Comment by ementally

https://spec.torproject.org/vanguards-spec/index.html

>A guard discovery attack allows attackers to determine the guard relay of a Tor client. The hidden service protocol provides an attack vector for a guard discovery attack since anyone can force an HS to construct a 3-hop circuit to a relay, and repeat this process until one of the adversary's middle relays eventually ends up chosen in a circuit. These attacks are also possible to perform against clients, by causing an application to make repeated connections to multiple unique onion services.