This attack could pose a serious security threat. You should take immediate action to stop any damage or prevent further damage from happening.
This signature detects attempts to download malicious Proxy Auto Config file on user's machine.
A Proxy Auto Config file as known as PAC file determines how browsers automatically choose a proxy server based on URL, User-Agent etc. Malicious PAC files can hijack user's browser and redirect them to malicious websites which steal credentials, identity etc.