Although these words are now widely attributed to Cicero, in fact they originated in the 20th century, with novelist Taylor Caldwell’s 1965 book A Pillar of Iron. Even though that work often drew directly from the recorded speeches and letters of Cicero for its dialogue, it was nonetheless a work of fiction, and the warning from Cicero about “treason from within” was an invention of Caldwell’s and not a reproduction of Cicero’s own words. https://www.snopes.com/fact-check/cicero-treason-quote/
“A nation can survive its fools, and even the ambitious. But it cannot survive treason from within. An enemy at the gates is less formidable, for he is known and carries his banner openly. But the traitor moves amongst those within the gate freely, his sly whispers rustling through all the alleys, heard in the very halls of government itself. For the traitor appears not as a traitor; he speaks in accents familiar to his victims, and he wears their face and their arguments, he appeals to the baseness that lies deep in the hearts of all men. He rots the soul of a nation, he works secretly and unknown in the night to undermine the pillars of the city, he infects the body politic so that it can no longer resist. A murder is less to fear. The traitor is the plague.”