---
id: "CVE-2023-33009"
kind: "vulnerability"
aliases: ["CVE-2023-33009"]
title: "A buffer overflow vulnerability in the notification function in Zyxel ATP series firmware versions 4.60 through 5.36\u2026"
severity: "critical"
cvss: 9.8
epss: null
exploited: true
status: "open"
category: "infrastructure"
products: [{"name": "Zyxel Multiple Firewalls", "fixed": [], "affected": "", "ecosystem": "vendor"}, {"name": "zyxel atp100 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel atp100 firmware", "fixed": [], "affected": "= 5.36", "ecosystem": "vendor"}, {"name": "zyxel atp200 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel atp200 firmware", "fixed": [], "affected": "= 5.36", "ecosystem": "vendor"}, {"name": "zyxel atp500 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel atp500 firmware", "fixed": [], "affected": "= 5.36", "ecosystem": "vendor"}, {"name": "zyxel atp100w firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel atp100w firmware", "fixed": [], "affected": "= 5.36", "ecosystem": "vendor"}, {"name": "zyxel atp700 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel atp700 firmware", "fixed": [], "affected": "= 5.36", "ecosystem": "vendor"}, {"name": "zyxel atp800 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel atp800 firmware", "fixed": [], "affected": "= 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 100 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 100 firmware", "fixed": [], "affected": "= 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 50 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 50 firmware", "fixed": [], "affected": "= 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 200 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 200 firmware", "fixed": [], "affected": "= 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 500 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 500 firmware", "fixed": [], "affected": "= 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 700 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 700 firmware", "fixed": [], "affected": "= 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 100w firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 100w firmware", "fixed": [], "affected": "= 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 50w firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 50w firmware", "fixed": [], "affected": "= 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg 20w-vpn firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg 20w-vpn firmware", "fixed": [], "affected": "= 5.36", "ecosystem": "vendor"}, {"name": "zyxel vpn100 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel vpn100 firmware", "fixed": [], "affected": "= 5.36", "ecosystem": "vendor"}, {"name": "zyxel vpn50 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel vpn50 firmware", "fixed": [], "affected": "= 5.36", "ecosystem": "vendor"}, {"name": "zyxel vpn300 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel vpn300 firmware", "fixed": [], "affected": "= 5.36", "ecosystem": "vendor"}, {"name": "zyxel vpn1000 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel vpn1000 firmware", "fixed": [], "affected": "= 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg20-vpn firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg20-vpn firmware", "fixed": [], "affected": "= 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg 40 firmware", "fixed": ["4.73"], "affected": ">= 4.60, < 4.73", "ecosystem": "vendor"}, {"name": "zyxel usg 40 firmware", "fixed": [], "affected": "= 4.73", "ecosystem": "vendor"}]
ecosystems: ["vendor"]
tags: ["exploited", "firewall", "infrastructure", "self-hosted", "zyxel"]
not_affected_if: ["unpatched-models-not-listed"]
published_at: "2023-05-24T13:15:09+00:00"
first_seen_at: "2026-10-23T19:11:51+00:00"
updated_at: "2026-10-23T22:29:04+00:00"
sources: [{"source": "kev", "url": "https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2023-33009", "first_seen_at": "2026-10-23T19:11:51+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2023-33009", "first_seen_at": "2026-10-23T22:29:04+00:00"}]
---

# A buffer overflow vulnerability in the notification function in Zyxel ATP series firmware versions 4.60 through 5.36…

Zyxel ATP, USG FLEX, USG20(W)-VPN, VPN and ZyWALL/USG firewalls have a buffer overflow in their notification function. An unauthenticated attacker can crash the device or run code on it remotely.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- vendor:Zyxel Multiple Firewalls affected ? fixed ?
- vendor:zyxel atp100 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel atp100 firmware affected = 5.36 fixed ?
- vendor:zyxel atp200 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel atp200 firmware affected = 5.36 fixed ?
- vendor:zyxel atp500 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel atp500 firmware affected = 5.36 fixed ?
- vendor:zyxel atp100w firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel atp100w firmware affected = 5.36 fixed ?
- vendor:zyxel atp700 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel atp700 firmware affected = 5.36 fixed ?
- vendor:zyxel atp800 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel atp800 firmware affected = 5.36 fixed ?
- vendor:zyxel usg flex 100 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel usg flex 100 firmware affected = 5.36 fixed ?
- vendor:zyxel usg flex 50 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel usg flex 50 firmware affected = 5.36 fixed ?
- vendor:zyxel usg flex 200 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel usg flex 200 firmware affected = 5.36 fixed ?
- vendor:zyxel usg flex 500 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel usg flex 500 firmware affected = 5.36 fixed ?
- vendor:zyxel usg flex 700 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel usg flex 700 firmware affected = 5.36 fixed ?
- vendor:zyxel usg flex 100w firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel usg flex 100w firmware affected = 5.36 fixed ?
- vendor:zyxel usg flex 50w firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel usg flex 50w firmware affected = 5.36 fixed ?
- vendor:zyxel usg 20w-vpn firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel usg 20w-vpn firmware affected = 5.36 fixed ?
- vendor:zyxel vpn100 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel vpn100 firmware affected = 5.36 fixed ?
- vendor:zyxel vpn50 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel vpn50 firmware affected = 5.36 fixed ?
- vendor:zyxel vpn300 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel vpn300 firmware affected = 5.36 fixed ?
- vendor:zyxel vpn1000 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel vpn1000 firmware affected = 5.36 fixed ?
- vendor:zyxel usg20-vpn firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel usg20-vpn firmware affected = 5.36 fixed ?
- vendor:zyxel usg 40 firmware affected >= 4.60, < 4.73 fixed 4.73
- vendor:zyxel usg 40 firmware affected = 4.73 fixed ?

## Fix

Apply the vendor update for your Zyxel model per Zyxel's advisory; CISA says to apply updates per vendor instructions. Specific fixed firmware versions: unknown

## Timeline

- 2026-10-23T19:11:51+00:00 kev https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2023-33009
- 2026-10-23T22:29:04+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2023-33009

## From the sources

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.

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