Rights of a non-admin user

Question

When I install an application being an administrator, is the non-admin user able to modify anything in the new program folder?
Vice-Versa: When I install an application which needs authentication being a non-admin user. Is this non-admin able to modify the program folder after installation with no authentication?
To what security risks may I be exposed to when I always use a non-admin account? What can a script or application do?
Can it cause code injactions or change my startup folder or anything unwated else(like hijacking my Safari or log my passwords)?
Regards, Clemens

Answer 1

the /Applications folder is only writable by administrators. Anything put inside cannot be modified by a non-administrator unless that non-administrator has been specifically given write access.
To what security risks may I be exposed to when I
always use a non-admin account? What can a script or
application do?
A script or application running under non-admin can only modify files that are writable by that user; i.e. the contents of the user's home folder and not much else.
Can it cause code injactions or change my startup
folder or anything unwated else(like hijacking my
Safari or log my passwords)?
Most of what you have listed are admin tasks; they can only be accomplished with an admin account, or from a non-admin account after admin authentication.
As far as password logging, a malware running under a non-admin account could theoretically install a keyboard logging app inside your home folder, and transmit your keystrokes out without your knowledge. That's why it's important to practice safe computing even when running as non-admin.
OTOH, a malware running under a non-admin account couldn't modify any existing applications to do this, whereas the same malware running under an admin account could. This is one more reason to save your admin account for tasks that need it and do everything else from a non-admin account.