an open invitation
1. An invitation to do something at any time or whenever someone is inclined. Just because I let your insufferable friend Paul to come over that one time doesn't mean he has an open invitation.
2. Something that provides an ostensible reason or excuse for someone to do something immoral or illegal. A lot of people fear that legalizing marijuana will create an open invitation for people to try, buy, or sell any drug they can think of.