When Internet users in China fire up TheAtlantic.com, check out product specifications on Sony Mobile, or add a Firefox plugin, well, too bad. The Chinese authorities have blocked those and thousands of other sites—and just as the People’s Republic hosts the World Internet Conference, to boot. All of these have one thing in common: They use edgecastcdn.net, the content-delivery network (CDN) of Verizon’s EdgeCast, says Greatfire.org, a group that promotes Chinese Internet freedom.

Charlie Smith of Greatfire says he’s seen “nothing on this scale ever before,” though that’s “because so many companies use EdgeCast for hosting.” (“Charlie Smith” is a pseudonym, used by a Greatfire member because of the Chinese government’s sensitivity to the group’s efforts to expose and undermine censorship.)

It’s not that The Atlantic or Sony had just uploaded subversive content. What Smith thinks is going on is that the Chinese government tried to block certain sites served up by EdgeCast. But EdgeCast serves up content for tens of thousands of sites, and the Chinese authorities seem to be blocking many others as well. (CDNs cache websites’ content on servers that are physically located closer to users to speed access times, absorb traffic spikes, and provide more efficient delivery of the data.)

EdgeCast said in a blog post that this week it’s seen China’s content “filtering escalate with an increasing number of popular web properties impacted and even one of our many domains being partially blocked … with no rhyme or reason as to why.”

That scattershot blocking risks making important commercial websites unavailable within China. Sony Mobile, for instance, likely won’t be able to communicate with its in-China users. Administrators of Drupal, a project website that many companies use, will also face disruptions as they update content and install extensions. (Sony Mobile didn’t immediately respond to a request for comment.)

This brings us to a flaw in the Great Firewall that Greatfire is trying to exploit. Though it’s frustrating for Sony and all the other sites with commercial activity in China—and readers, of course—that’s part of the strategy, something called “collateral freedom.” The idea is this: In addition to focusing on how to make the free flow of sensitive information harder to block, groups like Greatfire want to make it harder for the Chinese government to separate traffic flowing toward “sensitive content” from traffic to Sony, Drupal sites, and other neutral pages that the Chinese government wants to allow.

The wider-spread the use of a handful of huge CDNs like EdgeCast, the harder it can be for the Chinese government to cut off a site it dislikes without cutting off other innocuous ones. That could potentially make China a less appealing place to do business and foster citizen opposition to the censorship.

Alas, this site-blocking problem also makes it hard to guess which site the Chinese government was going after with this latest EdgeCast blackout. Smith offers one theory, though. Greatfire recently created a slew of mirror sites—meaning a clone of a site that the Chinese government had blocked, which Greatfire then promotes in the Chinese mainland through its GitHub site, among others, says Smith. Because this mirror-site clone is served to users by a global CDN alongside zillions of other commercial sites, the once-blocked site is effectively accessible in the mainland again, unless—as happened here—the government blocks whole swathes of the CDN’s traffic. (The authorities have also in the past blocked GitHub, but had to unblock it due to its importance for workers in the mainland’s tech industry.)

The latest mirror site created by Greatfire—which used EdgeCast—was for the Chinese-language news site Boxun. This, says Smith, “may have been the final straw for the authorities as Boxun has a history of breaking pretty big news stories in China, almost all of which have been censored by the authorities.”

EdgeCast says it has “put policies in place to help our customers mitigate the effects of this most recent filtering.” It adds, “For those of our customers who are frustrated by this, we share your frustration, as does the whole content delivery and hosting industry.”