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Full vulnerability report · 2023
CVE-2023-28771High confidence

Zyxel Multiple Firewalls OS Command Injection Vulnerability

Zyxel · Multiple Firewalls

9.8CriticalCVSS 3.1
Recommended action
Mitigate now

CISA confirms exploitation in the wild and lists 2023-06-21 as the remediation due date.

Mitigation available
R
Operational reassessment

Published severity in operational context

Open reassessment dashboard →
Published severityCriticalOperational priority:Critical, unchanged from published severity.unchanged

Evidence used

  • CISA confirms exploitation in the wild.
  • A structured source references public exploit or proof-of-concept material.
  • The selected CVSS metric records a network-reachable, unauthenticated path with no user interaction.
  • EPSS is 99.28% for the current model date.

Compensating controls

  • Apply the linked authoritative mitigation while planning the permanent fix.
  • Restrict the affected network interface to trusted sources where business-safe.
  • Increase monitoring for the attack path and post-exploitation behaviour described in the report.

Verification

  1. Confirm that the asset runs Zyxel Multiple Firewalls and falls inside the recorded affected range.
  2. Recheck the vendor advisory before scheduling a change because no verified fixed version is currently retained.
  3. Validate exposure, authentication requirements and compensating controls in the actual environment.
  4. Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
Mitigation target: Within 3 daysRemediation target: Within 90 days

This automated reassessment organises public evidence. It does not know asset exposure, business impact or control effectiveness and does not replace CVSS or a human risk decision.

Optional official sources

National CERT insights
?CERT means Computer Emergency Response Team; CSIRT is the closely related term Computer Security Incident Response Team.

Choose official national sources for this report. Each advisory shows its original language. Your selection is remembered on this device and included in shared links.

Official European source

ENISA European Vulnerability Database

Official EUVD identifiers, advisory evidence and known-exploited context. Missing fields are not treated as evidence of low risk.

1 current
ENISA EUVD identifier

EUVD-2023-32406

CISA KEV mirrored by ENISA

Improper error message handling in Zyxel ZyWALL/USG series firmware versions 4.60 through 4.73, VPN series firmware versions 4.60 through 5.35, USG FLEX series firmware versions 4.60 through 5.35, and ATP series firmware versions 4.60 through 5.35, which could allow an unauthenticated attacker to execute some OS commands remotely by sending crafted packets to an affected device.

EUVD state
Present in the current official mapping
Known exploitation
Recorded by ENISA since 31 May 2023. Evidence sources: cisa_kev.
ENISA score
9.8 · CVSS 3.1
Advisory evidence
1 linked advisory record
Recommended actionMitigate now

CISA confirms exploitation in the wild and lists 2023-06-21 as the remediation due date.

Mitigation available
01

What, why and how

Zyxel ATP, USG FLEX, VPN, and ZyWALL/USG firewalls allow for improper error message handling which could allow an unauthenticated attacker to execute OS commands remotely by sending crafted packets to an affected device.

What

Zyxel ATP, USG FLEX, VPN, and ZyWALL/USG firewalls allow for improper error message handling which could allow an unauthenticated attacker to execute OS commands remotely by sending crafted packets to an affected device.

Why

Untrusted input is incorporated into an operating-system command without safe parameter handling.

How

An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.

What

Zyxel ATP, USG FLEX, VPN, and ZyWALL/USG firewalls allow for improper error message handling which could allow an unauthenticated attacker to execute OS commands remotely by sending crafted packets to an affected device.

Why

Untrusted input is incorporated into an operating-system command without safe parameter handling.

How

An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.

02

Exploit reality and attack path

CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.

Observed exploitation
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
Confirmed in the wild

CISA added this CVE to its Known Exploited Vulnerabilities catalogue on 2023-05-31.

Public PoC / exploit material
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
Reference recorded

A structured CVE source labels at least one public reference as exploit material. BlackTree has not independently validated that it is safe, reliable or weaponised.

Likely attack path
a network path → Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection') → cause the confidentiality, integrity or availability impact described by the vendor
Attack surface
Network
Privileges required
None: unauthenticated exploitation is possible
User interaction
None
Attack complexity
Low: no specialised conditions are recorded
Security boundary
Unchanged: impact remains within the vulnerable component's security authority
Weakness
?CWE means Common Weakness Enumeration: a standard category for the underlying weakness.
CWE-78 ↗

CWE-78: Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection'). The product constructs all or part of an OS command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended OS command when it is sent to a downstream component.

CVSS vector
?CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Common Vulnerability Scoring System 3.1: the compact vector below is decoded into plain language.

AVNetworkAttack vector: The vulnerable component can be reached over a network.ACLowAttack complexity: No specialised conditions are required beyond attacker-controlled input.PRNonePrivileges required: The attacker does not need an account or existing privileges.UINoneUser interaction: No action by another user is required.SUnchangedScope: The security impact remains within the vulnerable component's authority.CHighConfidentiality impact: A successful attack can cause a major loss.IHighIntegrity impact: A successful attack can cause a major loss.AHighAvailability impact: A successful attack can cause a major loss.
Post-exploitation / living off the land
No specific living-off-the-land technique is confirmed in the structured sources. Monitor normal administration tools for activity inconsistent with the affected service's baseline.
NetworkUnauthenticatedCWE-78CISA KEVPublic exploit reference
A

Official authority intelligence

Only matched European and national findings are included. Language selectors and unavailable sources are omitted.

ENISA EUVD · EUVD-2023-32406Known-exploited evidence recorded

Improper error message handling in Zyxel ZyWALL/USG series firmware versions 4.60 through 4.73, VPN series firmware versions 4.60 through 5.35, USG FLEX series firmware versions 4.60 through 5.35, and ATP series firmware versions 4.60 through 5.35, which could allow an unauthenticated attacker to execute some OS commands remotely by sending crafted packets to an affected device.

Official EUVD record ↗
BSI · German · WID-SEC-2023-1070Zyxel Firewalls: Mehrere Schwachstellen

Ein Angreifer kann mehrere Schwachstellen in Zyxel Firewall ausnutzen, um beliebigen Programmcode auszuführen, Informationen offenzulegen, einen Denial of Service zu verursachen, einen Cross Site Scripting Angriff durchzuführen oder die Konfiguration zu manipulieren.

Official advisory ↗
Cyber Security Agency of Singapore · English · CSA-SB-20230426Security Bulletin 26 Apr 2023

The Cyber Security Agency of Singapore included this CVE in its official Security Bulletin 26 Apr 2023, published on 26 April 2023. Open the linked bulletin for the product, severity and reference information published in that issue.

Official advisory ↗
JVN iPedia · Japanese · JVNDB-2023-009227複数の ZyXEL 製品における OS コマンドインジェクションの脆弱性

ATP100 ファームウェア、ATP100W ファームウェア、ATP200 ファームウェア等複数の ZyXEL 製品には、OS コマンドインジェクションの脆弱性が存在します。

Official advisory ↗
KISA KrCERT/CC · Korean · KNVD-5937ZYXEL 제품 보안 업데이트 권고

Zyxel의 방화벽에서 발생하는 OS Command Injection 취약점 (CVE-2023-28771) [1]

Official advisory ↗
KISA KrCERT/CC · Korean · KNVD-5927현재 악용되고 있는 Exploit(Update. 2023-05-31)

Zyxel ATP, USG FLEX, VPN, and ZyWALL/USG firewalls allow for improper error message handling which could allow an unauthenticated attacker to execute OS commands remotely by sending crafted packets to an affected device.

Official advisory ↗
03

Patch and workaround

Operational remediation based on structured source evidence.

Status
?Patch availability is based on structured fixed-version fields and authoritative update references. If no fix is verified, check the vendor advisory before making a change.
Mitigation available
Affected
Fixed
No fixed version is explicitly recorded in the structured CVE data.
Action
ATP: ZLD V5.36 USG FLEX: ZLD V5.36 VPN: ZLD V5.36 ZyWALL/USG: ZLD V4.73 Patch 1
Workaround
No verified workaround is recorded. If business-safe, reduce exposure to the affected interface and allow only trusted sources until authoritative guidance is available.
04

Evidence and provenance

Published 25 Apr 2023 · Last source change 21 Oct 2025, 23:05 UTC · CWE-78 · Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')

CVE recordCVE.org · 5.1
CVSS sourceCNA
EPSS source
?The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.
FIRST · tracked since 2026-08-14
European sourceENISA EUVD · EUVD-2023-32406
Product sourceCISA KEV
Remediation sourceCVE/CNA references
CWE sourceCNA
NVD statusNVD enriched

Core structured fields are present and their contributing authorities are shown above.

Material change intelligence

What changed after publication

View recent updates ↗
  1. Vendor guidanceAuthoritative vendor guidance changed: removed remediation: knvd.krcert.or.kr/detail?id=648ff5bee54d111b872254ce.
    Before
    remediation: knvd.krcert.or.kr/detail?id=648ff5bee54d111b872254ce · vendor advisory: zyxel.com/zyxel-security-advisory-for-remote-command-injection-vulnerability-of-firewalls
    After
    vendor advisory: zyxel.com/zyxel-security-advisory-for-remote-command-injection-vulnerability-of-firewalls
    zyxel.com ↗
  2. Remediation statusRemediation status changed from Mitigation available to Awaiting fix.
    Before
    Mitigation available
    After
    Awaiting fix
    zyxel.com ↗
  3. Affected versionsThe structured affected or fixed version information changed.
    Before
    4.60 through 4.73; 4.60 through 5.35 · Fixed: No fixed version is explicitly recorded in the structured CVE data.
    After
    ZyWALL/USG series firmware: 4.60 through 4.73; VPN series firmware: 4.60 through 5.35; USG FLEX series firmware: 4.60 through 5.35; ATP series firmware: 4.60 through 5.35 · Fixed: No fixed version is explicitly recorded in the structured CVE data.
    CNA ↗
  4. Remediation statusRemediation status changed from Awaiting fix to Mitigation available.
    Before
    Awaiting fix
    After
    Mitigation available
    KISA KrCERT/CC ↗
Material fields only · duplicate refreshes suppressed · history retained for the configured operational retention period
CVE published
Added to CISA KEV
Remediation guidance recorded
ENISA EUVD · EUVD-2023-32406↗KISA KrCERT/CC remediation guidance↗CISA KEV record↗CVE authority or vendor reference↗Public exploit reference↗Official CVE record (CNA and ADP data)↗NVD record↗
Technical terms and abbreviations used in this report
CVE
Common Vulnerabilities and Exposures: the public identifier for one disclosed vulnerability.
CVSS
Common Vulnerability Scoring System: a technical severity framework; it is not patching priority by itself.
EPSS
Exploit Prediction Scoring System: FIRST's estimate of the probability that exploitation activity will be observed in the next 30 days; it is a forecast, not confirmation.
CWE
Common Weakness Enumeration: the standard category describing the underlying software or hardware weakness.
CNA
CVE Numbering Authority: an organisation authorised to assign and publish CVE records.
CISA ADP
Cybersecurity and Infrastructure Security Agency Authorized Data Publisher: structured enrichment added to a CVE record.
NVD
National Vulnerability Database: NIST's enrichment service for CVE records.
CERT / CSIRT
A computer security incident response team that publishes warnings or coordinates incident response.
PoC
Proof of concept: public material that demonstrates or helps reproduce exploitation.
CSAF
Common Security Advisory Framework: a machine-readable format for security advisories.
LoTL
Living off the land: abuse of legitimate tools or system functions during an attack.
Free version - for non-commercial use only.CVE-2023-28771 · cve.blacktree.nl