We're pleased to announce the availability of 'Socket Capable Browser Plugins Result In Transparent Proxy Abuse'. This document outlines the abuse case in CERT's VU #435052 advisory published last month.
"Transparent proxies allow organizations to influence and monitor the traffic from its users without their knowledge or participation. Transparent proxies act as intermediaries between a user and end destination, and aren't generally apparent to users sitting behind them. Enterprises, Hotels, and Internet Service Providers often use transparent proxy products to lower bandwidth consumption,speed up page loads for their users, and for monitoring and filtering of web surfing. When certain transparent proxy architectures are in use an attacker can achieve a partial Same Origin Policy Bypass resulting in access to any host reachable by the proxy via the use of client plug-in technologies (such as Flash, Applets, etc) with socket capabilities. This write up will describe this architecture, how it may be abused by Flash, its existence in various network layouts, and mitigations."
CERT Advisory: http://www.kb.cert.org/vuls/id/435052