Is CVE-2023-33009 (Zyxel Multiple Firewalls Buffer Overflow) being exploited in the wild?

What “being exploited in the wild” means for this Zyxel firewall CVE

For a perimeter firewall CVE, “in the wild” is the practical threshold that changes response priority. It means the issue is not just theoretically dangerous, it is already being used against real targets, so exposure review and remediation should move ahead of normal patch queues. Treat the question as an asset-and-compromise check, not a scoring exercise.

That distinction matters because internet-facing appliances often sit at a trust boundary and are difficult to monitor once exploited. If the product is exposed, the safer assumption is that exploitation may have been attempted already and that logs, management access, and adjacent systems deserve review alongside patching.

How to confirm whether your environment is exposed or already impacted

The first step is to identify every instance of the affected Zyxel firewall product, including forgotten appliances, lab gear, and devices reachable through remote administration. Validate not only whether the model is present, but whether it is reachable from untrusted networks, whether management interfaces are exposed, and whether it is still in active service.

If an appliance is on the internet or reachable through a vendor remote-management path, treat it as high priority. For edge-device issues, the most common failure is assuming that a firewall is “too internal” to be targeted. In practice, these products are often targeted precisely because they sit at the boundary and can be used to gain a foothold into the network.

For an operationally useful triage path, compare inventory data, vulnerability scan results, and configuration review. The Edge Device Security Guide is useful here because it frames management exposure, logging minimums, appliance credentials, and end-of-life risk as one combined problem rather than separate tasks.

What to do if patching must wait

If you cannot patch immediately, reduce exposure first. That usually means restricting management access to trusted networks, disabling unnecessary remote administration paths, enforcing strong administrative credentials, and tightening any access rules that allow the firewall to be reached from the internet or from broad internal networks.

When a fix is delayed, compensation should be specific and temporary, not a vague “monitor more.” Preserve logs, increase review of authentication and configuration-change events, and check for any unexpected new accounts, policy edits, or outbound connections from the appliance. If the firewall supports it, isolate it from nonessential management planes until the update is complete.

For exploit-driven remediation, prioritize the path of least privilege and the fastest blast-radius reduction. The Zero-Day and Exploited Vulnerabilities Guide is a strong companion because it focuses on emergency mitigation, hotpatching, and post-patch hunting when exploitation may already be underway.

Vendor advisories and the CISA Known Exploited Vulnerabilities Catalog should be the source of truth for remediation timing and required action. Use them to confirm the exact device guidance rather than relying on secondary summaries.

Practitioner Guidance

What to verify: Confirm whether the firewall is reachable from untrusted networks, whether administrative access is restricted, and whether the device has been used to terminate VPN, routing, or management traffic that would make compromise especially sensitive. If the appliance is exposed, assume it deserves urgent attention even before you know whether exploitation succeeded.

Decision rule: If the device is exposed and the vendor or CISA has flagged active exploitation, patch or mitigate first, then hunt for signs of compromise. If patching is blocked, document the exception, narrow access immediately, and plan credential and session review around the appliance’s admin plane.

What practitioners underestimate: Firewalls and similar edge devices are often treated as infrastructure rather than as high-value security assets. That mindset delays log review and compromise assessment, even though these devices can provide attackers a durable foothold and visibility into internal traffic.

Practitioner takeaway: Once a perimeter appliance CVE is confirmed as actively exploited, the correct order is exposure check, containment, remediation, and then compromise hunting, not the other way around.

Related resources from NHI Mgmt Group

• CVE-2023-33009 (Zyxel Multiple Firewalls Buffer Overflow)
• CVE-2023-33010 (Zyxel Multiple Firewalls Buffer Overflow)
• Is CVE-2025-20333 (Cisco Secure Firewall Adaptive Security Appliance (ASA) and Secure Firewall Threat Defense (FTD) Buffer Overflow) being exploited in the wild?
• CVE-2023-28771 (Zyxel Multiple Firewalls OS Command Injection)