The app has sought to allay fears that it could be used as a spying or propaganda tool by Beijing, saying it does not store data in the country and has restricted Chinese staff from accessing overseas data.
Mr Yu’s claims relate to how ByteDance operated before he was dismissed almost five years ago, and before TikTok became one of the world’s most popular apps.
He claimed that in the app’s early days, engineers at ByteDance would steal videos from Snapchat and Instagram and post them on the app in order to artificially stimulate growth on the app. He also accused the company of flooding the service with fake users.
He said he was fired for raising concerns about the practice and is suing for unfair dismissal.
Mr Yu, who said he was filing his case after negotiations with ByteDance failed, told the New York Times that engineers had “backdoor” access to user data wherever it was stored.
He also alleged that Zhang Yiming, ByteDance’s founder, arranged bribes to Chinese government officials.
Shou Zi Chew, TikTok’s chief executive, has claimed that the Chinese government has never demanded data from the app and said the company would refuse to hand it over if it was asked to do so. In a Congressional hearing in March, he admitted that ByteDance staff in China have access to users’ data.
The company is seeking to avoid further crackdowns by setting up US and European data centres and restricting how staff in China can access information. However, its efforts have not been enough to avoid government crackdowns.
In March, TikTok was banned from UK government devices as well as the Parliamentary Wi-Fi network, in what ministers said was a “precautionary” step.
The Biden administration has threatened a wider ban on use in the US if TikTok is not sold by ByteDance’s Chinese investors.
TikTok has been downloaded more than 2bn times and has more than 150m users across Europe, with a similar number in the US.
The company did not respond to a request for comment on the lawsuit and its allegations.
In response to US demands, China has said it would “firmly oppose” any forced sale of TikTok and that such an order would damage investors’ confidence in the US.
Mr Yu is seeking damages and lost earnings as well as the restoration of ByteDance share options that would be worth tens of millions of dollars, according to the New York Times.