CVE-2026-79981
Publiée le 2026-10-23 · Dernière modification le 2026-10-26
Le résolveur DNS (stub resolver) de cette pile réseau ne parvient pas à randomiser le port source des requêtes sortantes, utilisant un port unique et fixe pendant toute la durée du contexte du résolveur. Comme seul l'identifiant de transaction sur 16 bits change à chaque requête, l'entropie totale protégeant la requête est nettement inférieure aux normes de l'industrie. Un attaquant peut facilement découvrir ce port fixe en observant une seule requête ou en sondant la plage de ports UDP de l'appareil pour identifier le seul port qui ne renvoie pas de message ICMP « destination unreachable ».
Un attaquant distant (off-path) disposant d'un accès réseau peut inonder l'appareil de réponses DNS forgées afin de gagner la course contre le serveur légitime. S'il réussit, il peut rediriger votre trafic vers une adresse IP malveillante, compromettant ainsi l'intégrité de vos connexions réseau. Bien que cela ne nécessite ni authentification ni interaction de l'utilisateur, l'exécution fiable de cette attaque est difficile et elle est largement atténuée si vos applications utilisent la validation de certificats TLS pour vérifier l'identité du point de terminaison résolu.
DÉTAIL DU VECTEUR CVSS
RÉFÉRENCES
Description officielle du NVD (en anglais)
The DNS stub resolver in subsys/net/lib/dns/resolve.c binds one UDP socket per configured server when the resolver context is initialised (dns_resolve_init_locked() -> register_dispatcher() -> dns_dispatcher_register()), with the local port left at 0 so the stack picks an ephemeral port exactly once. dns_write() then sends every subsequent query from that same socket without re-binding, so the source port is constant for the lifetime of the resolver context while only the 16-bit transaction identifier (ctx->queries[i].id, drawn with sys_rand16_get()) varies per query. RFC 5452 section 9.2 requires both to vary; with a fixed port, an off-path attacker has to guess only the identifier. The stable port is cheap to learn. Any single observed query reveals it permanently, and it can also be found by probing the ephemeral UDP range: subsys/net/ip/connection.c answers unmatched unicast UDP with an ICMP port-unreachable, so the resolver's socket is the one port in that range that stays silent. Acceptance of a reply is decided solely by get_slot_by_id() matching the transaction identifier and a CRC-16 of the queried name — the reply's source address is never checked (dispatcher_cb() does ARG_UNUSED(addr)), so forged answers need not even be spoofed to appear to come from the configured server; they only need to reach the resolver's port. An attacker with UDP reachability to the device (same LAN, Wi-Fi or mesh segment, or a globally routable device address) can therefore flood answers for a predictable query name across the 16-bit identifier space during a query window and race the legitimate server. A win makes the application resolve an attacker-chosen address, and with CONFIG_DNS_RESOLVER_CACHE enabled the forged record is retained for its attacker-chosen TTL. The attack still requires winning that race, and applications that validate TLS certificates for the resolved endpoint are largely protected. The fix draws a fresh local port before each query to a server that has nothing outstanding: dns_randomize_source_port() unregisters the dispatcher, closes and reopens the socket, and re-registers it bound to port 0 so the system selects a new port, raising the per-query guessing space from roughly 16 to roughly 31 bits. Multicast DNS and LLMNR are excluded because their ports are protocol-fixed. The behaviour is controlled by CONFIG_DNS_RESOLVER_RANDOMIZE_SOURCE_PORT (default y).