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The NGAC (Next Generation Access Control) standard for attribute-based access control (ABAC) allows for run-time changes of the permission and prohibition configurations through administrative obligations triggered by access events. It makes access control more fine-grained and dynamic. However, it raises challenges for assuring the correctness of NGAC policies. As policy testing is an important technique for quality assurance, this paper presents an approach to mutation analysis of NGAC policies. It can evaluate the effectiveness of a testing method and reveal potential faults in an inadequately tested policy. The mutation analysis covers various types of potential faults in the assignments, associations, prohibitions, and obligations of NGAC policies. This paper also proposes an incremental testing approach that first validates the initial configuration of a policy and then the policy as a whole. It helps determine whether faults appear in the configuration or the obligations. To evaluate the work, we have developed four working policies and their test suites based on the current NGAC reference implementation. The empirical studies show that the mutation analysis can shed light on the strengths and weaknesses of the test suites. They also demonstrate the need for developing more cost-effective testing methods.