In this paper we introduce REST-Net, a novel Intrusion Detection System for Vehicular Ad Hoc Networks (VANETs) that helps to mitigate the integrity and authenticity challenges introduced by VANETs. At its core, REST-Net uses a dynamic detection engine that monitors and analyzes data sent in VANETs through plausibility checks to detect attacks in form of fake Messages. Unlike previous solutions REST-Net offers high detection rates, adaptive warning levels to prevent interruptions of drivers and a concept for the revocation of fake Messages once an attacker is detected. We present the design and components of REST-Net, discuss its security properties and provide results from an initial feasibility study with a micro-traffic simulator.