can dhcp be a security issue.
if every router is setup to forward the request to the next outer server, then your isp will get lots of dhcp requests.
if they wanted to show you why this is a bad idea, they could send your requests to someone else's dhcp server, locking your workstations out of your local network. ;)
in other words, no it isn't.
dhcp is a nic asking for config data. the network connection is not active until it gets the ip number. the dhcp server will have to be limited to not respond to external requests.
but the protocal itself is no more a risk than using static ip's