The Falcon and the Snowman (1985) IMDb
Seen in 2017.
In the 1970s, the son of an FBI agent gets a job routing US government intelligence data through the telex system. Disillusioned by the way the CIA undermines other democratic regimes, he starts selling secrets to the Soviets.
Significantly fictionalized, but the tone is suitably realistic. I especially appreciate the boredom inside the Black Vault at “RTX”.