In general terms, yes, it’s possible. Game clients receive data from their servers, which they expect to be in a particular format. If the server is modified to send mis-formatted data, the result could easily be to trigger a buffer overflow or other exploitable bug in the client.
See for example http://threatpost.com/researchers-discover-dozens-of-gaming-client-and-server-vulnerabilities/100744
solved Can I hack people connecting to my server? [closed]