How secure are your proxy servers?

Adventures of an Open Proxy Server discusses how hackers are exploiting proxy servers:

"Web traffic has grown at a phenomenal rate over the past 7 years. Companies and ISPs often turn to caching proxy servers to reduce the tremendous load on their networks. In order to satisfy the demands of their content-hungry users, these proxy servers are often configured to proxy any port, with little regard to security. If there are no access controls blocking connections from outside the network, it makes it possible to anonymously portscan the entire TCP port range of other outside systems. "

Via joatBlog