The study conducted by Harvard University's Berkman Center for Internet & Society showed that distributed denial-of-service (DDoS) attacks frequently knocked such sites offline.

Of the sites surveyed by the center, 62% were victimized by DDoS attacks in the last 12 months, and 61% experienced unexplained downtime.

DDoS attacks -- the kind at companies that withdrew services from the controversial group -- are launched from hundreds, thousands or even tens of thousands of computers simultaneous, or nearly so.

The aim: Flood the servers that host a site with bogus requests, or dupe that server into thinking it's overwhelmed. The result: The site goes black, or is pulled from the Web by its hosting provider to protect other sites from being crippled.

"Human rights and independent media sites are under constant attack," said Ethan Zuckerman, a senior researcher at the Berkman Center and one of the authors of the report, in an interview today. "DDoS attacks make it harder than ever for these groups to keep an online presence."