From HistoryExtra, the story of how the Soviet KGB penetrated the upper ranks of British government during and after World War II.
The CIA, in the person of James Angleton, was fooled as well.
[If you want to know more, see biography of Angleton, THE GHOST: The Secret Life of CIA Spymaster James Jesus Angleton.]
The five were recruited while students at the University of Cambridge in the 1930s and each would go on to have successful dual careers as British civil servants and Soviet spies. Kim Philby (1912–88) spent most of his career working for the British intelligence agency MI6, including a period as head of Soviet counterespionage and as MI6 liaison officer to the CIA in Washington DC. Donald Maclean (1913–83) had a successful career in the Foreign Office, working on atomic and military matters. Guy Burgess was…