Home/CVE/A command injection vulnerability in the Free Time WiFi hotspot feature of the Zyxel USG FLEX series firmware versions 4
CVE
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CVE-2023-34139

A command injection vulnerability in the Free Time WiFi hotspot feature of the Zyxel USG FLEX series firmware versions 4

A command injection vulnerability in the Free Time WiFi hotspot feature of the Zyxel USG FLEX series firmware versions 4.50 through 5.36 Patch 2 and VPN series firmware versions 4.20 through 5.36 Patch 2, could allow an unauthenticated, LAN-based attacker to execute some OS commands on an affected device.

HIGH · CVSS 8.8 EPSS 0.00759
EPSS exploitation odds0.76% · top 48%
Schedule remediation
  • CVSS base score ≥ 7.0
No Sigma yet — build one → YARA rules0
Look this up elsewhere - one-click external pivots
NVDCVE.orgCISAExploit-DBGitHub PoCVulnCheck KEVGreyNoise
▸ How to read a CVE - triage first, then detect and patch
This page is every public fact about CVE-2023-34139, cross-linked. Its job is to answer one question fast - does this need my attention now? - and then hand you the two things you do about it. Here is how an analyst reads it.
Triage: should I act now? Four signals, and they are not interchangeable:
CVSSseverity - how bad it is IF exploited, 0-10. A high CVSS alone is not urgency; a flaw can be a perfect 10 and never actually be attacked. EPSSprobability - a model’s estimate of the chance it is exploited in the next 30 days, 0-1. This is the “will it actually happen” signal. CISA KEVconfirmed - it is being exploited in the wild right now. The strongest signal on the page; KEV beats any score. Weaponisedavailability - public exploits / PoCs, and especially Metasploit modules rated Excellent / Great. Reliable, packaged exploit code means low-skill attackers can use it today.
How they combine: KEV, or a dependable Metasploit module, means patch now regardless of CVSS. High CVSS + low EPSS + no exploit is real but not an emergency - schedule it. Low CVSS but KEV-listed still gets patched now. The verdict above already weighed these for you; this is how it got there.
Then what - two workflows:
Detectwhen you cannot patch today, follow this CVE to the ATT&CK techniques it enables, then Build a SIEM detection (the green button) - author a rule, test it in Atomic, deploy it. That buys visibility while the patch waits. PatchAffected products / packages tell you if you are exposed; Fixed versions by distribution and Vendor advisories give the exact version that closes it.
Reading order for the panels below: verdict + badges, then Public exploits / Metasploit (is it weaponised), then ATT&CK techniques + Sigma / IDS rules (can I detect it), then Affected products / packages + Fixed versions (am I exposed, what patches it), then Threat actors / IOCs (who uses it), then Scoring & timeline / references (the evidence).
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Exploitation momentum

484 days of EPSS
dormant
Flat and low - no real exploitation pressure. This reads the direction and speed of EPSS over time, which can move before EPSS itself peaks or before CISA lists it.
Window
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Severity & exploitation scoring

View on NVD →
CVSS base score
8.8
HIGHCVSS v3.1 · [email protected]
EPSS exploitation probability
0.76%
Top 48%odds of exploitation in the next 30 days
CVSS metric silhouette
VectorComplexityPrivilegesInteractionScopeConfidentialityIntegrityAvailability
shape grows toward worst-case
SSVC triage · cisa-vulnrichment
Exploitation
none
Automatable
no
Tech impact
total
CVSS vector breakdown
Exploitability - how they get in
Attack Vector
Network Adjacent Local Physical
Attack Complexity
Low High
Privileges Required
None Low High
User Interaction
None Required
Scope
Unchanged Changed
Impact - what breaks
Confidentiality
None Low High
Integrity
None Low High
Availability
None Low High
VECTORCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Lifecycle
  1. 17 Jul 2023Published to NVD
  2. 17 Jun 2026Last modified
Every entry is a recorded date - NVD publish/modify, CISA KEV add, public exploit disclosure. No inferred events.
Attack path
Full kill chain
→
◆

ATT&CK techniques

1

Techniques this CVE enables. Pills with a solid outline are high confidence - named directly in ATT&CK or Nuclei, or human-curated by CTID; the rest are inferred from the weakness type using MITRE's CVE Mapping Methodology and the CWE → CAPEC chain. Broad, generic-weakness guesses are filtered out. A small N× marks a technique that N independent sources agree on.

T1059 · Command and Scripting Interpreter
▤ Build a SIEM detection for these techniques
▤

CAPEC attack patterns

5

Attack patterns this CVE enables - the bridge from weakness to ATT&CK technique.

CAPEC-CAPEC-108 · Command Line Execution through SQL Injection CAPEC-CAPEC-15 · Command Delimiters CAPEC-CAPEC-43 · Exploiting Multiple Input Interpretation Layers CAPEC-CAPEC-6 · Argument Injection CAPEC-CAPEC-88 · OS Command Injection
⬡

Weakness Classification

CWE-78Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')
CWE-78Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')
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Affected Products & Versions

15
zyxel usg 2200-vpn firmware>= 4.20 and < 5.37
zyxel usg flex 100 firmware>= 4.50 and < 5.37
zyxel usg flex 100w firmware>= 4.50 and < 5.37
zyxel usg flex 200 firmware>= 4.50 and < 5.37
zyxel usg flex 50 firmware>= 4.50 and < 5.37
zyxel usg flex 500 firmware>= 4.50 and < 5.37
zyxel usg flex 50w firmware>= 4.50 and < 5.37
zyxel usg flex 700 firmware>= 4.50 and < 5.37
zyxel zywall vpn100 firmware>= 4.20 and < 5.37
zyxel zywall vpn2s firmware>= 4.20 and < 5.37
zyxel zywall vpn300 firmware>= 4.20 and < 5.37
zyxel zywall vpn50 firmware>= 4.20 and < 5.37
zyxel zywall vpn 100 firmware>= 4.20 and < 5.37
zyxel zywall vpn 300 firmware>= 4.20 and < 5.37
zyxel zywall vpn 50 firmware>= 4.20 and < 5.37
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Related CVEs

8
CVEs linked to this one by a shared weakness (CWE) or affected product - joins on data already in the engine, with the reason shown per row.
CVE-1999-0067 phf CGI program allows remote command execution through shell metacharacters. same CWE-78 CVE-1999-0043 Command execution via shell metachars in INN daemon (innd) 1.5 using "newgroup" ... same CWE-78 CRITICAL CVE-2001-1583 lpd daemon (in.lpd) in Solaris 8 and earlier allows remote attackers to execute ... 4 same CWE-78 CVE-2002-0061 Apache for Win32 before 1.3.24, and 2.0.x before 2.0.34-beta, allows remote atta... 1 same CWE-78 CVE-2018-9129 ZyXEL ZyWALL/USG series devices have a Bleichenbacher vulnerability in their Int... same product MEDIUM CVE-2020-25014 A stack-based buffer overflow in fbwifi_continue.cgi on Zyxel UTM and VPN series... same product CRITICAL CVE-2022-0734 A cross-site scripting vulnerability was identified in the CGI program of Zyxel ... same product MEDIUM CVE-2022-0910 A downgrade from two-factor authentication to one-factor authentication vulnerab... same product MEDIUM
🔗

References & Sources

1
Source URLs (vendor pages, mailing lists, write-ups). Exploit/PoC links are in their own section above to avoid duplication.
https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-multiple-vulnerabilities-in-firewalls-and-wlan-controllers
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