CVE-2026-86003

### Summary CoreDNS accepted [RFC 2136](https://datatracker.ietf.org/doc/html/rfc2136) UPDATE messages over DoH, DoH3, DoQ, and DNS-over-gRPC, then allowed the `proxy`/`forward` plugin to send them unchanged to an upstream DNS server. UDP, TCP, and DoT rejected the same opcode before plugin dispatch. If an update-capable upstream trusts CoreDNS's source address or authenticated connection instead of requiring end-to-end TSIG, an unauthenticated client can use CoreDNS to add, replace, or delete DNS records. ### Details The affected listeners called `dns.Msg.Unpack` without the request policy used by the UDP/TCP server: - [DoH and DoH3](https://github.com/coredns/coredns/blob/18a58b9e898ccd95c3f8ee72a37b95b3d1e3e928/plugin/pkg/doh/doh.go#L134-L155) - [DoQ](https://github.com/coredns/coredns/blob/18a58b9e898ccd95c3f8ee72a37b95b3d1e3e928/core/dnsserver/server_quic.go#L212-L219) - [DNS-over-gRPC](https://github.com/coredns/coredns/blob/18a58b9e898ccd95c3f8ee72a37b95b3d1e3e928/core/dnsserver/server_grpc.go#L176-L184) CoreDNS routed the message using its Zone question without checking the opcode. [`forward`](https://github.com/coredns/coredns/blob/18a58b9e898ccd95c3f8ee72a37b95b3d1e3e928/plugin/forward/forward.go#L113-L118) then passed the original message to the [upstream](https://github.com/coredns/coredns/blob/18a58b9e898ccd95c3f8ee72a37b95b3d1e3e928/plugin/pkg/proxy/connect.go#L150-L167). By contrast, [`dns.DefaultMsgAcceptFunc`](https://github.com/miekg/dns/blob/v1.1.72/acceptfunc.go#L33-L57) allows only QUERY and NOTIFY. The fix applies that policy to the raw header via [`dnsutil.UnpackRequest`](https://github.com/coredns/coredns/blob/530b0a5ff2ad68cc0421f10dd93568945cc671c9/plugin/pkg/dnsutil/message.go#L13-L23) before any affected transport dispatches the request. ### PoC The reproducer starts a standard-library synthetic DNS upstream on loopback, sends an unsigned UPDATE over DoH, and reports whether the upstream received the record. It supports both UDP