diff -r a7221ada9368 -r 564e897f0790 plugins/muc/muc.lib.lua
--- a/plugins/muc/muc.lib.lua	Thu May 17 17:11:00 2018 +0200
+++ b/plugins/muc/muc.lib.lua	Thu May 17 17:11:25 2018 +0200
@@ -1076,6 +1076,9 @@
 	for jid, value in pairs(jid_affiliation) do
 		local affiliation = value["affiliation"];
 
+		if jid ~= jid_bare(jid) then
+			return false, "modify", "not-acceptable";
+		end
 		jid = jid_bare(jid);
 		if affiliation == "none" then affiliation = nil; end
 		if affiliation and affiliation ~= "outcast" and affiliation ~= "owner" and affiliation ~= "admin" and affiliation ~= "member" then