IPv6-related security issues
The following three possible scenarios indicate how IPv6 can affect your network security, even though you have never consciously deployed IPv6.
- Rogue IPv6 router attracts traffic
- Attacker bypasses IP based access control
- Client bypasses firewall with IPv6 tunnel
Read more about the different scenarios here - http://securityblog.switch.ch/2014/08/26/ipv6-insecurities-on-ipv4-only-networks/