The key is in the D. Since it is distributed, there's not a single server that can be attacked.
Here are a couple examples of attacks that DHT is vulnerable to:
http://www.cs.rice.edu/Conferences/IPTPS02/101.pdf http://www.cs.rice.edu/~twngan/doc/infocom06.pdf
The key is in the D. Since it is distributed, there's not a single server that can be attacked.