Emil Bobi, author of De Schattenstadt (The Void), cites the city's history as an espionage center and a lack of laws against the practice.
"Every second diplomat in Vienna can be linked to his or her country's intelligence agencies. Every embassy is overcrowded with personnel," he said.
Gert Rene Polli, former chief of Austria's counter-terrorism agency BVT, disputes Bobi's figures, suggesting they may be too low.
"Vienna is a stock exchange of information. We have the most liberal laws governing this activity in the world. It is also a nice place for spies to live and bring their families with good education and health services after difficult postings in Serbia, Iraq or Afghanistan," he said in an interview with the British newspaper The Telegraph.
The city was an intelligence hub during the Cold war, when Austria was officially neutral and both Allied and Warsaw Pact operatives could trade secrets relatively openly. Its history of espionage dates at least back to its prominence in the Austro-Hungarian Empire.
It remains the world leader in espionage by its proximity to the Balkans, the headquarters of the organization of Petroleum Exporting Countries, the International Atomic Energy Agency, and the Organization for Security and Cooperation in Europe, as well as some curious laws about the practice of spying.
Dating back to World War I and never amended, laws regarding spying involve only the direct targeting of Austria's state secrets. The laws have not changed, in part, because espionage is a part of Vienna's culture and tradition.
"Viennese society is built on secrets and people live to have secrets. If you have a secret in Vienna you are somebody," Bobi said.