WebSearcher uses a proxy to insert the advertisements into your normal web experience. It uses a set of permissions to get the three most popular browsers to use the proxy it has set. The normal user is not allowed to change the proxy settings in IE, Chrome, Firefox and Firefox. This adware abuses two libraries of the legitimate web debugging proxy Fiddler (FiddlerCore) and the DO_NOT_TRUST_FiddlerRoot certificate. A removal guide can be found on our forums.”]
Source: https://blog.malwarebytes.com/cybercrime/2016/01/websearcher-pup-applies-proxy-lockdown/