Windreader Sphinx — ruling 1

It doesn't matter who controls the creature with flying or which player or planeswalker that creature is attacking. Windreader Sphinx's controller is the player who may draw a card.

Windreader Sphinx — ruling 2

Windreader Sphinx's ability will trigger when it itself attacks.

Windreader Sphinx — ruling 3

The creature must have flying immediately after being declared as an attacking creature in order for Windreader Sphinx's ability to trigger. For example, attacking with Kitesail Corsair will cause the ability to trigger, and attacking with Pegasus Courser and a creature without flying will cause the ability to trigger only once.

Windreader Sphinx : règles, fonctionnement et interactions (MTG)

Windreader Sphinx is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flying Whenever a creature with flying attacks, you may draw a card.

How it works : Windreader Sphinx

Original English text — Wizards of the Coast / Scryfall

#1July 13, 2018· Wizards of the Coast
It doesn't matter who controls the creature with flying or which player or planeswalker that creature is attacking. Windreader Sphinx's controller is the player who may draw a card.
No other card is named in this ruling.
#2July 13, 2018· Wizards of the Coast
Windreader Sphinx's ability will trigger when it itself attacks.
No other card is named in this ruling.
#3July 13, 2018· Wizards of the Coast
The creature must have flying immediately after being declared as an attacking creature in order for Windreader Sphinx's ability to trigger. For example, attacking with Kitesail Corsair will cause the ability to trigger, and attacking with Pegasus Courser and a creature without flying will cause the ability to trigger only once.
No other card is named in this ruling.