Is it possible for an exit node to modify the data stream of a circuit, for example could a cell such as a padding cell be send down the stream after the data have been transmitted ?
I am thinking this is possible due to the binary modification attack, but am unsure if the exit node can only modify data, and not add cells
Thanks
Yes, the exit relay can do any of the behaviors you describe. It can modify the data that comes back from the website (or whatever the destination is), or it can simply pretend to be the destination and send you whatever it wants to. It can also send other Tor cells, like padding cells, if it wants.
Applications like Firefox rely on end-to-end encryption (like https) to recognize if data has been tampered by a man in the middle. In this case the exit relay is indeed 'in the middle' between the browser and the webserver. End-to-end encryption is a good idea whether you are using Tor or not.
The other questions listed here as 'related' look like more detailed duplicates of this question, so be sure to check them out too (and maybe even merge this question with one of them).
An exit node can not initiate a stream to a client or on behalf of a client. Only clients can initiate streams. Once a client has established a stream, if the service is plaintext, the exit node can monitor and manipulate the data in both directions.
