fair game

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic Actions permissible by the rules.
• noun idiomatic A goal or an object that may legitimately be sought.
• noun idiomatic An acceptable subject of criticism, scrutiny, or mockery.
• noun Used other than as an idiom: see fair,‎ game.: a game that is fair, that does not involve cheating, etc.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a person who is the aim of an attack (especially a victim of ridicule or exploitation) by some hostile person or influence