Kai-Petri wrote something about this spy 10 years ago in some topic but I had to "revive" this spy's work. Elyesa Bazna or simply "Cicero" was Albanian spy for Nazi Germany in the WWII. He was one of the most famous spies of World War II, who worked for Nazi Germany in 1943–44 while he was employed as valet to Sir Hughe Montgomery Knatchbull-Hugessen, British ambassador to neutral Turkey from 1939. He photographed secret documents from the embassy safe and turned the films over to the former German chancellor Franz von Pappen, at that time German ambassador in Ankara. For this service the Hitler government paid Cicero large sums in British pounds, most of it counterfeited in Germany. Despite the evident authenticity of the films, the Nazi officials in Berlin mistrusted Cicero and are said to have disregarded his information (some of which dealt with plans for the Allied invasion of Normandy on D-Day, June 6, 1944).
Spies are by definition untrustworthy, it requires a very peculiar mindset to successfully betray the people you work with every day and double or turned agents are a real possibility. So intelligence agencies rarely rely on a single source but usually cross index information from multiple sources to get confirmation. The Germans were flooded with conflicting reports about the where and when of overlord making he information they had useless for any practical decision making. I don't think an ambassador would have detailed plans but embassies will have some high level information as a major point of having embassies is providing an "authoritative voice" to the foreign government and an ambassador that is kept in the dark of major decision is unlikely to be perceived as such, embassies require information of policies and plans in advance in order not to "lose face".
Here's some more about Cicero from wikipedia if someone is interested. https://en.wikipedia.org/wiki/Elyesa_Bazna