---
id: CVE-2026-5856
title: >-
  Contiki-NG's DNS/mDNS resolver skip_name() in os/services/resolv/resolv.c
  walks DNS wire-format name labels with no packet-boundary check, and the
  caller in newdata() invokes it in a loop iterating nquestions times from the
  attacker-cont…
summary: >-
  Contiki-NG's DNS/mDNS resolver skip_name() in os/services/resolv/resolv.c
  walks DNS wire-format name labels with no packet-boundary check, and the
  caller in newdata() invokes it in a loop iterating nquestions times from the
  attacker-cont…
severity: high
cvss: 7.1
cvssVector: 'CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:L'
cwe:
  - CWE-125
published: '2026-08-06'
updated: '2026-09-16'
sourceUpdated: '2026-09-16T20:21:01.047'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-5856'
references:
  - url: 'https://github.com/contiki-ng/contiki-ng'
    label: disclosure@vulncheck.com
  - url: >-
      https://github.com/contiki-ng/contiki-ng/commit/04a3f0a0067d2ee87d1f297b6d0f4392d8c98ffe
    label: disclosure@vulncheck.com
  - url: 'https://github.com/contiki-ng/contiki-ng/pull/3169'
    label: disclosure@vulncheck.com
tags:
  - nvd
epss: 0.00291
epssPercentile: 0.19291
ingestedAt: '2026-09-16T21:05:36.835Z'
---

## Overview

Contiki-NG's DNS/mDNS resolver skip_name() in os/services/resolv/resolv.c walks DNS wire-format name labels with no packet-boundary check, and the caller in newdata() invokes it in a loop iterating nquestions times from the attacker-controlled DNS header before validating the transaction ID. An attacker who sets nquestions higher than the number of complete questions present causes skip_name() to walk past the UDP packet buffer, and the returned pointer is cast to struct dns_answer * for further memory reads. On builds with RESOLV_CONF_SUPPORTS_MDNS enabled, any peer on the local segment can trigger the read unauthenticated via a multicast UDP 5353 packet with no outstanding query required; on standard DNS builds an attacker who can inject a UDP response from port 53 during an outstanding query can trigger the same read. Impact is out-of-bounds read of uip_buf and adjacent memory, disclosing memory contents or crashing the resolver.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.