# CVE-2026-123603: Hickory DNS IPv4-mapped IPv6 canonicalization bypass

Source: https://junglewise.ai/threats/cve-2026-123603-hickory-proto-and-hickory-resolver-0-26-0-before-0-26-2-hickory  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-11-08. Updated: 2026-11-08  
License: CC BY 4.0 with a link back

CVE-2026-123603 · Severity: medium · CVSS 6.5 · Published 2026-11-08

Hickory DNS is a DNS resolver and server library. Several Hickory components fail to properly validate IPv4-mapped IPv6 addresses against access control rules, allowing attackers to bypass answer filters, reach denied nameservers, or access restricted server instances. This could expose DNS answers that should be filtered or allow unauthorized clients to connect to DNS servers.

Technologies: [Hickory-Dns Hickory-Proto](https://junglewise.ai/threats/technologies/hickory-dns-hickory-proto), [Hickory-Dns Hickory-Recursor](https://junglewise.ai/threats/technologies/hickory-dns-hickory-recursor), [Hickory-Dns Hickory-Resolver](https://junglewise.ai/threats/technologies/hickory-resolver). Vendors: [Hickory-Dns](https://junglewise.ai/threats/vendors/hickory-dns).

## Quick answers

**Is CVE-2026-123603 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-123603 being exploited in the wild.

**How severe is CVE-2026-123603?** Medium severity, with a CVSS score of 6.5, published 8 November 2026.

**Which products and versions are affected?** Hickory-Dns Hickory-Proto 0.26.0 before 0.26.2; Hickory-Dns Hickory-Recursor 0.25.0-alpha.3 through 0.26.0-alpha.1; Hickory-Dns Hickory-Resolver 0.26.0 before 0.26.2; Hickory-Dns Hickory-Server 0.25.0-alpha.1 before 0.26.2.

**Is there a fix for CVE-2026-123603?** Yes: fixed in hickory-proto 0.26.2, hickory-resolver 0.26.2, hickory-server 0.26.2. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-123603 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-123603?** Check whether you run Hickory-Dns Hickory-Proto, Hickory-Dns Hickory-Recursor, Hickory-Dns Hickory-Resolver or one of the others listed at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- Fixed in hickory-proto 0.26.2, hickory-resolver 0.26.2, hickory-server 0.26.2

## Technical details

The vulnerability stems from improper canonicalization of IPv4-mapped IPv6 addresses (e.g., ::ffff:192.0.2.1) when validating against deny_answers, deny_server, and AccessControlSet rules. Attackers can craft IPv6 addresses in IPv4-mapped form to bypass these filters. The issue affects hickory-proto, hickory-resolver, hickory-server, and hickory-recursor libraries when these access control features are enabled; affected products require non-default configuration or feature flags in most cases.

## Affected products

- hickory-dns hickory-proto 0.26.0 before 0.26.2
- hickory-dns hickory-resolver 0.26.0 before 0.26.2
- hickory-dns hickory-server 0.25.0-alpha.1 before 0.26.2
- hickory-dns hickory-recursor 0.25.0-alpha.3 through 0.26.0-alpha.1

## Timeline

- 2026-09-03: disclosed: Advisory GHSA-4jwp-xjwm-333g published
- 2026-11-08: advisory: CVE-2026-123603 assigned

## References

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

## Related threats

- [Hickory DNS hickory-proto denial of service in BinEncoder](https://junglewise.ai/threats/cve-2026-123608-hickory-dns-hickory-proto-from-0-3-1-before-0-26-1-contains-a)
- [Hickory DNS NSEC3 closest-encloser proof unbounded loop](https://junglewise.ai/threats/cve-2026-123607-hickory-dns-hickory-proto-0-25-0-alpha-3-through-0-25-2-and)
- [hickory-dns recursor glueless-NS referral amplification](https://junglewise.ai/threats/cve-2026-123606-the-hickory-dns-recursor-in-hickory-recursor-0-25-0-alpha-3)
- [Hickory DNS DNSSEC validation bypass via RRSIG Signer's Name](https://junglewise.ai/threats/cve-2026-123605-hickory-dns-hickory-proto-0-1-through-0-25-0-alpha-3-and-hickory)
- [Hickory DNS hickory-resolver unbounded retry loop resource exhaustion](https://junglewise.ai/threats/cve-2026-123604-hickory-dns-hickory-resolver-0-26-0-beta-1-before-0-26-2)