And even that isn't needed.
"According to Hallacy, a savvy network programmer could change his configuration file to intercept all data from other users on the attacker's local area or "node"."
I noticed that issue in the 90's as I could see other folk's PCs and printers on cable back then. So the issue is not just your cable modem. Luckily most folk install a router after the cable modem which helps a lot.