DETECTION OF EMULATED COMPUTER SYSTEMS USING VARIABLE DIFFICULTY CHALLENGES

Techniques are disclosed relating to detecting that a client system is an emulated computer system based on its computational performance of one or more challenge problems. In some embodiments, a server computer system may receive, from a client system, a request to access a web service. The server computer system may determine reported technical features of the client system and select a particular challenge problem to provide to the client system. The server computer system may determine an expected response time of the particular challenge problem for the client system. The server computer system may receive a challenge response from the client system that includes a proposed solution to the particular challenge problem. The server computer system may then determine whether to authorize the request based on a measured response time by the client system and the expected response time of the particular challenge problem for the client system.