Warren Buffett said that when he called the 2008 financial crisis an "economic Pearl Harbor" the description was not "strong enough."

"I found some things that were going on behind the scenes that I did not know," the Oracle of Omaha revealed in a CNBC "Squawk Box" interview, after reading an advanced copy of former Treasury Secretary Timothy Geithner's new book.

Read more from Buffett's Friday appearance:

Buffett doesn't expect big stock drop

Buffett blasts bitcoin: 'Stay away!'

Buffett on $1 billion March Madness contest