Reserving data authenticity in a hostile environment, where the sensor nodes may be compromised, is a critical security issue for wireless sensor networks. In such networks, once a real event is detected, nearby sensors generate data reports which are subsequently forwarded to the data collection point. However, the subverted sensors, which have access to the stored secret keys, can launch attacks to compromise data authenticity. They can act as sources for forged reports and inject an unlimited number of bogus reports that fabricate false events "happening" at arbitrary locations in the field. Such false reports may exhaust network energy and bandwidth resources, trigger false alarms and undesired reactions. The authors explain such attacks and that which can be roughly categorised as isolated attacks by each individual compromised node or colluding attacks by a group of collaborating impostors.