WASHINGTON (AP) — Former Yahoo CEO Marissa Mayer told lawmakers Wednesday that the threat from state-sponsored hackers has changed the playing field so dramatically that even the best-defended companies can fall victim.

Mayer joined former and current CEOs of Equifax in testifying before a Senate committee examining recent data breaches that affected millions of Americans. Lawmakers said they hoped to use the information learned to shape future legislation.

In Yahoo's case, stolen information for billions of users included names, email addresses, phone numbers, birthdates and security question...