From eab87ef07dd9461e38b716b7c4b4c95cedee0ca8 Mon Sep 17 00:00:00 2001
From: Neil Alexander <neilalexander@users.noreply.github.com>
Date: Tue, 2 Aug 2022 10:22:17 +0100
Subject: [PATCH] Stronger checks for `/send_join` (#2604)

---
 federationapi/routing/join.go | 32 ++++++++++++++++++++++++--------
 1 file changed, 24 insertions(+), 8 deletions(-)

diff --git a/federationapi/routing/join.go b/federationapi/routing/join.go
index 41004cf5..30406a15 100644
--- a/federationapi/routing/join.go
+++ b/federationapi/routing/join.go
@@ -202,6 +202,14 @@ func SendJoin(
 		}
 	}
 
+	// Check that the event is from the server sending the request.
+	if event.Origin() != request.Origin() {
+		return util.JSONResponse{
+			Code: http.StatusForbidden,
+			JSON: jsonerror.Forbidden("The join must be sent by the server it originated on"),
+		}
+	}
+
 	// Check that a state key is provided.
 	if event.StateKey() == nil || event.StateKeyEquals("") {
 		return util.JSONResponse{
@@ -216,6 +224,22 @@ func SendJoin(
 		}
 	}
 
+	// Check that the sender belongs to the server that is sending us
+	// the request. By this point we've already asserted that the sender
+	// and the state key are equal so we don't need to check both.
+	var domain gomatrixserverlib.ServerName
+	if _, domain, err = gomatrixserverlib.SplitID('@', event.Sender()); err != nil {
+		return util.JSONResponse{
+			Code: http.StatusForbidden,
+			JSON: jsonerror.Forbidden("The sender of the join is invalid"),
+		}
+	} else if domain != request.Origin() {
+		return util.JSONResponse{
+			Code: http.StatusForbidden,
+			JSON: jsonerror.Forbidden("The sender of the join must belong to the origin server"),
+		}
+	}
+
 	// Check that the room ID is correct.
 	if event.RoomID() != roomID {
 		return util.JSONResponse{
@@ -242,14 +266,6 @@ func SendJoin(
 		}
 	}
 
-	// Check that the event is from the server sending the request.
-	if event.Origin() != request.Origin() {
-		return util.JSONResponse{
-			Code: http.StatusForbidden,
-			JSON: jsonerror.Forbidden("The join must be sent by the server it originated on"),
-		}
-	}
-
 	// Check that this is in fact a join event
 	membership, err := event.Membership()
 	if err != nil {