CVE-2026-79981CVE-2026-79981

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