As a pentester that did exactly this in many corporate networks: this is extremely effective. Just announce with a router advertisement that there is a DHCPv6 server and start handing out link-local IPv6 addresses while you specify your own system as DNS-server.
Clients (both Windows and Linux) will prefer the DNS-server specified through IPv6 over the one from IPv4. Then you can spoof any DNS record and capture juicy NTLM hashes flying through the network or relay their authentication and get a free authenticated connection.
This is most effective in networks that were designed for only IPv4 and didn't consider IPv6 at all. But it is also effective in some networks that do use IPv6.
Mitigations? Either disable the IPv6-stack on all systems, or configure your switches to block the router advertisements and do not allow DHCPv6 traffic to the wrong systems.
Clients (both Windows and Linux) will prefer the DNS-server specified through IPv6 over the one from IPv4. Then you can spoof any DNS record and capture juicy NTLM hashes flying through the network or relay their authentication and get a free authenticated connection.
This is most effective in networks that were designed for only IPv4 and didn't consider IPv6 at all. But it is also effective in some networks that do use IPv6.
Mitigations? Either disable the IPv6-stack on all systems, or configure your switches to block the router advertisements and do not allow DHCPv6 traffic to the wrong systems.