In January, users were blocked from sending messages containing the characters for "nanfang zhoumo", Chinese for "Southern Weekly", a newspaper that was in open revolt against press control in Guangdong province.
Tencent's self-censorship, which has been shown to block sensitive messages sent outside China as well, may also affect its efforts to push WeChat outside China. The platform already has more than 100 million users outside China, and the company has signed soccer star Lionel Messi to promote it overseas.
"A big issue for Tencent would be convincing Americans and Europeans that they're not operating under the same self-censorship principles outside China as in China," said Doug Young, the Shanghai-based author of The Party Line. "Image-wise that could hurt them in their global expansion."
(Read more: We're reassured by China reforms: World Bank)
A spokesman for Tencent declined to comment on its censorship of WeChat because of the sensitivity of the issue.
"A conspicuous problem"
WeChat's relative newness means researchers are still studying how the app's censorship and monitoring work.
"If controls are present on the server side it makes it much more difficult to verify," said Masashi Crete-Nishihata, research manager of Citizen Lab, a research center at the University of Toronto. "And so far we suspect that's implemented on the server side."
Citizen Lab has already been able to show that Line, another smartphone messaging app created by Line, a Japanese subsidiary of South Korea's Naver, blocks a regularly updated list of banned phrases in China. Work they did with the University of New Mexico found that the use of censored words in the old Chinese version of Skype could also trigger remote surveillance.
Microsoft switched its Chinese partner for Skype in November and the program is now believed to be more secure.
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Weibo, the microblog, uses a computer system to scan each post before publication so sensitive ones can be flagged for censors employed by Sina, who decide whether to delete them.
"Just knowing how the censoring apparatus works, my personal guess would be they're going to use the same mechanisms," said Gary King, a Harvard professor who has researched how social media platforms are censored in China, but not WeChat.
Tencent's 15-year-old instant messaging service, QQ, is subject to active monitoring and censorship, according to Citizen Lab.
Speaking at a conference in London in November, Google Chairman Eric Schmidt recalled a meeting with President Xi and Premier Li Keqiang, just weeks after China passed its new regulations on "online rumors".
"The most interesting thing about talking to the government, from the president all the way to the governors, is that they are obsessed with the Internet," said Schmidt, without elaborating on their conversations.
(Read more: China's economic growth more like 4%: Marc Faber)
Authorities have signaled that they plan to increase their control of social media, including WeChat, and further "manage" China's Internet.
"If anything happens and it becomes explosive, everybody knows that Weixin will be the next target," said Jiang, of the University of North Carolina at Charlotte.