Add validation of packet control type.

Frameworks/MQTTnet.NetStandard/Serializer/MqttPacketReader.cs

@@ -12,7 +12,7 @@ namespace MQTTnet.Serializer
 {
     public static class MqttPacketReader
     {
-        public static async Task<MqttPacketHeader> ReadHeaderAsync(IMqttChannel stream, CancellationToken cancellationToken)
+        public static async Task<MqttPacketHeader> ReadHeaderAsync(IMqttChannel channel, CancellationToken cancellationToken)
         {
             if (cancellationToken.IsCancellationRequested)
             {
@@ -23,20 +23,26 @@ namespace MQTTnet.Serializer
             // some large delay and thus the thread should be put back to the pool (await). So ReadByte()
             // is not an option here.
             var buffer = new byte[1];
-            var readCount = await stream.ReadAsync(buffer, 0, buffer.Length, cancellationToken).ConfigureAwait(false);
+            var readCount = await channel.ReadAsync(buffer, 0, buffer.Length, cancellationToken).ConfigureAwait(false);
             if (readCount <= 0)
             {
                 return null;
             }
 
             var fixedHeader = buffer[0];
-            var controlPacketType = (MqttControlPacketType)(fixedHeader >> 4);
+            var controlPacketType = fixedHeader >> 4;
-            var bodyLength = await ReadBodyLengthAsync(stream, cancellationToken).ConfigureAwait(false);
+
+            if (controlPacketType < 1 || controlPacketType > 14)
+            {
+                throw new MqttProtocolViolationException($"The packet type is invalid ({controlPacketType}).");
+            }
+
+            var bodyLength = await ReadBodyLengthAsync(channel, cancellationToken).ConfigureAwait(false);
 
             return new MqttPacketHeader
             {
                 FixedHeader = fixedHeader,
-                ControlPacketType = controlPacketType,
+                ControlPacketType = (MqttControlPacketType)controlPacketType,
                 BodyLength = bodyLength
             };
         }