Changing your IP address or using proxy servers to access public websites you’ve been forbidden to visit is a violation of the Computer Fraud and Abuse Act (CFAA), a judge ruled Friday in a case involving Craigslist and 3taps.

The legal issue is similar to one in the Aaron Swartz case, in which there was debate over whether Swartz “had committed an unauthorized access under the CFAA when he changed his IP address to circumvent IP address blocking imposed by system administrators trying to keep Swartz off the network,” law professor Orin Kerr wrote yesterday on the Volokh Conspiracy blog.

The ruling in Craigslist v. 3taps (PDF) is the first “directly addressing the issue,” Kerr wrote. 3taps drew Craigslist’s ire by aggregating and republishing its ads, so Craigslist sent a cease-and-desist letter telling the company not to do that. Craigslist also blocked IP addresses associated with 3taps’ systems.

“3taps bypassed that technological barrier by using different IP addresses and proxy servers to conceal its identity and continued scraping data,” wrote Judge Charles Breyer of US District Court in Northern California. Craigslist subsequently accused 3Taps of violating the CFAA, which “imposes criminal penalties on any person who, among other prohibitions, ‘intentionally accesses a computer without authorization or exceeds authorized access, and thereby obtains… information from any protected computer.’”

3taps asked the court to “hold that an owner of a publicly accessible website has no power to revoke the authorization of a specific user to access that website” and argued that criminalizing its activity under the CFAA would create a slippery slope that could harm ordinary Internet users and allow Web companies to use anti-competitive practices.

More: Changing IP Address to Access Public Website Ruled Violation of US Law