We have implemented Clientless SSL VPN on an ASA5520 - OS 8.0.4(28). It now appears that there is a known bug when trying to use this feature through a proxy server. A workaround option is to configure smart tunnels, which we have tried but still without success. The Client log in screen comes up OK and we can login and select Application Access. The following screen says that the smart tunnel has started successfully but the local application we select (mstsc.exe) does not work. Has anyone successfully configured this workaround ? I would appreciate any assistance to resolve this issue.
Use the Proxies pane to configure the security appliance to use external proxy servers to handle HTTP requests and HTTPS requests. These servers act as an intermediary between users and the Internet. Requiring all Internet access via servers you control provides another opportunity for filtering to assure secure Internet access and administrative control.
Take a look here for configuring the use of external proxy sever using clientless SSL VPN: