11. A system comprising: one or more processors; one or more non-transitory computer-readable media comprising instructions that, when executed with the one or more processors, cause the system to at least: add, to a first network-based document from a network-based resource of a provider, an identifier of a second network-based document from the network-based resource and code based at least in part on a request for the first network-based document, the identifier comprising a network address of the second network-based document, the request received from a computing system of a user, the code configured at least to, upon execution: determine whether the network address and a cascading style sheets (CSS) attribute of the network address are present in a history stored at the computing system, and determine, based at least in part on presence of the network address and the CSS attribute in the history, that the second network-based document was accessed, the second network-based document accessed based at least in part on an identifier of the user; provide the first network-based document to the computing system based at least in part on the request; receive, from the computing system, an indication that the second network-based document was accessed prior to providing the first network-based document to the computing system, the indication received based at least in part on a determination of the presence of the network address and the CSS attribute in the history upon an execution of the code at the computing system; and authenticate the user based at least in part on the indication.
16. The system of claim 11 , wherein the second network-based document comprises a web page unique to the identifier of the user, wherein the second network-based document was accessed based at least in part on a rendering of the web page in an invisible frame at a browser of the computing system.