Against a backdrop of cyberattacks that have grown into full-fledged sabotage, Facebook chief security officer Alex Stamos is bringing a new message to hackers and security experts at the Black Hat conference. In short: It's time for hackers once known for relatively harmless mischief to shoulder responsibility for helping detect and prevent major attacks. The Black Hat security gathering, starting Wednesday in Las Vegas, follows a series of attacks and data breaches that have paralyzed hospitals, disrupted commerce, caused blackouts and interfered with national elections. Stamos, a keynote speaker, is calling for more emphasis on defense — and basic digital hygiene — over the thrilling hunt for undiscovered vulnerabilities. Stamos joined Facebook from Yahoo, which last year disclosed breaches of more than a billion user accounts.