Google Project Zero reports :
The transmission bittorrent client uses a client/server architecture, the user interface is the client which communicates to the worker daemon using JSON RPC requests.
As with all HTTP RPC schemes like this, any website can send requests to the daemon listening on localhost with XMLHttpRequest(), but the theory is they will be ignored because clients must prove they can read and set a specific header, X-Transmission-Session-Id.
Unfortunately, this design doesn’t work because of an attack called ‘DNS rebinding’. Any website can simply create a dns name that they are authorized to communicate with, and then make it resolve to localhost.
Exploitation is simple, you could set script-torrent-done-enabled and run any command, or set download-dir to /home/user/ and then upload a torrent for .bashrc.Read More