David Emberton writes at ActionScript.com : Flash MX Security Whitepaper
Twenty three pages of what you need to know about the Flash Player 6 sandbox security system, and how it can help you build better dynamic apps.
Flash Player 6 features a more sophisticated setup. Each movie loaded from a separate domain exists in its own “sandbox,” effectively cutting it off from additional domains unless you specificy them via the System.security object. The rules are slightly different for standalone players. As far as I can tell, System.security is undocumented anywhere except in this whitepaper.