The Washington Post spoke with current and former officials and analysts:

Not only does Trump have a history of disclosures, he checks the boxes of a classic counterintelligence risk: He is deeply in debt and angry at the U.S. government, particularly what he describes as the “deep state” conspiracy that he says tried to stop him from winning the White House in 2016 and what he falsely claims is an illegal effort to rob him of reelection.

Even without a grasp of specifics, Trump almost certainly knows information that would be valuable to adversaries, such as intelligence and military capabilities and any covert actions he authorized, The Post said. Also, he'll retain access to the classified records of his own administration.

Trump’s big mouth and his petty score-settling are pretty good evidence that he’ll never exercise the kind of discretion we normally assume from an ex-president.

Then there’s the worry Trump could sell state secrets. More from The Post:

Experts agreed that the biggest risk Trump poses out of office is the clumsy release of information. But they didn’t rule out that he might trade secrets, perhaps in exchange for favors, to ingratiate himself with prospective clients in foreign countries or to get back at his perceived enemies. When he leaves office, Trump will be facing a crushing amount of debt, including hundreds of millions of dollars in loans that he has personally guaranteed.

So what can President Joe Biden do? Not much as far as the secrets Trump already knows. Biden can break with tradition and refuse to give Trump any further intelligence briefings, which seems like a no-brainer. Also, once Trump leaves office, he is subject to criminal prosecution like every other private citizen. “The Espionage Act has been successfully used to convict current and former government officials who disclose information that damages U.S. national security,” The Post reported.

Let’s hope Biden would not hesitate to exercise all his powers to keep Trump from (further) selling out our country and/or punishing him if he does.

UPDATE: Sarah Kendzior reminds that the threat is not just from Trump but also his corrupt family and cronies:

This is a massive problem and it's not limited to Trump; Kushner and others will also sell state secrets. Even out of the White House, US nat sec is destroyed. And that outcome was inevitable the moment Trump was installed in power. https://t.co/OdqkTdcuyD

— Sarah Kendzior (@sarahkendzior) November 12, 2020