CVE-2023-33009
CVE-2023-33009 is a security flaw in Zyxel Multiple Firewalls, firewalls and routers for homes and small businesses. Attackers can run their own code on Zyxel Multiple Firewalls from anywhere, without logging in. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 5 Jun 2023.[1]
Zyxel Multiple Firewalls is in things households have: home and small-office routers, so the fix matters at home as much as at work. Zyxel has published a fix or advice.
Description
CISA calls it a buffer overflow in Zyxel Multiple Firewalls. In its words: “Zyxel ATP, USG FLEX, USG FLEX 50(W), USG20(W)-VPN, VPN, and ZyWALL/USG firewalls contain a buffer overflow vulnerability in the notification function that could allow an unauthenticated attacker to cause denial-of-service (DoS) conditions and remote code execution on an affected device.”[1]
The CVE record adds: “A buffer overflow vulnerability in the notification function in Zyxel ATP series firmware versions 4.60 through 5.36 Patch 1, USG FLEX series firmware versions 4.60 through 5.36 Patch 1, USG FLEX 50(W) firmware versions 4.60 through 5.36 Patch 1, USG20(W)-VPN firmware versions 4.60 through 5.36 Patch 1, VPN series firmware versions 4.60 through 5.36 Patch 1, ZyWALL/USG series firmware versions 4.60 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.”[2]
The underlying weakness is buffer overflow (CWE-120), in MITRE’s list of common software weaknesses.
It scores 9.8 out of 10 on the Common Vulnerability Scoring System, which rates it critical, scored by the organisation that published the CVE using CVSS 3.1.[2]
Affected versions
NVD’s record doesn’t list version ranges for it; Zyxel’s advisory has them[3]. CISA names the product as Zyxel Multiple Firewalls.
Every device with Zyxel Multiple Firewalls needs the update.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 5 Jun 2023, 12 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 26 Jun 2023 to fix it.[1]
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 28%. That’s in the top 2% of the flaws it scores.[4]
CISA assesses it as automatable: attackers can run it against many targets at once.
CISA doesn’t say who was attacked, or how.
Fix
Zyxel has published a fix or advice.[3] CISA’s required action: “Apply updates per vendor instructions.”[1]
- Update Zyxel Multiple Firewalls now.Open the router’s app, or its admin page in a browser, and look for a firmware update. Then turn on automatic updates, so the next fix arrives on its own.
- If the maker has stopped updating it, replace it.Routers stop getting fixes after a few years, and attackers keep a list of the ones that did. If there’s no newer firmware, it won’t be fixed.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 9.8.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at 28%.[4] |
See also
Other Zyxel Multiple Firewalls flaws
- CVE-2024-11667: path traversal ·
- CVE-2023-33010: buffer overflow ·
- CVE-2023-28771: OS command injection ·
- CVE-2022-30525: OS command injection ·
Other Zyxel flaws
- CVE-2026-7273, Zyxel GS1900 Series Switches: stack-based buffer overflow ·
- CVE-2024-40891, Zyxel DSL CPE Devices: DSL CPE OS command injection ·
- CVE-2024-40890, Zyxel DSL CPE Devices: DSL CPE OS command injection ·
- CVE-2017-6884, Zyxel EMG2926 Routers: command injection ·
- CVE-2017-18368, Zyxel P660HN-T1A Routers: command injection ·
Other buffer overflow flaws
- CVE-2025-43520, Apple devices: classic buffer overflow ·
- CVE-2022-37055, D-Link Routers: buffer overflow ·
- CVE-2025-20333, Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense: secure firewall adaptive security appliance (ASA) and secure firewall threat defense (FTD) buffer overflow ·
- CVE-2020-15069, Sophos XG Firewall: buffer overflow ·
- CVE-2023-41064, Apple iOS, iPadOS, and macOS: imageIO buffer overflow ·
Added the same day
- CVE-2023-33010, Zyxel Multiple Firewalls: buffer overflow ·
Every Zyxel flaw on CISA’s list · Everything added in June 2023
References
- ^ Cybersecurity and Infrastructure Security Agency. “Zyxel Multiple Firewalls Buffer Overflow Vulnerability”. Known Exploited Vulnerabilities Catalog, added 5 Jun 2023.
- ^ National Institute of Standards and Technology. “CVE-2023-33009 Detail”. National Vulnerability Database, published 24 May 2023.
- ^ Zyxel. Advisories and fixes: zyxel.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.281, percentile 98.1, 31 Oct 2026.
- CVE Program. “CVE-2023-33009”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.