> In high school, we had a Netware 3.12 environment, and the Guest account was enabled, albeit with very limited privileges. But for some reason, Guest could still use NET SEND, which popped up a little message in the bottom row of the destination machine's display. > ...Nobody noticed that these messages came from GUEST

You mention Netware, but as I recall the Netware function you describe was just "SEND" and "NET SEND" was a Microsoft networking thing. (But maybe there was some integration between the two after my experience with Netware, who knows.)

I mainly wanted to say, as someone who used/abused a Netware network in high school, I disassembled the SEND program and discovered that the username included in the message is not authenticated at all -- the IPX (or NETX, I forget which) software interrupt just took a string, and the SEND executable formatted the username into this string. So by crafting your own SEND program that used the software interrupt directly, you could easily forge any username you wanted. So you could very easily send a message from "ADMIN". :)

This should not be construed as a confession of any network shenanigans that may or may not have occurred at my high school. ;) :D :)