ipn/localapi: fix validHost parsing logic

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

ipn/localapi/localapi.go

@@ -179,6 +179,10 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+// validLocalHost allows either localhost or loopback IP hosts on platforms
+// that use token security.
+var validLocalHost = runtime.GOOS == "darwin" || runtime.GOOS == "ios" || runtime.GOOS == "android"
+
 // validHost reports whether h is a valid Host header value for a LocalAPI request.
 func validHost(h string) bool {
 	// The client code sends a hostname of "local-tailscaled.sock".
@@ -186,7 +190,9 @@ func validHost(h string) bool {
 	case "", apitype.LocalAPIHost:
 		return true
 	}
-	// Allow either localhost or loopback IP hosts.
+	if !validLocalHost {
+		return false
+	}
 	host, _, err := net.SplitHostPort(h)
 	if err != nil {
 		return false
@@ -194,7 +200,7 @@ func validHost(h string) bool {
 	if host == "localhost" {
 		return true
 	}
-	addr, err := netip.ParseAddr(h)
+	addr, err := netip.ParseAddr(host)
 	if err != nil {
 		return false
 	}

ipn/localapi/localapi_test.go

@@ -0,0 +1,34 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package localapi
+
+import (
+	"testing"
+
+	"tailscale.com/client/tailscale/apitype"
+)
+
+func TestValidHost(t *testing.T) {
+	tests := []struct {
+		host  string
+		valid bool
+	}{
+		{"", true},
+		{apitype.LocalAPIHost, true},
+		{"localhost:9109", validLocalHost},
+		{"127.0.0.1:9110", validLocalHost},
+		{"[::1]:9111", validLocalHost},
+		{"100.100.100.100:41112", false},
+		{"10.0.0.1:41112", false},
+		{"37.16.9.210:41112", false},
+	}
+
+	for _, test := range tests {
+		t.Run(test.host, func(t *testing.T) {
+			if got := validHost(test.host); got != test.valid {
+				t.Errorf("validHost(%q)=%v, want %v", test.host, got, test.valid)
+			}
+		})
+	}
+}