The Soviet Union, as a Totalitarian State could have much more invasive internal spy efforts then the Democratic United States and Great Britain. It is common knowledge of the immense repression that occurred in the Soviet Union, with millions intured in political prisons and Gulags. At the start of the Second World War, the Soviet Union was also still dealing with the effects of the Great Purge, in which Hundreds of Thousands died in a series of arrests, mock trials, and executions for suspicion of being against the Party. This culture of Paranoia both helped the Soviets to notice an attempt of espionage by a foreign party, and hurt the Soviets by having their peoples despising the regime.
The Soviet Union had a massive advantage in world espionage, as the Birthplace and Home of the “Worker’s Revolution” those across the world with communist sympathies or leanings were much more likely to help the Soviets in espionage. As opposed to race or nation based the Soviet Union, much like the United States, was ideology based and was able to draw on people's support across the world. This would be a major fear during the Cold War, then an American may support communism and be a fifth columnist or other threat.