Illegal means it's against the law, aka you'll face prosectution. No it is not illegal however, is it not allowed by the TOS.
Do you really have to ask this question? If you're too lazy to create geniune original content and make sites that people can use, you not only have no place in Adsense, you add no value to the internet. I can't believe you would ask such a ridiculous question on DP when you can just search for "MFA" and get your answer.