Please fill out the fields below so we can help you better. Note: you must provide your domain name to get help. Domain names for issued certificates are all made public in Certificate Transparency logs (e.g. crt.sh | example.com), so withholding your domain name here does not increase secrecy, but only makes it harder for us to provide help.
The operating system my web server runs on is (include version): Ubuntu 20.04.1
My hosting provider, if applicable, is: AWS
I can login to a root shell on my machine (yes or no, or I don't know): Yes
I'm using a control panel to manage my site (no, or provide the name and version of the control panel): Webmin
The version of my client is (e.g. output of certbot --version or certbot-auto --version if you're using Certbot): certbot 0.40.0
[https://domain.com]has a Free LetsEncrypt SSL using Virtualmin but when someone register on that website, it automatically create a sub-domain [https://ab.domain.com] and created sub-domain is shown as Non-Secure tag in URL with a invalid certificate.
Is there anyway to implement a Free SSL on sub-domain?
Hi, to get help from a volunteer here you need to explain what you've tried to do in detail, including any errors you've encountered, and things you've tried to do to fix it that didn't work.
I guess, the domain that are automatically created by my user on my webite when they register are NX Domain, those domains are not showing anywhere in virtualmin.
In theory, sure! However, it all depends on what your software can offer you.
Looking at your situation with randomly added subdomains to your domain, you might benefit from a wildcard certificate. However, while a wildcard for *.example.com would cover e.g. user.example.com, it would not cover another label deeper like www.user.example.com.