Bitcoin XT Fork Can ‘Blacklist’ Tor Exits, May Reveal Users’ IP Addresses / Juan S. Galt / 2015-08-20 09:37 AM

Bitcoin XT, the intentional fork that has been shaking up the bitcoin price, includes code branded as a hardcoded “blacklist” by critics. The code specifically targets Tor exit nodes, justifying the approach as protection against denial of service attacks. The code may also reveal the location of a Bitcoin XT node running through the Tor privacy network.

IPs revealed?

In a post to the Bitcoin core mailing list, a researcher by the name of ‘F L’ points out a ‘commit’ to the Bitcoin XT source code, with over a thousand lines, including a hard list of IP addresses that are loaded by default into clients. The code also asks a server for an updated list of IP addresses that it could disconnect from, upon the boot of the client. This, according to Peter Todd, could leak the Tor Bitcoin XT node’s IP address.

The approach is one of prioritizing some IP addresses over others, in the case that a node is overwhelmed with traffic from many sources in what is commonly known as a distributed denial of service attack (DDOS). If the maximum amount of connections a Bitcoin node can support is reached, the client would begin disconnecting from Tor exits, in an attempt to stop a DDOS attack.

The description of the commit created by Mike Hearn, reads: