fifth column

n.	1.	a group of persons inside the battle lines of a territory engaged in a conflict, who secretly sympathize with the enemy, and who engage in espionage or sabotage; - sometimes also referred to as a trojan horse.
	2.	Hence, any faction of persons within a group who secretly sympathize with an enemy, especially those who engage in activities harmful to the group; an enemy in one's midst; a group of traitors.

Noun

1.

fifth column - a subversive group that supports the enemy and engages in espionage or sabotage; an enemy in your midst Synonyms: Trojan horse

Judas-like, Trojan horse, Trojan-horse, betraying, collaborationist, collaborator, crypto, double-crossing, fellow traveler, fifth columnist, fraternizer, quisling, quislingistic, saboteur, security risk, subversive, traitorous, treasonable, treasonous, turncoat, two-timing, underground