diff --git a/modules/Makefile.am b/modules/Makefile.am
index 2a6fc8a45..ff6da3b38 100644
--- a/modules/Makefile.am
+++ b/modules/Makefile.am
@@ -263,6 +263,7 @@ federation_federation_make_leave_la_SOURCES = federation/make_leave.cc
 federation_federation_send_leave_la_SOURCES = federation/send_leave.cc
 federation_federation_backfill_la_SOURCES = federation/backfill.cc
 federation_federation_backfill_ids_la_SOURCES = federation/backfill_ids.cc
+federation_federation_event_auth_la_SOURCES = federation/event_auth.cc
 federation_federation_federation_la_SOURCES = federation/federation.cc
 
 federation_module_LTLIBRARIES = \
@@ -282,6 +283,7 @@ federation_module_LTLIBRARIES = \
 	federation/federation_send_leave.la \
 	federation/federation_backfill.la \
 	federation/federation_backfill_ids.la \
+	federation/federation_event_auth.la \
 	federation/federation_federation.la \
 	###
 
diff --git a/modules/federation/event_auth.cc b/modules/federation/event_auth.cc
new file mode 100644
index 000000000..198472223
--- /dev/null
+++ b/modules/federation/event_auth.cc
@@ -0,0 +1,145 @@
+// Matrix Construct
+//
+// Copyright (C) Matrix Construct Developers, Authors & Contributors
+// Copyright (C) 2016-2018 Jason Volk <jason@zemos.net>
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice is present in all copies. The
+// full license for this software is available in the LICENSE file.
+
+using namespace ircd;
+
+mapi::header
+IRCD_MODULE
+{
+	"federation event_auth (undocumented)"
+};
+
+resource
+event_auth_resource
+{
+	"/_matrix/federation/v1/event_auth/",
+	{
+		"federation event_auth",
+		resource::DIRECTORY,
+	}
+};
+
+conf::item<size_t>
+event_auth_flush_hiwat
+{
+	{ "name",     "ircd.federation.event_auth.flush.hiwat" },
+	{ "default",  16384L                                   },
+};
+
+resource::response
+get__event_auth(client &client,
+                const resource::request &request)
+{
+	if(request.parv.size() < 1)
+		throw m::NEED_MORE_PARAMS
+		{
+			"room_id path parameter required"
+		};
+
+	m::room::id::buf room_id
+	{
+		url::decode(request.parv[0], room_id)
+	};
+
+	if(request.parv.size() < 2)
+		throw m::NEED_MORE_PARAMS
+		{
+			"event_id path parameter required"
+		};
+
+	m::event::id::buf event_id
+	{
+		url::decode(request.parv[1], event_id)
+	};
+
+	const m::event::fetch event
+	{
+		event_id
+	};
+
+	const m::room room
+	{
+		room_id, event_id
+	};
+
+	if(!room.visible(request.node_id))
+		throw m::ACCESS_DENIED
+		{
+			"You are not permitted to view the room at this event"
+		};
+
+	const m::room::state state
+	{
+		room
+	};
+
+	const unique_buffer<mutable_buffer> buf
+	{
+		96_KiB
+	};
+
+	resource::response::chunked response
+	{
+		client, http::OK
+	};
+
+	const auto flush{[&response]
+	(const const_buffer &buf)
+	{
+		response.write(buf);
+		return buf;
+	}};
+
+	json::stack out
+	{
+		buf, flush, size_t(event_auth_flush_hiwat)
+	};
+
+	json::stack::object top{out};
+	json::stack::member auth_chain_m
+	{
+		top, "auth_chain"
+	};
+
+	json::stack::array auth_chain
+	{
+		auth_chain_m
+	};
+
+	const auto append{[&auth_chain]
+	(const m::event &event)
+	{
+		if(json::get<"event_id"_>(event))
+			auth_chain.append(event);
+	}};
+
+	const auto append_with_sender{[&append, &state, &event]
+	(const m::event &event_)
+	{
+		append(event_);
+		if(json::get<"sender"_>(event_) != json::get<"sender"_>(event))
+			state.get(std::nothrow, "m.room.member", json::get<"sender"_>(event_), append);
+	}};
+
+	state.get(std::nothrow, "m.room.create", "", append);
+	state.get(std::nothrow, "m.room.join_rules", "", append);
+	state.get(std::nothrow, "m.room.power_levels", "", append_with_sender);
+	state.get(std::nothrow, "m.room.member", json::get<"sender"_>(event), append);
+	return {};
+}
+
+resource::method
+method_get
+{
+	event_auth_resource, "GET", get__event_auth,
+	{
+		method_get.VERIFY_ORIGIN
+	}
+};