With a PAC file, the attackers can be a lot more choosy, redirecting requests to a group of sites or to one site in particular.The attack resembles DNSChanger, the widespread malware that changed a specific file on victim's systems to reroute all domain name lookups to an attacker-controlled server.

Preventing PAC files from compromising browsers is not a simple task, as client-side security software will likely find it difficult to detect whether a give PAC file is a valid change or a malicious attack.