Documents and images shared by users outside China on WeChat, the country’s most popular social media platform, are being monitored and cataloged for use in political censorship in China, a new report says.

Citizen Lab, the University of Toronto online watchdog, says in Thursday’s report that WeChat users outside of China are thus unwittingly contributing to censorship. Content they share that censors deem inappropriate is thus barred from being seen by users inside China.

WeChat’s parent, Tencent, did not immediately respond to an online inquiry seeking comment. Tencent declined comment when contacted by CNBC.

Tencent is a major investor in Cary-based Epic Games, which is publisher of the globally popular Fortnite.

Key Findings

Citizen Lab highlighted findings of the report:

• We present results from technical experiments which reveal that WeChat communications conducted entirely among non-China-registered accounts are subject to pervasive content surveillance that was previously thought to be exclusively reserved for China-registered accounts.
• Documents and images transmitted entirely among non-China-registered accounts undergo content surveillance wherein these files are analyzed for content that is politically sensitive in China.
• Upon analysis, files deemed politically sensitive are used to invisibly train and build up WeChat’s Chinese political censorship system.
  From public information, it is unclear how Tencent uses non-Chinese-registered users’ data to enable content blocking or which policy rationale permits the sharing of data used for blocking between international and China regions of WeChat.
• Tencent’s responses to data access requests failed to clarify how data from international users is used to enable political censorship of the platform in China.

100 million users outside China

Until now, WeChat was not known to be subjecting accounts registered outside of China to the same pervasive surveillance as domestic accounts. An estimated 100 million people use WeChat outside China, according to the Munich firm MessengerPeople.

Citizen Lab says its findings are based on technical experiments. It says it did not detect censorship in communications among accounts registered outside China. But it says it did identify surveillance of content being sent exclusively between such accounts.

Tencent does not clearly state in its terms of service that it is surveilling accounts registered outside of China, Citizen Lab says.

The researchers say they first contacted WeChat in January asking about their findings. They said they have not received a response despite WeChat’s acknowledgment in February that it had received their questions.

With more than a billion users, WeChat is the world’s No. 3 messaging app behind Facebook’s WhatsApp and Messenger.

Within China, WeChat is censored and expected to adhere to content restrictions set by authorities.