VulnerabilityPublished 24 May 2023Updated 21 Oct 2025

CVE-2023-33009: Zyxel ATP series firmware · Classic Buffer Overflow

Affects Zyxel ATP series firmwareSource CVE Database V5

Exploited in the wild · KEV due 26 Jun 2023 · 28% EPSS : apply the vendor's fix or mitigations to Zyxel ATP series firmware now.

Data as of 26 Oct 2026, 11:07 UTC

CVSS 3.1 base score

9.8Critical

How severe the flaw is on the 0–10 CVSS scale.

KEV deadline: 26 Jun 2023 US federal agencies must patch by this date; a sensible deadline for everyone else.
EPSS: 28.1% chance of exploitation in 30 days – higher than 98% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: High
Integrity: High
Availability: High

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

t = 0 is the CVE’s publication on cve.org

Disclosed: evidence found.
CVE published 24 May 2023
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: evidence found.
Listed in CISA KEV 12 days after disclosure
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

CVE ID reserved
by Zyxel Corporation
17 May 2023, 7 days before disclosure
CVE published
on cve.org
24 May 2023, t = 0
Added to CISA KEV (opens in a new tab)
exploitation confirmed
5 Jun 2023, 12 days after disclosure
KEV remediation due
US federal deadline
26 Jun 2023, 33 days after disclosure

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), CISA KEV (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Apply updates per vendor instructions.

Affected software

Product	Affected	Fixed in
Zyxel ATP series firmware	Affected >=4.60 <=5.36	Fixed in–

Check the firmware or software version for Zyxel ATP series firmware in the device's admin interface against the vendor advisory.

Description

CVE-2023-33009 is a critical buffer overflow vulnerability in multiple Zyxel firewall and VPN device firmware versions (4.60 through 5.36 Patch 1). It affects Zyxel ATP, USG FLEX, USG20(W)-VPN, VPN series, and ZyWALL/USG series devices. The flaw exists in the notification function and can be exploited remotely without authentication or user interaction. Successful exploitation can lead to denial-of-service or remote code execution, compromising confidentiality, integrity, and availability. No known exploits are currently reported in the wild, but the high CVSS score (9.8) indicates severe risk. European organizations using these Zyxel devices should prioritize patching and implement network-level protections. Countries with significant Zyxel deployments and critical infrastructure reliance on these devices are at higher risk.

Impact

For European organizations, this vulnerability poses a critical risk to network security infrastructure. Zyxel devices are widely used in small to medium enterprises and some larger organizations for firewalling and VPN connectivity. Exploitation could lead to complete compromise of perimeter defenses, allowing attackers to bypass security controls, intercept sensitive data, disrupt business operations, or launch further attacks within the network. Critical sectors such as finance, healthcare, government, and energy that rely on these devices for secure remote access and network segmentation are particularly vulnerable. The ability to execute code remotely without authentication increases the likelihood of automated exploitation attempts once public details become widespread. Denial-of-service attacks could also disrupt essential services, impacting availability. The lack of current known exploits provides a window for proactive mitigation, but the critical nature demands immediate attention to prevent potential breaches.

Mitigation

1. Monitor Zyxel’s official channels closely for firmware updates addressing CVE-2023-33009 and apply patches immediately upon release. 2. Until patches are available, restrict network access to management interfaces of affected devices using firewall rules and VPNs to limit exposure. 3. Implement network segmentation to isolate Zyxel devices from critical internal systems, reducing potential lateral movement if compromised. 4. Employ intrusion detection/prevention systems (IDS/IPS) with updated signatures to detect anomalous traffic patterns targeting Zyxel devices. 5. Conduct regular vulnerability scans and penetration tests focusing on perimeter devices to identify exploitation attempts. 6. Disable or limit the notification function if feasible, as it is the vulnerable component, to reduce attack surface. 7. Enforce strict access control policies and monitor logs for unusual activity related to Zyxel devices. 8. Educate IT staff about this vulnerability to ensure rapid response and incident handling readiness. 9. Consider deploying additional layers of security such as endpoint detection and response (EDR) solutions to detect post-exploitation behaviors.

CWE-120Buffer Copy without Checking Size of Input ('Classic Buffer Overflow') (opens in a new tab)
Base weaknessLikelihood of exploit: HighCWE Top 25 (2025): #11
The product copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer.
Requirements · Language Selection
Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
Architecture and Design · Libraries or Frameworks
Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
Operation · Build and Compilation · Environment Hardening
Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index…
OWASP Cheat Sheets
Docker Security (opens in a new tab)
XML Security (opens in a new tab)
OWASP ASVS 4.0.3
V5.4.1 (opens in a new tab)
V14.1.2 (opens in a new tab)
V13.3.1 (opens in a new tab)
Attack patterns
Buffer Overflow via Environment Variables (opens in a new tab)
Overflow Buffers (opens in a new tab)
Client-side Injection-induced Buffer Overflow (opens in a new tab)
Filter Failure through Buffer Overflow (opens in a new tab)
MIME Conversion (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

Timeline

What to do

Affected software

Description

AI analysisMachine-generated, updated 28 Oct 2025, 23:48 UTC

WeaknessesCWE-120

Technical details

References

Community reviews

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