We've already seen Islamic Jihadis use the Syrian migrant crisis as cover to get into Europe and carry out attacks in Paris.
Err, except that all but one of them were native Europeans iirc? And really, I think it's totally over egging it when people talk about ISIS slipping in with the migrants....I mean, I guess they would have the funding and means to find far better ways of slipping in than dodgy boat crossings and where they are going to be under scrutiny.