In this paper we study how to fulfill proof of violation (POV) for availability of IaaS systems in the cloud. A POV scheme enables a user or a service provider to produce a precise proof of either the occurrence of the violation of properties or the innocence of the service provider. POV schemes are solutions for obtaining mutual nonrepudiation between users and the service provider in the cloud. To the best of our knowledge, it is the first scheme that can perform availability auditing according to cryptographic proofs. Experimental results are presented that demonstrate the feasibility of the proposed scheme. Service providers can use the proposed scheme to provide a mutual nonrepudiation guarantee for availability in their service-level agreements.