http://ttevhjjsjxz6uqqcjkbig5cycd7n7xv7cmd6f5fcvrqaaa7f3bj36wad.onion/technology/security/pow/index.html
¶ There's a lot of computation needed in order to establish an Onion Service
connection between a client and a service. This computation consumes processing
time, memory, storage and energy in the server, opening the door to a
vulnerability known as Denial of Service (DoS), where an attacker spawns
multiple connection attempts in order to overload the service and render it
inaccessible.