In an example aspect, a method includes receiving a plurality of login attempts from a network address over a length of time, querying log data to determine, for the network address, an average number of login failures of the plurality of login attempts over the length of time, calculating a failure rate metric based on the average number of login failures, determining that the failure rate metric exceeds a reference number of login failures for the length of time, the reference number of login failures based on a historical average number of login failures for the length of time, and based in part on the determining, adding the network address to a system deny list.