Operator mistakes are a signifcant source of unavailability in Internet services. In our previous work, we proposed operator action validation as an approach for detecting mistakes while hiding them from the service and its users. Previous validation strategies have limitations, however, including the need for instances of correct behavior for comparison. In this paper, we propose a novel model-based validation strategy that addresses these limitations and complements our previous techniques. Model-based validation calls for service engineers to define models of Internet services that can be used to differentiate between correct and incorrect confgurations and behaviors. These models are then used to guide the specifcation of validation assertions that check the correctness of operator actions before they are exposed. We have implemented a prototype modelbased validation system for two services, the Web crawler of a commercial search engine (Ask.com) and an academic yet realistic online auction service. Experimentation with model-based validation demonstrates that it is highly effective at detecting and hiding both activated and latent mistakes.