
I am reading the book called "My Silent War" by Kim Philby. It was the autobiography by a spy.
Philby told his life story on espionage which spanned across 1940s to 1960s.
Philby was a British citizen. He was brought up and educated under the British education system. In Cambridge, he was exposed to communism. He became a Communist then.
He was under the payroll of MI6, The British Intelligence Service. He climbed up the career ladder and became a high-ranking officer in MI6. Behind the scene, he was a Soviet agent.
Philby confessed that he was not a double agent. He was actually a penetration agent in the MI6 working for the interest of the communism.
Penetration agent is nothing new.
Lai Teck (the one time chairman of Communist Party of Malaya) was actually a spy planted by the British in the CPM. Through deception, Lai Teck gained the trust of the committee members of CPM and ascended to become the chairman of the party. He continually sold out party members, so that the leaders were arrested by the British colonial government one after another before the Second World War.

Another good movie on the penetration agent was the "Wu Jian Dao" (Infernal Affairs). The convention practice is the police force sending the under-cover agents into the secret societies. "Wu Jian Dao" was a story on how the secret societies sending the penetration agents into the Hong Kong police force.