In one aspect, the present disclosure relates to a method for reducing fraud in computer networks, the method including receiving, from each of a plurality of user devices, a request to block an ad displayed within a web browser installed on the user device, the request comprising image data and a forwarding URL associated with the ad; storing crowdsourced ad blocking data based on the received requests to block ads; receiving a request for a list of blocked ads; generating a list of blocked ads based on analyzing the crowdsourced ad blocking data, wherein analyzing the crowdsourced ad blocking data comprises identifying ads blocked by at least a threshold number of users; and sending the list of blocked ads to a first user device, the first user device comprising a browser extension configured to prevent ads within the list of blocked ads from being rendered in a browser.