curl supports communicating using the TELNET protocol and as a part of this it offers users to pass on username and "telnet options" for the server negotiation.
Due to lack of proper input scrubbing and without it being the documented functionality, curl would pass on username and telnet options to the server as provided. This could allow users to pass in carefully crafted content that pass on content or do option negotiation without the application intending to do so. In particular if an application for example allows users to provide the data or parts of the data.