Computer systems and methods in various embodiments are configured for improving the security and efficiency of server computers interacting through an intermediary computer with client computers that may be executing malicious and/or autonomous headless browsers or “bots”. In an embodiment, a computer system comprises a memory; one or more processors coupled to the memory; a processor logic coupled to the memory and the one or more processors, and configured to: intercept, from a server computer, one or more original instructions to be sent to a browser of a client computer; send the one or more original instructions to the browser and one or more telemetry instructions, wherein the telemetry instructions are configured, when executed, to generate a set of telemetry data indicating one or more objects that were referenced by the browser and to send the set of telemetry data to the intermediary computer; receive the set of telemetry data and determine whether the browser is legitimate or illegitimate based on the set of telemetry data.