How severe is CVE-2026-39409?

No CVSS score has been assigned to CVE-2026-39409 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-39409?

CVE-2026-39409 affects the following packages: hono (npm). Ecosystems affected: npm.

How do I fix CVE-2026-39409?

Update hono to 4.12.12 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-39409 is resolved across your whole dependency graph.

How do I detect CVE-2026-39409 in my npm dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for hono, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-39409 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2026-39409 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-39409 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-39409?

CVE-2026-39409 has an EPSS (Exploit Prediction Scoring System) score of 0.3%, placing it in the 29th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-39409?

CVE-2026-39409 is classified as CWE-180 (CWE-180). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-39409 published, and has it been updated?

CVE-2026-39409 was published on April 8, 2026 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

📦 npm

Not in CISA KEV

CVE-2026-39409 — hono

Fix: honojs/hono@48fa223

CVE-2026-39409 is a CWE-180 vulnerability in hono. A fix is available for hono — see the affected versions and patch details below.

Hono has incorrect IP matching in ipRestriction() for IPv4-mapped IPv6 addresses

Also known asGHSA-xpcf-pg52-r92g

Published

Apr 8, 2026

Updated

Oct 28, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 7, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-39409.

EPSS Exploitation Probability

via FIRST.org ↗

0.3%probability of exploitation in next 30 days

Lower Risk+0.01%

Lower risk than most CVEs29th percentile — riskier than 29% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

1 pkg affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.

7Kother npm packages depend on this — each one inherits the vulnerability until it's patched upstream

hononpm

104.5Mdownloads / week

Description

Summary

ipRestriction() does not canonicalize IPv4-mapped IPv6 client addresses (e.g. ::ffff:127.0.0.1) before applying IPv4 allow or deny rules. In environments such as Node.js dual-stack, this can cause IPv4 rules to fail to match, leading to unintended authorization behavior.

Details

The middleware classifies client addresses based on their textual form. Addresses containing ":" are treated as IPv6, including IPv4-mapped IPv6 addresses such as ::ffff:127.0.0.1. These addresses are not normalized to IPv4 before matching.

As a result:

IPv4 static rules (e.g. 127.0.0.1) do not match because the raw string differs
IPv4 CIDR rules (e.g. 127.0.0.0/8, 10.0.0.0/8) are skipped because the address is treated as IPv6

For example, with:

denyList: ['127.0.0.1']

a request from 127.0.0.1 may be represented as ::ffff:127.0.0.1 and bypass the deny rule.

This behavior commonly occurs in Node.js environments where IPv4 clients are exposed as IPv4-mapped IPv6 addresses.

Impact

Applications that rely on IPv4-based ipRestriction() rules may incorrectly allow or deny requests.

In affected deployments, a denied IPv4 client may bypass access restrictions. Conversely, legitimate clients may be rejected when using IPv4 allow lists.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`hono`	all versions	4.12.12`npm install hono@4.12.12`

Affected Products

1 product · 1 configurations

Application

honohono

≤ 4.12.11

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for hono, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update hono to 4.12.12 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-39409 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

## Summary `ipRestriction()` does not canonicalize IPv4-mapped IPv6 client addresses (e.g. `::ffff:127.0.0.1`) before applying IPv4 allow or deny rules. In environments such as Node.js dual-stack, this can cause IPv4 rules to fail to match, leading to unintended authorization behavior. ## Details The middleware classifies client addresses based on their textual form. Addresses containing "`:`" are treated as IPv6, including IPv4-mapped IPv6 addresses such as `::ffff:127.0.0.1`. These addresses are not normalized to IPv4 before matching. As a result: * IPv4 static rules (e.g. `127.0.0.1`)

O3 Security · Impact-Aware SCA

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