---
id: "CVE-2023-28771"
kind: "vulnerability"
aliases: ["CVE-2023-28771"]
title: "Improper error message handling in Zyxel ZyWALL/USG series firmware versions 4.60 through 4.73, VPN series firmware\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 atp100w firmware", "fixed": ["5.35"], "affected": ">= 4.60, < 5.35", "ecosystem": "vendor"}, {"name": "zyxel atp200 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel atp500 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel atp700 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel atp800 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 100 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 100w firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 200 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 50 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 500 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 50w firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel usg flex 700 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel vpn100 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel vpn1000 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel vpn300 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel vpn50 firmware", "fixed": ["5.36"], "affected": ">= 4.60, < 5.36", "ecosystem": "vendor"}, {"name": "zyxel zywall usg 310 firmware", "fixed": ["4.73"], "affected": ">= 4.60, < 4.73", "ecosystem": "vendor"}, {"name": "zyxel zywall usg 310 firmware", "fixed": [], "affected": "= 4.73", "ecosystem": "vendor"}, {"name": "zyxel zywall usg 100 firmware", "fixed": ["4.73"], "affected": ">= 4.60, < 4.73", "ecosystem": "vendor"}, {"name": "zyxel zywall usg 100 firmware", "fixed": [], "affected": "= 4.73", "ecosystem": "vendor"}]
ecosystems: ["vendor"]
tags: ["exploited", "firewall", "infrastructure", "self-hosted", "zyxel"]
not_affected_if: ["no-zyxel-firewall"]
published_at: "2023-04-25T02:15:08+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-2023-28771", "first_seen_at": "2026-10-23T19:11:51+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2023-28771", "first_seen_at": "2026-10-23T22:29:04+00:00"}]
---

# Improper error message handling in Zyxel ZyWALL/USG series firmware versions 4.60 through 4.73, VPN series firmware…

Zyxel ATP, USG FLEX, VPN and ZyWALL/USG firewalls mishandle error messages, letting an unauthenticated remote attacker run OS commands by sending crafted packets. CISA lists it as actively exploited in the wild.

_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 atp100w firmware affected >= 4.60, < 5.35 fixed 5.35
- vendor:zyxel atp200 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel atp500 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel atp700 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel atp800 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel usg flex 100 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel usg flex 100w firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel usg flex 200 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel usg flex 50 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel usg flex 500 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel usg flex 50w firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel usg flex 700 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel vpn100 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel vpn1000 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel vpn300 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel vpn50 firmware affected >= 4.60, < 5.36 fixed 5.36
- vendor:zyxel zywall usg 310 firmware affected >= 4.60, < 4.73 fixed 4.73
- vendor:zyxel zywall usg 310 firmware affected = 4.73 fixed ?
- vendor:zyxel zywall usg 100 firmware affected >= 4.60, < 4.73 fixed 4.73
- vendor:zyxel zywall usg 100 firmware affected = 4.73 fixed ?

## How to check

Identify Zyxel ATP, USG FLEX, VPN or ZyWALL/USG devices in your network and compare firmware to the vendor advisory; the sources do not give a version check command.

## Fix

Apply the vendor firmware update for the affected Zyxel models per the vendor instructions; the sources do not list fixed versions.

## Timeline

- 2026-10-23T19:11:51+00:00 kev https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2023-28771
- 2026-10-23T22:29:04+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2023-28771

## From the sources

Improper error message handling in Zyxel ZyWALL/USG series firmware versions 4.60 through 4.73, VPN series firmware versions 4.60 through 5.35, USG FLEX series firmware versions 4.60 through 5.35, and ATP series firmware versions 4.60 through 5.35, which could allow an unauthenticated attacker to execute some OS commands remotely by sending crafted packets to an affected device.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_