI or body.
      • Remote access: permit external administration only via VPN with MFA, never through the exposed web UI.
      • Configuration hardening: disable unused services (FTP, Telnet/SSH, DLNA, download service, cloud apps) and run the web UI over HTTPS only where possible.

    3. Runbook step 3

      Detection rules

      • Network telemetry (Zeek/Suricata): HTTP requests from external or unusual internal sources to NAS IPs containing injection patterns. Zeek http.log example: uri or post_body matches wget|curl|busybox|/bin/sh|chmod 777 where id.resp_h is in the NAS segment.
      • Outbound connections from NAS: new internet connections from NAS devices, especially to unknown IPs on high ports, IRC/Telnet ports, or HTTP fetches of ELF binaries. KQL sketch: CommonSecurityLog | where SourceIP in (nas_ips) and DestinationIP !in (allowlist) | summarize count() by DestinationIP, DestinationPort.
      • Scanning from the storage VLAN: NAS initiating bursts of connection attempts to TCP 23/2323/80/8080 on external or internal hosts — a classic botnet propagation indicator.
      • Device logs (enable syslog forwarding): unexpected reboots, new accounts, config changes or disabled logging. Sigma shape: logsource: product: zyxel_nas; detection: keywords: ['wget', 'tftp', 'chmod +x', '/tmp/'] ; condition: keywords.
      • Resource anomalies: sustained high CPU or outbound volume during idle periods (cryptomining, DDoS participation) against SNMP/NetFlow baselines.

    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

    The pre-authentication command injection vulnerability in the Zyxel NAS326 firmware versions prior to V5.21(AAZF.14)C0, NAS540 firmware versions prior to V5.21(AATB.11)C0, and NAS542 firmware versions prior to V5.21(ABAG.11)C0 could allow an unauthenticated attacker to execute some operating system (OS) commands remotely by sending a crafted HTTP request.

    Source: CSA/SingCERT, pwn.watch, 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
    ZyxelNAS326 firmware
    • < V5.21(AAZF.14)C0
    • V5.21(AAZF.14)C0
    ZyxelNAS540 firmware
    • < V5.21(AATB.11)C0
    • V5.21(AATB.11)C0
    ZyxelNAS542 firmware
    • < V5.21(ABAG.11)C0
    • V5.21(ABAG.11)C0

    Vendors and products

    Affected operating systems

    Affected operating systems
    Otherzyxelnas326_firmware—
    Otherzyxelnas540_firmware—
    Otherzyxelnas542_firmware—

    Weakness classes (CWE)

    • CWE-78Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')

      Base

      The product constructs all or part of an OS command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended OS command when it is sent to a downstream component.

      Entry at MITRE CWE-78

    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

    References & sources

    • zyxel.comVendor advisory: Zyxel Multiple Network-Attached Storage (NAS) DevicesPatch, Vendor advisory, pwn-watch
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2023-27992government-resource, US government resource
    • pwn.watchpwn.watch: Zyxel Multiple Network-Attached Storage (NAS) Devices: Command Injection (CVE-2023-27992)pwn-watch
    • NVD (NIST)NVD: CVE-2023-27992pwn-watch
    • cisa.govCISA Known Exploited Vulnerabilities Catalogpwn-watch
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