In an embodiment, a computer system is configured to improve security of server computers interacting with client computers through an intermediary computer, and comprising: a memory comprising processor logic; one or more processors coupled to the memory, wherein the one or more processors execute the processor logic, which causes the one or more processors to: intercept, from a server computer, one or more original instructions to be sent to a browser being executed on a client computer; inject, into the one or more original instructions, one or more browser detection instructions, which when executed cause one or more operations to be performed by an execution environment on the client computer and send a result that represents an internal state of the execution environment after performing the one or more operations to the intermediary computer; send the one or more original instructions with the one or more browser detection instructions to the browser; receive the result and determine whether the browser is a legitimate browser, or a headless browser, based, at least in part, on the result.