Hours after security researchers at Citizen Lab reported that some Zoom calls were routed through China, the video conferencing platform has offered an apology and a partial explanation.

To recap, Zoom has faced a barrage of headlines this week over its security policies and privacy practices, as hundreds of millions forced to work from home during the coronavirus pandemic still need to communicate with each other.

The latest findings landed earlier today when Citizen Lab researchers said that some calls made in North America were routed through China — as were the encryption keys used to secure those calls. But as was noted this week, Zoom isn’t end-to-end encrypted at all, despite the company’s earlier claims, meaning that Zoom controls the encryption keys and can therefore access the contents of its customers’ calls. Zoom said in an earlier blog post that it has “implemented robust and validated internal controls to prevent unauthorized access to any content that users share during meetings.” The same can’t be said for Chinese authorities, however, which could demand Zoom turn over any encryption keys on its servers in China to facilitate decryption of the contents of encrypted calls.

Zoom now says that during its efforts to ramp up its server capacity to accommodate the massive influx of users over the past few weeks, it “mistakenly” allowed two of its Chinese data centers to accept calls as a backup in the event of network congestion.

From Zoom’s CEO Eric Yuan:

During normal operations, Zoom clients attempt to connect to a series of primary datacenters in or near a user’s region, and if those multiple connection attempts fail due to network congestion or other issues, clients will reach out to two secondary datacenters off of a list of several secondary datacenters as a potential backup bridge to the Zoom platform. In all instances, Zoom clients are provided with a list of datacenters appropriate to their region. This system is critical to Zoom’s trademark reliability, particularly during times of massive internet stress.”

In other words, North American calls are supposed to stay in North America, just as European calls are supposed to stay in Europe. This is what Zoom calls its data center “geofencing.” But when traffic spikes, the network shifts traffic to the nearest data center with the most available capacity.

China, however, is supposed to be an exception, largely due to privacy concerns among Western companies. But China’s own laws and regulations mandate that companies operating on the mainland must keep citizens’ data within its borders.

Zoom said in February that “rapidly added capacity” to its Chinese regions to handle demand was also put on an international whitelist of backup data centers, which meant non-Chinese users were in some cases connected to Chinese servers when data centers in other regions were unavailable.

Zoom said this happened in “extremely limited circumstances.” When reached, a Zoom spokesperson did not quantify the number of users affected.

Zoom said that it has now reversed that incorrect whitelisting. The company also said users on the company’s dedicated government plan were not affected by the accidental rerouting.

But some questions remain. The blog post only briefly addresses its encryption design. Citizen Lab criticized the company for “rolling its own” encryption — otherwise known as building its own encryption scheme. Experts have long rejected efforts by companies to build their own encryption, because it doesn’t undergo the same scrutiny and peer review as the decades-old encryption standards we all use today.

Zoom said in its defense that it can “do better” on its encryption scheme, which it says covers a “large range of use cases.” Zoom also said it was consulting with outside experts, but when asked, a spokesperson declined to name any.

Bill Marczak, one of the Citizen Lab researchers that authored today’s report, told TechCrunch he was “cautiously optimistic” about Zoom’s response.

“The bigger issue here is that Zoom has apparently written their own scheme for encrypting and securing calls,” he said, and that “there are Zoom servers in Beijing that have access to the meeting encryption keys.”

“If you’re a well-resourced entity, obtaining a copy of the internet traffic containing some particularly high-value encrypted Zoom call is perhaps not that hard,” said Marcak.

“The huge shift to platforms like Zoom during the COVID-19 pandemic makes platforms like Zoom attractive targets for many different types of intelligence agencies, not just China,” he said. “Fortunately, the company has (so far) hit all the right notes in responding to this new wave of scrutiny from security researchers, and have committed themselves to make improvements in their app.”

Zoom’s blog post gets points for transparency. But the company is still facing pressure from New York’s attorney general and from two class-action lawsuits. Just today, several lawmakers demanded to know what it’s doing to protect users’ privacy.

Will Zoom’s mea culpas be enough?

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