CVE-2026-5856 Contiki-NG DNS/mDNS Resolver Out-of-Bounds Read via Unchecked skip_name Traversal Before Transaction-ID Validation
An attacker can trigger an out-of-bounds read in the Contiki-NG DNS resolver, potentially disclosing memory contents or causing a denial of service.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-5856?
CVE-2026-5856 is a high-severity vulnerability in Contiki-NG, published on 6 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
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.
Weakness: CWE-125
Which versions are affected by CVE-2026-5856?
How do I fix CVE-2026-5856?
No fix has been published for CVE-2026-5856 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-5856 being exploited?
There is no known exploitation of CVE-2026-5856 in the wild and no public exploit code.
Sources
Also known as GHSA-8m55-g479-xj8c.