Possibility to write a listener running on crm instance

Hi,

I need to receive a message sent by another application to dynamics crm. This message cannot be sent in a format which is understood by dynamics crm, so I need to convert the message first.

I know that this is possible using a middleware which would do the converting part.

Are there any possibility to build something which runs on the crm instance itself? An aspx page or something like that? The page itself should be public so it should be accessible without authentication.

Thanks for the ideas