the_dunk_tank

Aldrich Ames in CIA and Robert Hanssen in FBI, though they worked much later and neither was a communist (Hanssen might be or at least antiimperialist, but he never admitted that). Hanssen pissed the pigs so much they given him 15 life sentences.

In the Manhattan project, beside Rosenbergs there were at least three other Soviet spies:

• Georg Koval, an engineer working for GRU, most likely real communist, USA do not know till today what exactly he gave to GRU and the fact he was a spy was only really revealed in 2007 when Putin gave him the Hero of Russian Federation for that.
• Harry Gold, courier, he was a communist but apparently very shallow one. Recruited by NKVD he was only a minor asset. Didn't worked in MP directly, but was courier for Fuchs.
• Klaus Fuchs, physicist, one of the main theoretical fathers of both nuclear and hydrogen bombs, passed tons of info about US, UK and Canadian nuclear projects to Soviets over the 9 years. He was at first member of SPD, from which he was kicked out for associating with Thalmann, then he joined KPD. Later in life he lived in GDR and was a member of cnetral comitee of SED, so definitely real deal communist.

All that said, it is actually still unknown and most likely exaggerated how much all those agents improved the Soviet nuclear project, most likely the data they provided served mostly to compare with western projects because Soviet nuclear project had no real problems with theory or engineering, they were mostly restricted by the amount of enriched uranium and plutonium they could produce because in the first piles it went really slow, and USA had significant time advantage in that.