CVE-2023-6399

Authenticated service interruption via crafted VPN hostname

Published Feb 20, 2024 · Updated Aug 2, 2024

Format string injection in Zyxel firewall firmware allows remote authenticated users to interrupt the deviceid daemon with a crafted VPN hostname. The deviceid daemon uses an IPSec VPN user’s hostname as an externally controlled format string; Zyxel has not disclosed the affected function. Device Insight must be enabled and valid IPSec VPN access is required; the reported consequence is limited to denial of service.

CVSS severity5.7
Medium
EPSS probability0.65%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Format string injection in Zyxel firewall firmware allows remote authenticated users to interrupt the deviceid daemon with a crafted VPN hostname. The deviceid daemon uses an IPSec VPN user’s hostname as an externally controlled format string; Zyxel has not disclosed the affected function. Device Insight must be enabled and valid IPSec VPN access is required; the reported consequence is limited to denial of service.

The record

CVE
CVE-2023-6399
Published
Feb 20, 2024
Updated
Aug 2, 2024
Vendor
Zyxel Networks
Product
Zyxel ATP Series Firmware
Classifications
CWE-134, T1499.004
Attack vector
adjacent
Privileges
authenticated

Timeline

How it unfolded

  1. Feb 20, 2024CVE 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=version 4.32 through 5.37 Patch 1

What conditions does exploitation require?

Attack vectoradjacent
Required privilegesauthenticated

What is affected?

Zyxel Networks · Zyxel ATP Series Firmwareversion=version 4.32 through 5.37 Patch 1
Zyxel Networks · USG FLEX H Series uOS Firmwareversion=version 1.10 through 1.10 Patch 1
Zyxel Networks · Zyxel USG FLEX Series Firmwareversion=version 4.50 through 5.37 Patch 1
Zyxel Networks · USG FLEX 50/50W Seriesversion=version 4.16 through 5.37 Patch 1

Published CVSS scores

5.7Zyxel CorporationCVSS:3.1/AV:A/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectoradjacent
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-134Use of Externally-Controlled Format String
T1499.004Endpoint Denial of Service: Application or System Exploitation

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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