The CLIENTSECID is an invisible component that may be used to manage an additional layer of security. It corresponds with the server side class CLIENTSECIDBinding.

The server side instance of CLIENTSECIDBinding creates a unique id with every response. This id is kept in the client component and is sent back to the server side with the next communication round trip. The server side checks if this id was one which was actually sent to the client side and if it is not sent a second time. The id that is sent back and forth is part of the https-content and is encrypted as consequence.