Homeland Security Secretary Janet Napolitano on Wednesday warned that some of the largest U.S. financial institutions "are actively under attack" from cyber hackers.

While Napolitano sounded the alarm about the attacks at a cybersecurity event hosted by The Washington Post, she declined to provide any details about them.

"Right now, financial institutions are actively under attack. We know that. I'm not giving you any classified information," she said. "I will say this has involved some of our nation's largest institutions. We've also had our stock exchanges attacked over the last [few] years, so we know ... there are vulnerabilities. We're working with them on that."