WeChat is censoring messages that go beyond China's borders, according to a report by Citizen Lab.

The report said that WeChat is banning messages with keywords that mention sensitive subject like the Tiananmen Square massacre, Tibet, or the banned spiritual movement Falun Gong both in private and group chats without users' knowledge. Citizen Lab found 174 keywords that are censored in WeChat.

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Previously, the Tencent-owned chat app would notify users when their messages were not delivered because it contained controversial subjects such as Tiananmen Square massacre. Currently, WeChat no longer sends notification to either sender or recipient of the messages. The latest censorship in China extends to users living abroad with an account originally registered using a Chinese phone number, even if their account was switched to an international number.

“By removing notice of censorship, WeChat sinks deeper into a dark hole of unaccountability to its users,” Citizen Lab director Ron Deibert said.

Jason Ng, a researcher from Citizen Lab, said that the app censorship is being done via phone number because it allows WeChat to target all Chinese users even if they are using virtual private network (VPN) or other means to avoid the vast censorship apparatus called the Great Firewall of China.

Aside from the controversial keywords, Citizen Lab report found that WeChat is also implementing URL filtering system in its built-in browser. It uses various lists of blacklisted and whitelisted websites for Chinese and International accounts. The report has determined 41 of the websites that were blocked in accounts that were registered under mainland Chinese phone numbers.

The latest report from Citizen Lab was developed from its previous study in 2015 where it found that WeChat censorship started heavily targeting politics and rumor discussions. Last year’s report discovered that the Chinese-owned app deleted sensitive posts using its public accounts feature.

WeChat has over 806 million monthly active users, and most of them are located in China. It is built as an instant messaging service but expanded into video conferencing, voice messaging, media file sharing, and a built-in Web client for surfing the Internet.

TagsWeChat, Tencent, china, censorhip, Citizen Lab, China censorship, messaging app