No

A cyber attack is a cyber operation, whether offensive or defensive, that is reasonably expected to cause injury or death to persons or damage or destruction to objects.

Michael Schmitt et al; Tallinn Manual 1.0, pp 92
  • Cyber Operation?
  • Expected to cause death or injury to people or objects?
Russian Intrusion of Yahoo does not match this definition. Here's a list of actual cyberattacks and and why this matters.

What happened?

Yahoo revealed that it an intrusion into its network in late 2014 had given hackers access to 500 million users’ usernames, email addresses, phone numbers, dates of birth, passwords, and a mix of encrypted and plaintext security questions and answers. The company’s CIO claimed the attack was perpetrated by a state-sponsored actor.