Internet Explorer has a peculiar behavior in handling cross frame scripting when a proxy is involved.

Whenever two frames that communicate one with each other are located on two different servers (e.g. and, as happens with Lightstreamer web client library when the "External Deployment" approach is used) and only one of those is behind a proxy, even if the document.domain property of the two pages is changed to be equal, the communication is not possible and a "permission denied" error is thrown.

This is an uncommon situation as usually a PC is connected directly to the Internet or through a proxy and so from its point of view both servers are behind the proxy or they aren't. But that could happen if both servers and the client PC are on the same LAN and an exception on the proxy setting lets IE connect to one server directly and to the other using the proxy.

This issue was seen on IE7 and IE6.