The mastermind of the Silk Road anonymous black market website, known for selling guns and drugs, was captured by authorities after he mistakenly integrated the traceable bot protection service Captcha with the address, according to the FBI.
During the prosecution of the site's head Ross William Ulbricht, who used the alias ‘Dread Pirate Roberts’, a former FBI investigator said the site's IP address was insufficiently hidden due to a "leaky" Captcha prompt not being correctly syncronised with the anonymous router Tor - a tool designed to conceal users' browsing histories from ISPs.
Christopher Tarbell, former FBI agent, said in a statement, released by the Southern District New York District Court: “The IP address leak we discovered came from the Silk Road user log-in interface.
"Upon examining the individual packets of data being sent back from the website, we noticed that the headers of some of the packets reflected a certain IP address not associated with any known Tor node as the source of the packets."
Tarbell added: "This IP address was the only non-Tor source IP address reflected in the traffic we examined.”
When the analyst entered the IP address into a conventional search engine, the Silk Road’s log-in page,with the bot security system Captcha prompt, appeared. As a result, the site was closed in October last year during a raid which saw $3.6m worth of Bitcoins seized.
The state last week denied that it had illegally seized evidence against the defendant, releasing the following statement: "Ulbricht conjures up a bogeyman - the NSA - which Ulbricht suspects, without any proof whatsoever, was responsible for locating the Silk Road server, in a manner that he simply assumes somehow violated the Fourth Amendment.
"The facts are not at all what Ulbricht imagines them to be... the Silk Road server was located not by the NSA but by the Federal Bureau of Investigation, using perfectly lawful means."
Ulbricht denied the charges against him and is awaiting a court decision on the legality of the state evidence.
Last month the head of Tor, Andrew Lewman, implicated GCHQ and the NSA in helping the router - by providing anonymous solutions to the site's complex bugs.