Abstract: A server computer system programmed to generate a first challenge credential to the client computer, that corresponds to a first response credential in a first challenge-response credential pair; render first dynamic-credential instructions causing the client to generate the first response credential; send the first challenge credential and the first dynamic-credential instructions, but not the first response credential; receive a request that includes a first test-challenge credential and a first test-response credential; determine whether the first test-challenge credential and the first test-response credential are the first challenge-response credential pair; if the first test-response credential is the first response credential, determine that a count is associated with the first challenge-response credential pair, and determine whether the count satisfies a first threshold; if the count does not satisfy the first threshold, determine that the first request is not a replay request and assign a second cou

Abstract: 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”.