{"id":"CVE-2026-5856","summary":"Contiki-NG DNS/mDNS Resolver Out-of-Bounds Read via Unchecked skip_name Traversal Before Transaction-ID Validation","details":"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.","modified":"2026-08-12T03:51:43.062467748Z","published":"2026-08-06T18:47:38.653Z","database_specific":{"cwe_ids":["CWE-125"],"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/5xxx/CVE-2026-5856.json","unresolved_ranges":[{"extracted_events":[{"fixed":"04a3f0a0067d2ee87d1f297b6d0f4392d8c98ffe"}],"source":"AFFECTED_FIELD"}],"cna_assigner":"VulnCheck"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/5xxx/CVE-2026-5856.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-5856"},{"type":"REPORT","url":"https://github.com/contiki-ng/contiki-ng/pull/3169"},{"type":"FIX","url":"https://github.com/contiki-ng/contiki-ng/commit/04a3f0a0067d2ee87d1f297b6d0f4392d8c98ffe"},{"type":"PACKAGE","url":"https://github.com/contiki-ng/contiki-ng"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://github.com/contiki-ng/contiki-ng","events":[{"introduced":"0"},{"fixed":"04a3f0a0067d2ee87d1f297b6d0f4392d8c98ffe"}],"database_specific":{"source":"REFERENCES"}}],"versions":["develop/v5.1","develop/v5.0","develop/v4.9","develop/v4.8","develop/v4.7","develop/v4.6","develop/v4.5","develop/v4.4","develop/v4.3","develop/v4.2","develop/v4.1","develop/v4.0","fork","develop/v3.x-fork","old/v2.6","2.6-rc0","2.6","2.5-release","old/v2.4","2.4","old/v2.3","2.3","old/v2.2.3","2.2.3","old/v2.2.2","2.2.2","old/v2.2.1","2.2.1","old/v2.2","2.2","old/v2.1","2.1","old/v2.0","2.0"],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-5856.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","score":"CVSS:4.0/AV:A/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:L/SC:N/SI:N/SA:N"}]}