Hackers are increasingly hitting the Web sites of human rights and independent media groups in an attempt to silence them, says a new study released this week by Harvard University's Berkman Center for Internet & Society.

Based on a survey of 45 groups, the report "Distributed Denial of Service Attacks Against Independent Media and Human Rights Sites" found that a large percentage said they've been targeted by distributed denial-of-service (DDoS) attacks from those who disagree with their viewpoints. The Web sites typically have been knocked offline for short periods of time but in some cases have been down for days.

Overall, the survey uncovered reports of 329 attacks against more than 815 Web sites going back to 1998. But more recently, in the 12 months from September 2009 to August 2010, the center received reports of 140 attacks against more than 280 sites. That number likely represents only a small subset of all DDoS attacks, according to the center, since most attacks aren't reported in the media.

DDoS attacks have been much in the news lately as a result of the attacks against PayPal, Visa, and MasterCard by "hacktivists" from the Anonymous group in support of WikiLeaks. In September, the Web site of the Motion Picture Association of America was brought down by users of the Internet bulletin board 4chan, which targeted the MPAA for its actions against online file-sharing sites and individuals.

One example cited in the Berkman Center's study was that of Russian independent newspaper Novaya Gazeta. The paper's executive editor, Sergey Sokolov, reported that his Web site has been hit by sustained DDoS attacks many times in the past year, once keeping it offline for more than a week. Though Sokolov can't be sure who's behind the attacks, he believes the culprits are government-sponsored "Kremlin Youth" organizations.

Many of the organizations who've reported DDoS attacks don't have the resources--financial and otherwise--to effectively combat them. Ethan Zuckerman, one of the authors of the study, suggests that such groups consider moving their sites to such hosts as Blogger, WordPress, or other large hosting platforms that offer certain protection against DDoS attacks. In addition, Zuckerman advises groups to have mirror sites in place in case their main site goes down, in the same way that activists recently set up mirror sites for WikiLeaks.

To compile its study, the Berkman Center said it invited 317 independent media and human rights sites around the world to join in on the survey focusing on DDoS attacks against their groups. Though only 45, or 14 percent, of the sites responded, the center believes such a response rate is acceptable and understandable given the difficulty in reaching the right people and the sensitivity of the topic.