Malware authors have developed a new tactic that uses a feature built into all of the modern browsers to redirect users to malicious sites without their knowledge or action. Malware writers, particularly the creators of Brazilian banker Trojans, have latched onto PAC files as a way to push victims to phishing sites. PAC files are designed to enable browsers to automatically select which proxy server to use to get a specific URL. Not only Internet Explorer users are affected, but also users of Firefox and Chrome.
Source: https://threatpost.com/attackers-using-malicious-pac-files-phishing-attacks-041410/73827/