Let's take a deeper view of it. The following image shows the general idea about the implemented attack on a proxy server (BTW they setup a SQUID proxy and the registered it on public proxy registries).
Again I don't see any interesting technique in this. BUT I do see the beauty of this study in capturing the "stakeholders". If you follow on reading the paper, authors show who used this proxy and what he did with it. Obviously most of the performed operations by exploiting the free (and hacked) proxy were with malicious intents. One of the most interesting proves that authors provided is about Scam and people who answered back by giving personal informations.
Most of the stakeholders come from Ex-URS, Brasil and USA. Many of them from Cina, only few of them from Europe. Beside normal stats on where users come from, understandings how malicious hackers use proxies to attack is really interesting. Another little but significative theoric brick could be added to all the knowledge we had from honey-net project.