Public Proxy Servers are a Risk
If you configure your web client to use a public proxy server, you are instantly trusting all your traffic to the administrator of that server. Most web traffic is in clear text but even the SSl encrypted traffic can be intercepted by a proxy server administrator.
So all your login names, accounts, passwords plus a full list of every web server you visit is accessible to the owner of this proxy server.
And do you know what ?
Most people don't even know anything about the public proxy server they are using - not a good idea !!! Remember this very simple point, running an anonymous proxy server is very expensive - there are server costs, support and bandwidth charges. Why would the owner of the server let it out for free to everyone and incur huge costs. There's usually a simple explanation, the owners of these public proxy servers don't even know they are being used until tehy get the bill. They are left open by accident or not secured and opened up by hacking groups. Of course often the hackers will use these public proxy servers to install malware or viruses to users, or simply steal their details as the traffic flows through the proxies.