Unfortunately this simple and smart proxy technique are being largely used by brazilian malware writers to redirect infected users to malicious hosts serving phishing pages of financial institutions. A .pac script URL is configured in the browser, in the field “Use automatic configuration script”:
Here an example of a malicious .pac file in the wild:
A lot of the Brazilian malware is using this trick nowadays. Not only Internet Explorer users are affected, but also users of Firefox and Chrome. The malware changes the file prefs.js, inserting the malicious proxy in it:
2010 Sep 03, 07:09
Re: Does this family affect users of Mac OS X?
No, this trojan was made initially to Windows