diff --git a/apps/emqx/src/emqx_frame.erl b/apps/emqx/src/emqx_frame.erl
index a86bb0155..10f0aa135 100644
--- a/apps/emqx/src/emqx_frame.erl
+++ b/apps/emqx/src/emqx_frame.erl
@@ -288,11 +288,15 @@ parse_connect(FrameBin, StrictMode) ->
 parse_connect2(
     ProtoName,
     <<BridgeTag:4, ProtoVer:4, UsernameFlag:1, PasswordFlag:1, WillRetain:1, WillQoS:2, WillFlag:1,
-        CleanStart:1, 0:1, KeepAlive:16/big, Rest2/binary>>,
+        CleanStart:1, Reserved:1, KeepAlive:16/big, Rest2/binary>>,
     StrictMode
 ) ->
+    case Reserved of
+        0 -> ok;
+        1 -> ?PARSE_ERR(reserved_connect_flag)
+    end,
     {Properties, Rest3} = parse_properties(Rest2, ProtoVer, StrictMode),
-    {ClientId, Rest4} = parse_utf8_string_with_hint(Rest3, StrictMode, invalid_username),
+    {ClientId, Rest4} = parse_utf8_string_with_hint(Rest3, StrictMode, invalid_clientid),
     ConnPacket = #mqtt_packet_connect{
         proto_name = ProtoName,
         proto_ver = ProtoVer,
@@ -324,7 +328,7 @@ parse_connect2(
         <<>> ->
             ConnPacket1#mqtt_packet_connect{username = Username, password = Password};
         _ ->
-            ?PARSE_ERR(malformed_connect_payload)
+            ?PARSE_ERR(malformed_connect_data)
     end;
 parse_connect2(_ProtoName, _, _) ->
     ?PARSE_ERR(malformed_connect_header).
@@ -340,6 +344,7 @@ parse_packet(
     <<AckFlags:8, ReasonCode:8, Rest/binary>>,
     #{version := Ver, strict_mode := StrictMode}
 ) ->
+    %% Not possible for broker to receive!
     case parse_properties(Rest, Ver, StrictMode) of
         {Properties, <<>>} ->
             #mqtt_packet_connack{
diff --git a/apps/emqx/test/emqx_frame_SUITE.erl b/apps/emqx/test/emqx_frame_SUITE.erl
index f6296359f..158318b07 100644
--- a/apps/emqx/test/emqx_frame_SUITE.erl
+++ b/apps/emqx/test/emqx_frame_SUITE.erl
@@ -57,7 +57,13 @@ groups() ->
             t_serialize_parse_v5_connect,
             t_serialize_parse_connect_without_clientid,
             t_serialize_parse_connect_with_will,
-            t_serialize_parse_bridge_connect
+            t_serialize_parse_bridge_connect,
+            t_parse_invalid_remaining_len,
+            t_parse_malformed_properties,
+            t_malformed_connect_header,
+            t_malformed_connect_payload,
+            t_reserved_connect_flag,
+            t_invalid_clientid
         ]},
         {connack, [parallel], [
             t_serialize_parse_connack,
@@ -636,18 +642,32 @@ t_parse_malformed_properties(_) ->
         emqx_frame:parse(<<2:4, 0:4, 3:8, 1:8, 0:8, 0:8>>)
     ).
 
-t_parse_malformed_connect(_) ->
+t_malformed_connect_header(_) ->
     ?assertException(
         throw,
         {frame_parse_error, malformed_connect_header},
-        emqx_frame:parse(<<16, 11, 0, 6, 77, 81, 73, 115, 110, 112, 3, 130, 1, 6>>)
-    ),
+        emqx_frame:parse(<<16, 11, 0, 6, 77, 81, 73, 115, 100, 112, 3, 130, 1, 6>>)
+    ).
+
+t_malformed_connect_payload(_) ->
     ?assertException(
         throw,
-        {frame_parse_error, malformed_connect_payload},
-        emqx_frame:parse(
-            <<16, 21, 0, 6, 77, 81, 73, 115, 110, 112, 3, 130, 1, 6, 0, 0, 2, 67, 49.49>>
-        )
+        {frame_parse_error, malformed_connect_data},
+        emqx_frame:parse(<<16, 15, 0, 6, 77, 81, 73, 115, 100, 112, 3, 0, 0, 0, 0, 0, 0>>)
+    ).
+
+t_reserved_connect_flag(_) ->
+    ?assertException(
+        throw,
+        {frame_parse_error, reserved_connect_flag},
+        emqx_frame:parse(<<16, 15, 0, 6, 77, 81, 73, 115, 100, 112, 3, 1, 0, 0, 1, 0, 0>>)
+    ).
+
+t_invalid_clientid(_) ->
+    ?assertException(
+        throw,
+        {frame_parse_error, #{hint := invalid_clientid}},
+        emqx_frame:parse(<<16, 15, 0, 6, 77, 81, 73, 115, 100, 112, 3, 0, 0, 0, 1, 0, 0>>)
     ).
 
 parse_serialize(Packet) ->