To improve security, we can fix the owner of the post and the time when it was created. We can prevent changes to the owner and the timeCreated and slug fields by adding an additional deny rule to our Posts collection, as follows:

if(Meteor.isServer) {

  // Allow rules

  Posts.deny({
    update: function (userId, docs, fields, modifier) {
      // Can't change owners, timeCreated and slug
      return _.contains(fields, 'owner') || _.contains(fields, 'timeCreated') || _.contains(fields, 'slug');
    }
  });
}

This rule will simply check whether the fields argument contains one of the restricted fields. If it does, we deny the update to this post. So, even if our previous allow rules have passed, our deny rule ensures that the document doesn't change.

We can try the deny rule by going to our browser's console, and when we are at a post page, typing the following commands:

Posts.update(Posts.findOne()._id, {$set: {'slug':'test'}}); 

This should give you an error stating update...