CVE-2023-35139

Session cookie theft via stored ZTP-log XSS

Published Nov 28, 2023 · Updated Aug 2, 2024

Stored XSS in the ZTP-log CGI of affected Zyxel firewall firmware allows remote attackers on the LAN to steal browser cookies. The CGI stores attacker-supplied script content without neutralizing it and later emits that content when the ZTP-log dump page is viewed. Exploitation requires unauthenticated LAN access and a user to visit the affected CGI, where the script executes in that user's browser.

CVSS severity5.2
Medium
EPSS probability0.46%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Stored XSS in the ZTP-log CGI of affected Zyxel firewall firmware allows remote attackers on the LAN to steal browser cookies. The CGI stores attacker-supplied script content without neutralizing it and later emits that content when the ZTP-log dump page is viewed. Exploitation requires unauthenticated LAN access and a user to visit the affected CGI, where the script executes in that user's browser.

The record

CVE
CVE-2023-35139
Published
Nov 28, 2023
Updated
Aug 2, 2024
Vendor
Zyxel Networks
Product
Zyxel USG FLEX 50(W) Series Firmware
Classifications
CWE-79, T1539
Attack vector
adjacent
Privileges
unauthenticated

Timeline

How it unfolded

  1. Nov 28, 2023CVE publishedPublication date reported by the CVE source.
  2. Aug 2, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=versions 5.10 through 5.37

What conditions does exploitation require?

Attack vectoradjacent
Required privilegesunauthenticated

What is affected?

Zyxel Networks · Zyxel USG FLEX 50(W) Series Firmwareversion=versions 5.10 through 5.37
Zyxel Networks · Zyxel ATP Series Firmwareversion=versions 5.10 through 5.37
Zyxel Networks · Zyxel USG20(W)-VPN Series Firmwareversion=versions 5.10 through 5.37
Zyxel Networks · Zyxel VPN Series Firmwareversion=versions 5.00 through 5.37
Zyxel Networks · Zyxel USG FLEX Series Firmwareversion=versions 5.00 through 5.37

Published CVSS scores

6.1NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
5.2Zyxel CorporationCVSS:3.1/AV:A/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectoradjacent
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1539Steal Web Session Cookie

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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