Historical Rules · 25 September 2014
Rule 508.5
Official source text from the 25 September 2014 Rules release
If an ability of an attacking creature refers to a defending player, or a spell or ability refers to both an attacking creature and a defending player, then unless otherwise specified, the defending player it’s referring to is the player that creature was attacking at the time it became an attacking creature that combat, or the controller of the planeswalker that creature was attacking at the time it became an attacking creature that combat.