@Nils

Tor path selection avoids using relays from the same /16 subnet, and I thought it considered the Autonomous System (AS) as well. However now I'm not finding concrete evidence that path selection looks at AS. I found some older academic papers on the subject [1], but nothing in the current specification [2].

Avoiding using two nodes from the same AS would seemingly go a long way toward mitigating the attack vector you mentioned though.

1) http://freehaven.net/anonbib/cache/DBLP:conf/ccs/EdmanS09.pdf

2) https://gitweb.torproject.org/torspec.git/plain/path-spec.txt