"The investigation marks a milestone in China's efforts to handle national security issues related to data, and it could affect how future probes will be carried out."

The Cyberspace Administration of China laid the legal groundwork last year for such probes and is now enforcing new rules, Song said.

According to measures that took effect a year ago, entities subject to cybersecurity reviews should receive the initial results within 45 days, but the results from complicated cases take longer to process.

Song said: "It is high time for companies, Chinese and foreign, to comb through their data assets. Instead of paying lip service, they must sit down and devote considerable resources to identifying crucial data and how to process and store it."

Experts said Didi, Full Truck Alliance and Boss Zhipin boast sizable pools of data that include not just information about individual users, but also basic data related to crucial infrastructure such as roads and transportation－significant aspects of national security.

Li Keshun, deputy director of the Jiangsu Provincial Big Data Transaction and Circulation Engineering Laboratory, said these companies, as leading platforms in the fields of daily travel, online freight transportation and job hunting, hold at least 80 percent of the in-depth data in their respective industries.

"Such data can directly or indirectly reflect flows of population, commercial goods, business operations and other key information in many regions across China," Li said.

In the prospectus for its initial public offering, or IPO, Didi said it has 493 million active riders annually and processes 41 million daily transactions on average.

In 2017, a Didi unit obtained a license to carry out high-definition mapping for autonomous driving technologies in China, which can be closely related to national geographic information security.

Full Truck Alliance's two truck-hailing platforms carry crucial data related to logistics and expressways, as their services are available in more than 300 cities nationwide. According to the company, it is now the largest highway logistics internet information platform in China.

Boss Zhipin boasts 30.6 million active users a month on its online employment platform and served more than 6.3 million certified companies as of March, according to its IPO prospectus.

Didi, Full Truck Alliance and Kanzhun, which operates Boss Zhipin, debuted on US bourses last month.