Outgoing Homeland Security Secretary Janet Napolitano warned her successor on Tuesday to move quickly to prepare for an inevitable large-scale cyberattack against the United States.

Napolitano, delivering her farewell address at the National Press Club, said her successor should move fast to strengthen the nation’s cyber defenses.

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“Our country will, at some point, face a major cyber event that will have a serious effect on our lives, our economy and the everyday functioning of our society,” said Napolitano.

“While we have built systems, protections and a framework to identify attacks and intrusions, share information with the private sector and across government, and develop plans and capabilities to mitigate the damage, more must be done, and quickly.”

Napolitano said that while the terrorist threat to the country has not been eliminated, the ability for the U.S. to thwart attacks rapidly increases with each uncovered plot.

“For every attack we experience, every threat we face and every piece of intelligence we come across, we learn; we assess our preparations and capabilities; we make changes; we become more flexible in the actions we take; and we get stronger and more nimble,” said Napolitano, who is leaving her post to become the head of the University of California school system.

As evidence, she pointed to the country’s response to the bombing at the Boston Marathon earlier this year and the stronger local and state relationships that have been forged while she has led the department.

Napolitano also lauded the department’s move — under direction from President Obama — to allow children brought to the U.S. illegally to remain in the country.

The Deferred Action for Childhood Arrivals (DACA) process has allowed 430,000 young people to stay in the United States.

Napolitano stressed that the prosecutorial discretion, which many Republicans have boisterously objected to, has made the country’s immigration system more sound, but that comprehensive legislation still needs to be passed.

Obama has not weighed in on who Napolitano's successor will be, but several names have been floated as strong contenders. Chief among them is New York City Police Commissioner Ray Kelly, who has remained mum about whether he's been in talks about the post.

Former Sen. Joseph Lieberman (I-Conn.), who headed the Senate Homeland Security and Governmental Affairs Committee, which played a key role in creating the department, has also been suggested for the post, as have former Deputy Secretary Jane Lute and Bill Bratton, the former head of police for Boston, Los Angeles, and New York City.

While secretary, Napolitano has remained relatively unscathed by congressional investigations and potential scandals within the department. Many Republicans, though they disagree with some of her policy decisions, have applauded her tenure in the demanding position, calling attention to the “thankless” nature of her job.

Only 10 years old, the Department of Homeland Security is the largest agency in the country, encompassing a wide range of federal responsibilities and offices that range from immigration and border enforcement to port and aviation security.

Napolitano said it was imperative that her future successor continues to “move to a more risk-based, intelligence-driven security system” with the nation’s approach to aviation security.

She also highlighted chemical, biological, radiological and nuclear security measures; securing large-scale public events, such as the Inauguration and the national party conventions; and the Coast Guard, as areas that will need continued attention under the department’s new leadership.

This story was updated at 11:05 a.m.