With ever increasing sophistication, printers are becoming more computer-like every day. High-end printers effectively contain computers; with processors, lots of memory, and even disk drives, these printers need to be secured in a manner similar to typical computers. While the need to secure printers is aimed primarily at those printers which connect directly to a network, even printers connected directly to a desktop computer may be vulnerable (depending on any resource sharing which may be set up on that particular computer).
The degree to which a non-secured printer may be vulnerable depends somewhat on the sophistication of the printer. On less sophisticated printers, a compromised printer may be simply an annoyance as someone else may be able to take control of your printer and block anyone else (including you!) from using it or may waste resources (i.e., make the printer print blank pages or, worse yet, waste ink or toner by printing solid pages). With more sophisticated printers, we've had instances at Lehigh where printers have been hacked. In these instances someone broke into the computer within the printer and used it for unscrupulous purposes such as sending SPAM or distributing copyrighted files.
The degree to which a printer can be secured also depends on the sophistication of the printer. For printers plugged directly into desktop computers, security is typically more of a function of the computer into which the computer is plugged. If the computer doesn't allow print sharing, the printer is most likely secure; if it does allow print sharing, that sharing should be limited through a password or limited to known computer systems.
For printers plugged directly into a network connection, security typically starts with a password. While setting a password for the control of the printer settings won't necessarily prevent someone from sending junk to the printer, it will keep anyone else from easily taking control of that printer. More sophisticated printers may provide more sophisticated security measures. High-end printers containing computers (with operating systems) may even need to be patched just as a typical desktop computer may need the occasional patch.
Check the documentation which came with the printer for any initial security settings. Register and periodically check with the printer manufacturer's web page for any security patches and upgrades which may become available for that printer and, once they become available, protect your printer by applying any additional security in a timely manner.