[sudo-users] Sudo - i without password

[Velko Kanev]

Hello,

I didn't know where to post my question.
I' m working with devices with embedded custom build linux distro. What i
found is pretty easy privilege escalation using "sudo-i" command. It never
asks for password, just gives me  root access ( switches user to "root"). I
checket sudoers file in / etc/sudoers there is nothing like "NO PASSWORD"
for root or any other user.  There is no #include directive in the file
neither. The group file also shows that avery user is in separate group.
The user  which i i initially have access with is "admin" but it is not
part of root group.
The question is why i gain access to root without knowing the root password
and without password prompt?

Have all the best!
Velko Kanev