---
id: "CVE-2026-123603"
page: "https://infrastack.ai/live/CVE-2026-123603"
kind: "vulnerability"
aliases: ["CVE-2026-123603"]
title: "Hickory DNS: IPv4-mapped IPv6 addresses bypass answer and access filters"
severity: "medium"
cvss: 6.9
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["dns", "hickory", "package", "rust", "self-hosted"]
not_affected_if: ["ipv4-only", "no-deny-rules", "not-using-hickory"]
published_at: "2026-11-08T02:16:55+00:00"
first_seen_at: "2026-11-08T02:21:14+00:00"
updated_at: "2026-11-08T02:24:37+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-123603", "first_seen_at": "2026-11-08T02:21:14+00:00"}]
exploit_sources: []
---

# Hickory DNS: IPv4-mapped IPv6 addresses bypass answer and access filters

Link for people: https://infrastack.ai/live/CVE-2026-123603 (this .md is for agents)

Several Hickory DNS Rust crates fail to normalize IPv4-mapped IPv6 addresses when checking deny_answers, deny_server and access control rules. An attacker can use such addresses to slip past DNS answer filtering, reach denied nameservers, or query a dual-stack hickory-server from clients it should block.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the crate versions in `Cargo.lock` for `hickory-proto`, `hickory-resolver`, `hickory-server` and `hickory-recursor`.
- Look for `deny_answers`, `deny_server` or `AccessControlSet` in your Hickory configuration and code.
- Check whether the server listens on IPv6 dual-stack sockets.

## Fix

- `hickory-proto` and `hickory-resolver`: `0.26.2` (from 0.26.0 before 0.26.2)
- `hickory-server`: `0.26.2` (from 0.25.0-alpha.1 before 0.26.2)
- `hickory-recursor`: unknown (affected range 0.25.0-alpha.3 through 0.26.0-alpha.1; the sources do not name a fixed version)

## Timeline

- 2026-11-08T02:21:14+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-123603

## Documents

- [advisory] Vendor advisory: github.com https://github.com/hickory-dns/hickory-dns/security/advisories/GHSA-4jwp-xjwm-333g
- [advisory] Third-party advisory: www.vulncheck.com https://www.vulncheck.com/advisories/hickory-dns-missing-ipv4-mapped-ipv6-canonicalization-access-control-bypass

## From the sources

hickory-proto and hickory-resolver 0.26.0 before 0.26.2, hickory-server 0.25.0-alpha.1 before 0.26.2, and hickory-recursor 0.25.0-alpha.3 through 0.26.0-alpha.1 fail to canonicalize IPv4-mapped IPv6 addresses when checking deny_answers, deny_server, or AccessControlSet rules. Attackers can use IPv4-mapped addresses to evade answer filtering, trigger queries to denied nameservers, or reach hickory-server library deployments on dual-stack IPv6 sockets from denied clients.

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_Free to read, with fair use of about 60 pages a minute. For higher limits or API access: sales@infrastack.ai_