mole

An agent of one organisation who penetrates a particular intelligence service by getting a job in it.

The International Spy Museum glossary notes that the term was popularised by John le Carré.¹

The best-known cases in FBI records: CIA officer Aldrich Ames, a specialist in Soviet intelligence, offered his services to the KGB on 16 April 1985; his treachery compromised more than 100 US operations and led to the deaths of 10 sources. FBI agent Robert Hanssen, who in 1985 headed a counterintelligence section in the New York field office, spied for the KGB and later the SVR until his arrest on 18 February 2001. Both were sentenced to life imprisonment.^2,3

In a looser sense, any agent inside an adversary’s institution may be called a mole – for example the members of the Cambridge Five in the British services and government departments.⁴