CriticalCVE-2023-33009

    Zyxel Multiple Firewalls Buffer Overflow Vulnerability (CVE-2023-33009)

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

    Share

    What is affected, what to do

    A buffer overflow vulnerability in the notification function of Zyxel ATP, USG FLEX, USG FLEX 50(W), USG20(W)-VPN, VPN, and ZyWALL/USG firewalls allows an unauthenticated attacker to cause denial-of-service (DoS) conditions and remote code execution on affected devices. This affects firmware versions 4.60 through 5.36 Patch 1 for the ATP and USG series, and versions 4.60 through 4.73 Patch 1 for the ZyWALL/USG series. The vulnerability is actively exploited in ransomware campaigns since June 2023 and is listed in the CISA KEV catalog. Immediate patching to the latest firmware version is critical to mitigate this risk.

    The vulnerability is being actively exploited; treat it as a priority. The available sources name no fixed version. Check the vendor guidance in the references below. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Zyxel ATP series firmware eq 4.60 through 5.36 Patch 1..4.60 through 5.36 Patch 1
    • Zyxel USG FLEX series firmware eq 4.60 through 5.36 Patch 1..4.60 through 5.36 Patch 1
    • Zyxel USG FLEX 50(W) firmware eq 4.60 through 5.36 Patch 1..4.60 through 5.36 Patch 1
    • Zyxel USG20(W)-VPN firmware eq 4.60 through 5.36 Patch 1..4.60 through 5.36 Patch 1
    • Zyxel VPN series firmware eq 4.60 through 5.36 Patch 1..4.60 through 5.36 Patch 1
    • Zyxel ZyWALL/USG series firmware eq 4.60 through 4.73 Patch 1..4.60 through 4.73 Patch 1

    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

    28.1 %

    EPSS score

    Elevated — model estimates 10-50% exploitation likelihood.

    Source: FIRST EPSS, percentile 98.1

    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
    228.71 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
    28.1 %
    Elevated — model estimates 10-50% exploitation likelihood.
    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
    98.1
    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-120

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This is a memory-corruption flaw in a perimeter appliance. It is reachable over the network without authentication or user interaction and can lead to full code execution on the firewall, which puts it in the most severe class with a very high likelihood of exploitation. Edge devices like these are typically mass-scanned and exploited automatically, either to enlist them in DDoS botnets or to use them as a foothold into the internal network. SMBs, branch offices and retail networks are the typical targets, since they often run Zyxel units without centralised patch management. CISA does not flag known ransomware use, but that does not lower the urgency for exposed devices. Treat any unit with a reachable WAN interface as critical and patch or isolate it within 24 hours; internal-only units can follow in the next regular maintenance window.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Inventory: Find every Zyxel ATP, USG FLEX, USG FLEX 50(W), USG20(W)-VPN, VPN and ZyWALL/USG appliance and record its firmware version (GUI dashboard or CLI show version). The vulnerable ranges are 4.60 through 5.36 Patch 1 (ATP, USG FLEX, USG FLEX 50(W), USG20(W)-VPN, VPN) and 4.60 through 4.73 Patch 1 (ZyWALL/USG).
      • Upgrade firmware: Move to a release later than 5.36 Patch 1 or 4.73 Patch 1. A patch is available from the vendor — look up the exact fixed version in the Zyxel security advisory. Download firmware only from the official Zyxel portal and verify the checksum.
      • Internet-facing units first: Prioritise devices whose WAN interface is reachable from the internet (remote admin, SSL VPN, IPsec). If you cannot get an immediate maintenance window, remove WAN access to the management services now under "System > WWW/SSH/FTP".
      • Compromise check before patching: Preserve the configuration and logs. Then look for unknown admin accounts, altered firewall or NAT rules, new VPN users and unexplained reboots. If you find anomalies, treat the device as compromised: reflash it, restore a known-good config, and rotate all local admin, VPN, RADIUS/LDAP bind credentials and pre-shared keys.
      • Reporting obligations: If you see signs of exploitation on a perimeter device, start the NIS2 reporting workflow (24-hour early warning to BSI or your competent authority).

    2. Runbook step 2

      Mitigation layers

      • Network segmentation / attack surface: Restrict the management services (HTTPS GUI, SSH) to a dedicated management VLAN or jump host. On the WAN side, expose only the services you actually need, such as IPsec with source-IP allowlisting where your partner setup allows it. Disable unused features like SSL VPN, UPnP and cloud management.
      • Upstream filtering: Where an upstream router or ISP filter exists, apply an ACL so only known admin ranges can reach the appliance's management ports. Geo-blocking does not replace patching, but it cuts down the noise from mass scanning.
      • IPS signatures: Keep the ET rulesets current on upstream Suricata/Snort sensors and filter for CVE-2023-33009 or "Zyxel" signatures. Also update Zyxel's own IPS signatures on ATP and USG FLEX units if you are licensed for them.
      • Egress control for the appliance itself: Limit connections that originate from the firewall itself to NTP, DNS, the Zyxel update servers and your syslog/SIEM. This makes it harder to fetch a second-stage payload or enlist the device in a botnet after code execution.
      • IAM / least privilege: Keep local admin accounts to a minimum, disable default accounts, and enable 2FA for administration where the firmware supports it. Run LDAP/RADIUS bind accounts with minimal rights.
      • Resilience: The flaw also enables denial of service, so have HA pairs and out-of-band console access ready to recover a crashed unit.

    3. Runbook step 3

      Detection rules

      • Reboots and crashes (syslog): Watch the Zyxel syslog for unexpected reboots, watchdog events or process crashes, especially repeated ones or any outside maintenance windows. SPL: index=network sourcetype=zyxel* ("reboot" OR "system start" OR "crash" OR "watchdog") | stats count by host
      • Device-originated outbound traffic (Zeek/NetFlow): Flag sessions whose source IP is the firewall itself (WAN or management IP) going to non-allowlisted destinations, such as HTTP downloads, IRC or odd high ports. KQL: CommonSecurityLog | where SourceIP in (fw_ips) and DestinationIP !in (allowlist). In Zeek, check conn.log where id.orig_h is the firewall IP.
      • Configuration changes: Alert on syslog events for user creation, admin privilege changes, new NAT or firewall rules, or remote management being enabled without a matching change ticket. Sigma shape: logsource: product: zyxel with keywords: ['user add', 'admin', 'configuration changed'], correlated against the change calendar.
      • Appliance DNS lookups: Watch for DNS queries from the firewall itself to dynamic-DNS domains, paste sites or newly registered domains, using resolver logs or Zeek dns.log.
      • External scanning and exploit attempts: Review Zyxel-related IDS alerts on WAN segments. A high volume from many distinct source IPs against the appliance services points to automated mass exploitation.

    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

    A buffer overflow vulnerability in the notification function in Zyxel ATP series firmware versions 4.60 through 5.36 Patch 1, USG FLEX series firmware versions 4.60 through 5.36 Patch 1, USG FLEX 50(W) firmware versions 4.60 through 5.36 Patch 1, USG20(W)-VPN firmware versions 4.60 through 5.36 Patch 1, VPN series firmware versions 4.60 through 5.36 Patch 1, ZyWALL/USG series firmware versions 4.60 through 4.73 Patch 1, could allow an unauthenticated attacker to cause denial-of-service (DoS) conditions and even a remote code execution on an affected device.

    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
    ZyxelATP series firmware
    • 4.60 through 5.36 Patch 1
    • —
    ZyxelUSG FLEX 50(W) firmware
    • 4.60 through 5.36 Patch 1
    • —
    ZyxelUSG FLEX series firmware
    • 4.60 through 5.36 Patch 1
    • —
    ZyxelUSG20(W)-VPN firmware
    • 4.60 through 5.36 Patch 1
    • —
    ZyxelVPN series firmware
    • 4.60 through 5.36 Patch 1
    • —
    ZyxelZyWALL/USG series firmware
    • 4.60 through 4.73 Patch 1
    • —

    Vendors and products

    Affected operating systems

    Affected operating systems
    Otherzyxelatp100_firmware5.36
    Otherzyxelatp100_firmware—
    Otherzyxelatp100w_firmware5.36
    Otherzyxelatp100w_firmware—
    Otherzyxelatp200_firmware5.36
    Otherzyxelatp200_firmware—
    Otherzyxelatp500_firmware5.36
    Otherzyxelatp500_firmware—
    Otherzyxelatp700_firmware5.36
    Otherzyxelatp700_firmware—
    Otherzyxelatp800_firmware5.36
    Otherzyxelatp800_firmware—
    Otherzyxelusg20-vpn_firmware5.36
    Otherzyxelusg20-vpn_firmware—
    Otherzyxelusg_20w-vpn_firmware5.36
    Otherzyxelusg_20w-vpn_firmware—
    Otherzyxelusg_40_firmware4.73
    Otherzyxelusg_40_firmware—
    Otherzyxelusg_40w_firmware4.73
    Otherzyxelusg_40w_firmware—
    Otherzyxelusg_60_firmware4.73
    Otherzyxelusg_60_firmware—
    Otherzyxelusg_60w_firmware4.73
    Otherzyxelusg_60w_firmware—

    Weakness classes (CWE)

    • CWE-120Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')

      Base

      The product copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer.

      Entry at MITRE CWE-120

    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 FirewallsVendor advisory, pwn-watch
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2023-33009government-resource, US government resource
    • pwn.watchpwn.watch: Zyxel Multiple Firewalls: 2 exploited vulnerabilities (CVE-2023-33009, CVE-2023-33010)pwn-watch
    • NVD (NIST)NVD: CVE-2023-33009pwn-watch
    • NVD (NIST)NVD: CVE-2023-33010pwn-watch
    • cisa.govCISA Known Exploited Vulnerabilities Catalogpwn-watch
    Embed badge

    Show the severity and KEV status of this vulnerability on your site – with a link back here. The image is updated hourly.

    Badge preview for CVE-2023-33009
    <a href="https://intel.neosec.eu/en/cve/CVE-2023-33009"><img src="https://intel.neosec.eu/badge/cve/CVE-2023-33009.svg?lang=en" alt="CVE-2023-33009 – NEOSEC Intel" height="20"></a>