This paper presents the results of a demonstration based on the use of the Navigation Message Authentication (NMA) technique, which represents an effective countermeasure against counterfeit Global Navigation Satellite System (GNSS) signals and is one of the possibilities considered for the adoption by Galileo. An intermediate spoofing attack has been simulated in a vehicular case study, considering both a commercial GPS receiver and a modified software receiver. The obtained results demonstrate the potential vulnerability of current commercial receivers and the added value of NMA, which has been exploited in the software receiver for implementing a simple but effective spoofing detection method.