iOS device UDID

How is this possible?

Getting the UDID this way is possible through Apple's Over-the-Air Profile Delivery Concepts. Read more

Is getting my UDID through this way secure?

It is definitely not secure. The profile you are installing could be infected with malware and point your device to malicious servers and install malicious certificates (read more). Also your device information including UDID could be taken and potentially result in privacy violation.

I have already installed a profile from an untrusted site. What should I do now?

If the configuration profile is removed, the harmful changes will be erased.