One of the problems related to the simulation of attacks against critical infrastructures is the lack of adequate tools for the simulation of malicious software (malware). Malware attacks are the most frequent in the Internet and they pose a serious threat against critical networked infrastructures. To address this issue we developed Mobile Agent Malware Simulator (MAISim). The framework uses the technology of mobile agents and it aims at simulation of various types of malicious software (viruses, worms, malicious mobile code). Moreover it can be flexibly deployed over computer network of an arbitrary information system.