There is provided a method of operating a security token, said security token comprising a secure element and a microcontroller unit being coupled to said secure element, wherein: the secure element receives an authentication command from a host device while the microcontroller unit is in a first sleep state; the secure element decodes the authentication command, sends a corresponding authentication request to the microcontroller unit and subsequently enters into a second sleep state; the microcontroller unit wakes up upon receiving the authentication request and subsequently determines an amount of available power; the microcontroller unit processes the authentication request only if the amount of available power exceeds a threshold. Furthermore, a corresponding computer program product and a corresponding security token are provided.