Not necessarily advised, though

You can flip-off a cop, legally. The law, specifically the Constitution, is on your side. At least according to this article in Time. As indicated, this is not always your best course of action, because the police might issue you a "disorderly conduct" ticket or arrest you, regardless of whether you have the right to or not. Obviously, it's up to you to decide whether the possible court proceedings, etc, will be worth it. Probably better to keep your cool.