A recent AH question on how Axis and Allied nations communicated with each other got me curious about whether Axis or Allied powers infiltrated the governments of Switzerland, Spain, or other neutral nations to discover what their erstwhile friends were asking their enemies, such as potential peace feelers that might undermine the war effort. I understand that Spain was a general hotbed of spies, and both the US and UK were concerned at various times about just how keen the other was at remaining in the war.
Short answer is yes.
Long one : I'll take here the example of Switzerland.
So yes both sides infiltrate neutral nations, here Swiss Confederation, during WWII. Switzerland was literally a spy nest for both sides. You can find in the Swiss Federal Archives, diplomatic telegrams asking the Swiss government to stop exporting weapons to Ally and Axis. German asking Swiss gov. to stop exporting weapon to England and England, asking Swiss gov. to stop supplying Germany. Those letters are quite explicit on the presence of agents on swiss ground (you can check document n°47175 or the 17904).
These letters are here. This website contains some documents (not all) on the diplomatic history of Switzerland. Professional historians (they are located in the same building as the Federal Archives) read and select some documents that they deemed “relevant” and publish them online. The rest is physically available in Bern.
Otherwise, you can consult the Rapport Bergier published in the early 2000s. A group of historian led by the Pr. Bergier studied all the archives available in order to make light on the role of Switzerland during WWII. You'll find some chapters about foreign interference (not sure).
The report can be found on uek.ch
Finally, there is a book specifically on this topic (only in French). Secret War.
I can't affirm that the Swiss government was infiltrated, because I do not know. We know for a fact that foreign agents were on Swiss ground, but in the government I don't know. And to be honest, the confederation has largely collaborated with the Nazi regime. This was clearly demonstrated by the Bergier report. It is worth asking whether the presence of agents in the government was really necessary. But, afterwards, it is not impossible that state agents played a double game.