CVE-2026-39409Network attack vector

Authorization bypass via uncanonicalized IPv4-mapped IPv6 addresses

Published Apr 8, 2026 · Updated Apr 8, 2026

IP restriction bypass in Hono before 4.12.12 allows remote attackers to access denied resources via IPv4-mapped IPv6 addresses. The ipRestriction() matcher classifies colon-containing addresses as IPv6, compares their raw text against static rules, and skips IPv4 CIDR rules without first converting mapped addresses to IPv4. Exploitation requires IPv4 allow or deny rules and an environment that presents an attacker's IPv4 source as mapped IPv6; denied clients can reach protected endpoints, while allowlisted clients can be rejected.

CVSS severity6.3
Medium
EPSS probability0.34%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

IP restriction bypass in Hono before 4.12.12 allows remote attackers to access denied resources via IPv4-mapped IPv6 addresses. The ipRestriction() matcher classifies colon-containing addresses as IPv6, compares their raw text against static rules, and skips IPv4 CIDR rules without first converting mapped addresses to IPv4. Exploitation requires IPv4 allow or deny rules and an environment that presents an attacker's IPv4 source as mapped IPv6; denied clients can reach protected endpoints, while allowlisted clients can be rejected.

The record

CVE
CVE-2026-39409
Published
Apr 8, 2026
Updated
Apr 8, 2026
Vendor
Hono
Product
Hono
Classifications
CWE-180, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 8, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 8, 2026Record 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=< 4.12.12

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Hono · Honoversion=< 4.12.12

Published CVSS scores

6.3GitHubCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N
5.3NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/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)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-180Incorrect Behavior Order: Validate Before Canonicalize
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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