Securing social networking services is challenging and becomes even more complex when third-party applications are able to access user data. Still, adequate security and privacy solutions are imperative in order to build and maintain trust in such extensible social platforms. This paper discusses security issues in the context of OpenSocial-instrumented social networking services. It shows that the OpenSocial specification is far from being comprehensive in respect to security. Resulting weaknesses and shortcomings are emphasized and discussed. Finally, the paper attempts to fill these gaps by proposing extensions to the OpenSocial specification and recommendations for social networks that implement OpenSocial.