For a case where I want to log a string to a text file, say in C:\log.txt, is this possible in UMDF driver? considering that UMDF drivers have limited UAC.
You cannot log to C:\. Standard users are not allowed to write to that location.
The reason the default security settings on C:\ prevent standard users from writing to C:\ is to stop applications from writing to C:\.
You can write to locations you can write to, e.g.