The destination address of HSRP hello packets is the all routers multicast address (224.0.0.2) and the port is UDP 1985, so it could be susceptible to malicious activities.
Another weak defense against this, other than the one you suggested, is to use a HSRP password. An example command is "standby 1 authentication cisco". However, this is sent in clear text, so a sniffer could defeat this. I would use priority and passwords to help protect your HSRP.
VTP is also vulnerable to this sort of thing (a PC can become root and sniff your network) as well so you can use VTP password as well.
In general common network defenses you can use are: disabling unused ports, using dedicated native vlans on trunks (ie vlans that aren't used by users), port security, bpdu guards and private vlans.
Hope it helps.
Steve