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

Mayer is joining former and current CEOs of Equifax in testifying before a Senate committee examining recent data breaches that affected millions of Americans.

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

Mayer says the thefts occurred during her nearly five-year tenure and she wants to “sincerely apologize to each and every one of our users.”

Mayer says it’s clear that no company, individual or even government agency is immune from attack. She also says strong collaboration between the public and private sectors is essential to battling cybercrime.