The focus of this paper is on automating the security testing of RESTful APIs. The testing stage of this specific kind of components is often performed manually, and this is yet considered as a long and difficult activity. This paper proposes an automated approach to help developers generate test cases for experimenting with each service in isolation. This approach is based upon the notion of test case mutation, which automatically generates new test cases from an original test case set. Test case mutation operators perform slight test case modifications to mimic possible failures or to test the component under test with new interactions. In this paper, we examine test case mutation operators for RESTful APIs and define 18 operators specialised in security testing. Then, we present our test case mutation algorithm. We evaluate its effectiveness and performance on four web service compositions