Block custom internal endpoint that should never be called

- Is not spec'd
- Will not be spec'd
- Is 100% internal as per its authors

src/main/java/io/kamax/mxisd/HttpMxisd.java

@@ -21,6 +21,7 @@
 package io.kamax.mxisd;
 
 import io.kamax.mxisd.config.MxisdConfig;
+import io.kamax.mxisd.http.undertow.handler.InternalInfoHandler;
 import io.kamax.mxisd.http.undertow.handler.OptionsHandler;
 import io.kamax.mxisd.http.undertow.handler.SaneHandler;
 import io.kamax.mxisd.http.undertow.handler.as.v1.AsNotFoundHandler;
@@ -117,6 +118,9 @@ public void start() {
                 .put(AsTransactionHandler.Path, asTxnHandler)
                 .put("/transactions/{" + AsTransactionHandler.ID + "}", asTxnHandler) // Legacy endpoint
 
+                // Banned endpoints
+                .get(InternalInfoHandler.Path, SaneHandler.around(new InternalInfoHandler()))
+
         ).build();
 
         httpSrv.start();

src/main/java/io/kamax/mxisd/http/undertow/handler/InternalInfoHandler.java

@@ -0,0 +1,50 @@
+/*
+ * mxisd - Matrix Identity Server Daemon
+ * Copyright (C) 2019 Kamax Sarl
+ *
+ * https://www.kamax.io/
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package io.kamax.mxisd.http.undertow.handler;
+
+import io.undertow.server.HttpServerExchange;
+
+import java.util.concurrent.ThreadLocalRandom;
+
+public class InternalInfoHandler extends BasicHttpHandler {
+
+    /*
+     * This endpoint should never be called as being entierly custom as per instructions of New Vector,
+     * the author of that endpoint.
+     *
+     * Used for the first time at https://github.com/matrix-org/synapse/pull/4681/files#diff-a73c645c44a17da6ab70f256da6b60afR41
+     *
+     * Full context: https://matrix.to/#/!YkZelGRiqijtzXZODa:matrix.org/$15510967621328WMKVu:kamax.io?via=matrix.org
+     * Room name: #matrix-spec
+     * Room alias: #matrix-spec:matrix.org
+     */
+    public static final String Path = "/_matrix/identity/api/{version}/internal-info";
+
+    @Override
+    public void handleRequest(HttpServerExchange exchange) throws Exception {
+        // We will return a random status code in all possible error codes
+        int type = ThreadLocalRandom.current().nextInt(4, 6) * 100; // Random 4 or 5, times 100
+        int status = type + ThreadLocalRandom.current().nextInt(0, 100); // Random 0 to 99
+
+        respond(exchange, status, "M_FORBIDDEN", "This endpoint is under quarantine and possibly wrongfully labeled stable.");
+    }
+
+}