I did some research on zzhc.vnet.cn and what its purpose might be. Zzhc is probably an abbreviation of 自注册, meaning self-registration. There is plenty of documentation (in Chinese) on how to implement it (e.g. [1]), but so far I haven't been able to figure out what it's actually good for.

You can find implementations by Qualcomm and Mediatek on GitHub, the Mediatek one even comes with a minimal README [2]. That seems to indicate that it's gated by a feature flag "MTK_CT4GREG_APP" and is only supposed to be active when explicitly selected while the phone is in developer mode. That makes it likely that sending the data was only due to a misconfiguration.

Considering the long list of manufacturers starting at page 10 of [1], it's also possible that others are leaking data in the same way.

[1] https://wenku.baidu.com/view/c2eaa9fc5022aaea998f0f7f.html

[2] https://github.com/griffins-testing-ground/android_vendor_mt...