Affected: Go: github.com/coredns/coredns

### Summary

CoreDNS's DNS-over-HTTPS (DoH) GET path accepts oversized `dns=` query values and performs substantial request parsing, query unescaping, base64 decoding, and message unpacking work before returning `400 Bad Request`.

A remote, unauthenticated attacker can repeatedly send oversized DoH GET requests to `/dns-query?dns=...` and force high CPU usage, large transient allocations, elevated garbage-collection pressure, and increased resident memory consumption even though the requests are ultimately rejected.

This is a denial-of-service issue caused by expensive pre-validation processing on the DoH GET path.

### Details

The vulnerable flow is in `plugin/pkg/doh/doh.go`:

- `RequestToMsg()` dispatches GET requests to `requestToMsgGet()`: - `plugin/pkg/doh/doh.go:79-89` - `requestToMsgGet()` calls `req.URL.Query()`, extracts `dns`, and passes it directly to `base64ToMsg()`: - `plugin/pkg/doh/doh.go:99-108` - `base64ToMsg()` decodes the full attacker-controlled value via `b64Enc.DecodeString()` and only then attempts to unpack it into a DNS message: - `plugin/pkg/doh/doh.go:121-130`

Relevant snippet:

```go func requestToMsgGet(req *http.Request) (*dns.Msg, error) { values := req.URL.Query() b64, ok := values["dns"] if !ok { return nil, fmt.Errorf("no 'dns' query parameter found") } if len(b64) != 1 { return nil, fmt.Errorf("multiple 'dns' query values found") } return base64ToMsg(b64[0]) }

func base64ToMsg(b64 stri…