Attributes That Has No Rules Can Be Massive Asigned

I’ve came across a issue,my activeForm object has six attributes I wrote rules for three of them,the rest of them have no rules at all,PHP warning occured when I was trying to run this app----failed to asign unsafe attributes.This is the result that it was surposed to be.

After that I wrote rule for another attribute which means there still had 2 out of 6 attributes has no rules.But all warning disappeared and validate passed.I do not know why it can pass validation while 2 attributes has no rules attached.

Since those 2 attributes do not have any rules set, then they are completely ignored (and values are not set), which results in validation pass.