In an embodiment, a computer system configured to: generate a first challenge credential to be sent to a client computer; render one or more first dynamic-credential instructions, which when executed by the client computer, cause the client computer to generate a first dynamic credential that corresponds to the first challenge credential; modify a first set of instructions, which define one or more original operations, to produce a second set of instructions, wherein the second set of instructions include the first challenge credential and the one or more first dynamic-credential instructions, and which when executed by the client computer, cause the first challenge credential to be included in the one or more requests sent from the client computer; send the second set of instructions to a second computer.