Abstract: A method of fuzzy testing a software system, wherein the software system comprises a plurality of callable units and is arranged to receive input for the software system to process, the method comprising: determining, for each callable unit of the plurality of callable units, based on one or more security vulnerability metrics, a target number of times that callable unit is to be tested; initializing a ranked plurality of queues, each queue for storing one or more seeds, said initializing comprising storing one or more initial seeds in a corresponding queue of the ranked plurality of queues; performing a sequence of tests, wherein performing each test comprises: obtaining a seed from the highest ranked non-empty queue; performing a mutation process on the obtained seed to generate a test seed; providing the test seed as input to the software system for the software system to process; and evaluating the processing of the test seed by the software system to generate a result for the test; wherein each queue in