Zyxel Multiple Firewalls Buffer Overflow Vulnerability (CVE-2023-33010)
Published In CISA KEV since Sources: SingCERT, pwn.watch, CVE.org
What is affected, what to do
A buffer overflow vulnerability in the ID processing function in Zyxel ATP series firmware versions 4.32 through 5.36 Patch 1, USG FLEX series firmware versions 4.50 through 5.36 Patch 1, USG FLEX 50(W) firmware versions 4.25 through 5.36 Patch 1, USG20(W)-VPN firmware versions 4.25 through 5.36 Patch 1, VPN series firmware versions 4.30 through 5.36 Patch 1, ZyWALL/USG series firmware versions 4.25 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.
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 4.32 through 5.36 Patch 1..4.32 through 5.36 Patch 1 - Zyxel USG FLEX series firmware
eq 4.50 through 5.36 Patch 1..4.50 through 5.36 Patch 1 - Zyxel USG FLEX 50(W) firmware
eq 4.25 through 5.36 Patch 1..4.25 through 5.36 Patch 1 - Zyxel USG20(W)-VPN firmware
eq 4.25 through 5.36 Patch 1..4.25 through 5.36 Patch 1 - Zyxel VPN series firmware
eq 4.30 through 5.36 Patch 1..4.30 through 5.36 Patch 1 - Zyxel ZyWALL/USG series firmware
eq 4.25 through 4.73 Patch 1..4.25 through 4.73 Patch 1
- Zyxel ATP series firmware
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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
29.0 %
EPSS score
Elevated — model estimates 10-50% exploitation likelihood.
Source: FIRST EPSS, percentile 98.2
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- 228.79 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- 29.0 %
- 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- 98.2
- 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 16 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-120
Response and mitigation
Why act now?
Prioritisation rationale
This flaw is network-reachable without authentication and sits on the perimeter device itself. Successful exploitation gives an attacker control of the network edge, the VPN access paths and the credentials stored on the box. This generation of Zyxel firewalls was mass-scanned and exploited by automated tooling shortly after disclosure, notably to enlist devices into DDoS botnets. Typical victims are SMBs, branch offices and distributed sites with internet-facing VPN. Exploitation likelihood is very high, and although CISA lists no known ransomware use, that does not lower urgency for exposed units. Treat any vulnerable device with a reachable IKE service as critical, and patch or isolate it within 24 hours.
Runbook step 1
Immediate response (0-24 h)
- Inventory: Identify every Zyxel ATP, USG FLEX, USG FLEX 50(W), USG20(W)-VPN, VPN and ZyWALL/USG unit and confirm its firmware via the Dashboard or CLI
show version. Vulnerable versions:- ATP 4.32–5.36 Patch 1
- USG FLEX 4.50–5.36 Patch 1
- USG FLEX 50(W) and USG20(W)-VPN 4.25–5.36 Patch 1
- VPN 4.30–5.36 Patch 1
- ZyWALL/USG 4.25–4.73 Patch 1
- Upgrade the firmware: Move to a release later than 5.36 Patch 1, or later than 4.73 Patch 1 for ZyWALL/USG. A patch is available from the vendor — look up the exact fixed version in the Zyxel security advisory. Download only from the official Zyxel portal and verify checksums.
- Shrink exposure now: Until the upgrade is in place, restrict WAN-side IPsec/IKE (UDP 500 and 4500) to known peer IPs. Disable unused VPN gateways and policies. Block WAN access to HTTPS/SSH management entirely.
- Hunt for compromise: Check for unexpected reboots, crashes, unknown admin accounts, and new firewall, NAT or forwarding rules. Do this both before and after patching.
- If compromise is suspected: Treat the device as untrusted. Reflash the firmware and restore configuration from a known-good backup. Then rotate all secrets, including admin passwords, IPsec PSKs, certificates, LDAP/RADIUS bind accounts and VPN user passwords.
- Reporting obligations: NIS2/KRITIS entities should assess the early-warning obligation to the national CSIRT/BSI (24 h) if indicators of compromise are found.
- Inventory: Identify every Zyxel ATP, USG FLEX, USG FLEX 50(W), USG20(W)-VPN, VPN and ZyWALL/USG unit and confirm its firmware via the Dashboard or CLI
Runbook step 2
Mitigation layers
- Network segmentation: Make management interfaces reachable only from a dedicated admin network (OOB VLAN or jump host). Never expose management to the internet.
- Upstream filtering: Where an upstream router or ISP ACL exists, permit UDP 500/4500 only from defined IPsec peers. Consider geo- or IP-allowlisting for remote-access users until the patch is applied.
- IPS rule: On an upstream Suricata/Snort sensor, alert on or drop IKE packets (UDP 500/4500) carrying abnormally large Identification payloads. One approach:
alert udp $EXTERNAL_NET any -> $HOME_NET [500,4500] (msg:"Zyxel IKE oversized ID payload"; dsize:>1000; …). Tune the threshold to your baseline. - Egress control for the device itself: Allow the firewall to reach only required destinations, such as Zyxel update servers, NTP, syslog and RADIUS/LDAP. Block arbitrary outbound connections from the appliance to hinder botnet enrolment and reverse shells.
- Lifecycle: Verify vendor support status for ZyWALL/USG units on the 4.x branch. Replace devices that no longer receive security updates.
- Logging resilience: Forward device syslog to the SIEM so evidence survives a crash or reboot.
Runbook step 3
Detection rules
- Device syslog: Alert on unexpected reboots, crash or core-dump messages and IKE daemon restarts. Example SPL:
index=netfw vendor=zyxel ("reboot" OR "core dump" OR "ike" AND "restart") | stats count by host. Clusters of crashes point to DoS or failed exploitation attempts. - Network telemetry (Zeek/Suricata): Watch for oversized or malformed IKE packets to UDP 500/4500 from non-peer sources. Also watch for bursts of IKE traffic from many sources in a short window, which indicates mass scanning.
- Outbound connections from the firewall: Review NetFlow or Zeek
conn.logwhere the source IP is the firewall's WAN or management address. Flag connections to unknown destinations, especially non-standard ports, IRC/Telnet, or HTTP GETs fetching shell scripts or ELF binaries. - Config and account changes: Alert on new admin users, changes to firewall, NAT or VPN policies, and WAN management being enabled outside change windows. Sigma shape:
logsource: product: zyxel; selection: msg|contains: ['user added','configuration changed']; condition: selection and not change_window. - External exposure checks: Regularly verify from the outside, via your own scans or an attack-surface-management service, whether management ports or unexpected IKE endpoints are reachable.
- Device syslog: Alert on unexpected reboots, crash or core-dump messages and IKE daemon restarts. Example SPL:
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 ID processing function in Zyxel ATP series firmware versions 4.32 through 5.36 Patch 1, USG FLEX series firmware versions 4.50 through 5.36 Patch 1, USG FLEX 50(W) firmware versions 4.25 through 5.36 Patch 1, USG20(W)-VPN firmware versions 4.25 through 5.36 Patch 1, VPN series firmware versions 4.30 through 5.36 Patch 1, ZyWALL/USG series firmware versions 4.25 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 versions | Fixed in | ||
|---|---|---|---|
| Zyxel | ATP series firmware |
|
|
| Zyxel | USG FLEX 50(W) firmware |
|
|
| Zyxel | USG FLEX series firmware |
|
|
| Zyxel | USG20(W)-VPN firmware |
|
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| Zyxel | VPN series firmware |
|
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| Zyxel | ZyWALL/USG series firmware |
|
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Vendors and products
Affected operating systems
| Other | zyxel | atp100_firmware | 5.36 |
| Other | zyxel | atp100_firmware | — |
| Other | zyxel | atp100w_firmware | 5.36 |
| Other | zyxel | atp100w_firmware | — |
| Other | zyxel | atp200_firmware | 5.36 |
| Other | zyxel | atp200_firmware | — |
| Other | zyxel | atp500_firmware | 5.36 |
| Other | zyxel | atp500_firmware | — |
| Other | zyxel | atp700_firmware | 5.36 |
| Other | zyxel | atp700_firmware | — |
| Other | zyxel | atp800_firmware | 5.36 |
| Other | zyxel | atp800_firmware | — |
| Other | zyxel | usg20-vpn_firmware | 5.36 |
| Other | zyxel | usg20-vpn_firmware | — |
| Other | zyxel | usg_20w-vpn_firmware | 5.36 |
| Other | zyxel | usg_40_firmware | 4.73 |
| Other | zyxel | usg_40_firmware | — |
| Other | zyxel | usg_40w_firmware | 4.73 |
| Other | zyxel | usg_40w_firmware | — |
| Other | zyxel | usg_60_firmware | 4.73 |
| Other | zyxel | usg_60_firmware | — |
| Other | zyxel | usg_60w_firmware | 4.73 |
| Other | zyxel | usg_60w_firmware | — |
| Other | zyxel | usg_flex_100_firmware | 5.36 |
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.
| CISA KEV Catalog | CISA Known Exploited Vulnerability (opens in a new tab)cisa.gov | actively exploited |
References & sources
- zyxel.comVendor advisory: Zyxel Multiple FirewallsVendor advisory, pwn-watch
- cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2023-33010government-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
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