Ya’akov Riftin ...
This article, dated 28 October 2016, says:
In the beginning of the 1950s, the Russians launched a wide-scale operation codenamed "Trest" to infiltrate Mapam (the United Workers Party). According to the Mitrokhin documents, they were quite successful: The KGB recruited Ya'akov Riftin, who was a Mapam member of Knesset from 1949 to 1965.
One of the documents mentions that Riftin was a member of the Knesset's Foreign Affairs and Defense Committee. According to the Mitrokhin documents, he regularly passed on classified documents, including top secret ones, to the Soviet Embassy.
See this Wikipedia (English) article