Circumventing Access Control Lists by Transparent Proxy - A Case Study

Open proxy servers are a well-known security problem that threatens the security of the Internet by making it possible for unscrupulous people to mask their identity while performing any number of malicious acts. Reading about the open proxy problem, one would come to believe that a serious configuration error such as lack of access controls is necessary for a proxy to be abused. This paper describes a method used in an actual case to circumvent seemingly adequate access controls by using the transparent caching mechanism of the WCCP protocol to abuse an otherwise protected network for the purposes of sending spam and connecting anonymously to unsavory sites. We will also see how this vulnerability could have been used to bypass a firewall and gain access to a protected network.