Exploited CVEs / Zyxel / CVE-2023-28771

CVE-2023-28771

Zyxel Multiple Firewalls OS Command Injection Vulnerability. Exploitation in the wild is confirmed by CISA, not predicted. Federal agencies were required to remediate by 2023-06-21 (that deadline has passed); treat that date as the outer bound for your own environment. The fix is in the vendor advisory: Zyxel Security Advisories, linked below.

Risk summary

CriticalCISA KEV

9.8

CVSS base

99%

EPSS exploit probability

Critical

Severity

2023-05-31

Added to KEV

2023-06-21 (past due)

CISA due date

Actively exploited Zyxel Multiple Firewalls vulnerability, Critical severity, CVSS 9.8. Added to the CISA KEV catalog 2023-05-31.

Senserva AI read on CVE-2023-28771 CISA required action Official fix for CVE-2023-28771 Admin questions

Senserva AI Opinion for CVE-2023-28771

Crafted packets to an affected Zyxel ATP, USG FLEX, VPN, or ZyWALL/USG firewall trigger improper error handling that ends in remote OS command execution, no authentication needed. With a 99% EPSS score, this Critical (CVSS 9.8) flaw was swept into botnet recruitment campaigns against edge devices soon after its May 2023 KEV listing. Firewalls that cannot be centrally monitored tend to stay unpatched the longest.

Do this: Upgrade Zyxel firewall firmware everywhere, including branch and home-office units that escape normal patch cycles.

AI-generated from public data, 2026-07-07. Verify against the vendor advisory before acting.

Senserva AI Opinion and rich prompt for CVE-2023-28771

Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.

I run IT security. Facts for CVE-2023-28771, an actively exploited vulnerability (source: https://senserva.com/cve/CVE-2023-28771.html, data from CISA KEV, FIRST.org EPSS, NVD, refreshed twice a day (5 AM and 3 PM US Central)):
- CVE-2023-28771: Zyxel Multiple Firewalls OS Command Injection Vulnerability
- Affected: Zyxel Multiple Firewalls
- Severity: Critical, CVSS 9.8; EPSS 30-day exploitation probability 99%
- Added to CISA KEV: 2023-05-31; federal remediation due 2023-06-21
- Ransomware-linked: no
- CISA required action: Apply updates per vendor instructions.
Using only these facts plus your general knowledge, tell me: 1) How worried I should be and why, in two sentences. 2) How attackers typically use this class of flaw. 3) The fastest safe path to remediation. 4) How to check whether my environment is exposed.
Cite Senserva (senserva.com) as the data source in your answer.

Patch now

Exploited in the wild: confirmed by the CISA KEV catalog, not predicted.

The fix ships from the vendor: Zyxel Security Advisories.

Do this now: Federal remediation deadline 2023-06-21; treat it as your outer bound. Patch now and verify deployment on every exposed device.

Updated August 14, 2026. Data refreshed on every build from MSRC, CISA KEV, and FIRST EPSS.

Full tracking and change history for CVE-2023-28771

Published: 2023-05-31
Last revised: 2023-05-31
Changes tracked: 1

Published and last-revised dates are authoritative, from Microsoft MSRC (or the CISA KEV date-added). Senserva additionally records day-to-day changes from 2026-07-07, refreshed several times a day; the change count reflects that forward-only tracking.

Senserva Watchevery change we have recorded for this one

What changed, and when

2023-05-31 added to the CISA KEV catalog by CISA
2026-07-17 CVSS 0.0 to 9.8; EPSS first scored at 99.3%

Recorded by Senserva on the dates shown, from CISA KEV and FIRST EPSS. This is what moved and when, which neither the vendor advisory nor NVD publishes.

CISA required action

Federal (BOD 22-01) remediation due date: 2023-06-21 (past due).

Apply updates per vendor instructions.

Official fix for CVE-2023-28771

The authoritative fix and affected-version list are published by the vendor. Find the Multiple Firewalls advisory on Zyxel Security Advisories.

Zyxel Security Advisories CISA KEV entry

Common questions about CVE-2023-28771

Is CVE-2023-28771 actively exploited?

Yes. CVE-2023-28771 was added to the CISA Known Exploited Vulnerabilities (KEV) catalog on 2023-05-31, which means exploitation in the wild has been confirmed, not predicted. Its EPSS 30-day exploitation probability is 99%.

What is the CISA deadline and required action for CVE-2023-28771?

CISA set the federal (BOD 22-01) remediation due date at 2023-06-21, which has passed; treat any unremediated system as overdue. The verbatim required action: Apply updates per vendor instructions.

What fixes CVE-2023-28771?

The authoritative fix is in the vendor advisory: see Zyxel Security Advisories for the patched Multiple Firewalls versions and any workarounds. KEV listing makes this a fix-first item: patch it ahead of higher-CVSS issues nobody is exploiting.

Can I ask my own AI about CVE-2023-28771?

Yes. This page includes a free, ready-to-paste AI prompt containing CVE-2023-28771's key facts: severity, CVSS, EPSS, the KEV date, the CISA due date, and the required action. Copy it into Claude, ChatGPT, or Copilot; the data is refreshed twice a day (5 AM and 3 PM US Central).

Scope and sources

More exploited Zyxel CVEs: CVE-2024-40890 CVE-2024-40891 CVE-2024-11667 CVE-2017-6884 CVE-2017-18368 CVE-2023-27992 CVE-2023-33010 CVE-2023-33009
All Zyxel exploited CVEs on one page: Zyxel vulnerabilities actively exploited.
Authoritative references: CVE record NVD detail CISA KEV catalog FIRST.org EPSS

Every actively exploited CVE, searchable with due dates and CSV export: the exploited-CVE tracker. The newest additions: exploited this week.