HTTP requests initiated from a web browser of a client computer system are proxied prior to release to a router, such as a home router. HTTP requests identifying a referrer URL corresponding to routable, public IP address and a target URL corresponding to a non-routable, private IP address are determined to be indicative of a drive-by pharming attack, and are blocked from sending to the router. HTTP requests not identifying a referrer URL corresponding to a routable, public IP address and a target URL corresponding to a non-routable, private IP address, the HTTP request are not determined to be indicative of a drive-by pharming attack, and are released for sending to the router. In some embodiments, an HTTP response received in response to a released HTTP request is proxied prior to release to the web browser. An HTTP response having content of type text/html or script is modified as indicated to prevent malicious activity and released to the web browser.