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I'm curious as to what happens to a file if let's say, it's permissions only allow the owner to read and write, and the group and everyone have no access at all. (700, or rwx------).

Let's say the file is on an external drive, so it's possible that the owner is no longer available because we have disconnected the drive and connected it to another computer. How am I able to access the files in this case? Can I still access them by logging in as a superuser?

On one hand, it'd be nice to be able to login to a superuser on any computer to access the file, because at least there's a backup plan if the original computer ever dies, etc. On the other hand, it seems like a security concern that anyone can technically access data on an external drive, or even an internal drive perhaps, by declaring themselves as a superuser.

Gary
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  • On systems with more advanced rights management (e.g. most Unix and Windows NT versions), the administrator cannot access the file directly, but needs to take ownership first, and cannot give ownership back. This step can be put in audit logs which the admin can't alter either. In this way, security can be breached, but it leaves permanent traces. – MSalters Aug 08 '13 at 09:02

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The superuser will always be able to read any file. And will be able to change owner or permissions of any file, regardless of the original owner.

So yes, you could access those files using the superuser, and you could change permissions or owner to be able to acces them with your own regular user.

This is indeed very useful, for example, to make backups, root can make a backup of the whole drive regardless of any permission, and you will be thankful for that later if you happen to lose anything.

Regarding your security concern, anyone who has physical access to any drive, will be able one way or the other to access the data in it. If you really are worried about this, you coul encrypt the data, so even if the superuser can access it, there won't be anything useful if he doesn't know the key.

Chikitulfo
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  • Excellent thanks, that all makes sense. So I guess file permissions are more meant to "direct traffic" on a computer, in a way, rather than actually protect any data? – Gary Aug 08 '13 at 06:52
  • @Gary It protects data, but it can't protect data from super users. If you have a multi-user system and those users are not super users the data is safe. (I am including not being able to boot off a knoppix cd and login as root under that as "not super user", mainly situations where file permissions can be "trusted" are scenarios where the user does not have physical access to the machine (web/file/terminal servers for example)) – Scott Chamberlain Aug 08 '13 at 07:14
  • Right, remote servers are a very good point and of course that's one of the best examples of where file permissions work well (just like when I connect to my website's FTP server). I feel like on a personal computer, the security advantage isn't as strong then. Actually, the reason I am curious about this is because lately, I've noticed that some files on my Mac have the permission of 700, and I thought that was a problem, but it shouldn't be if I can just act as a superuser whenever necessary to access them (from other computers). – Gary Aug 08 '13 at 07:19
  • @Gary I case of a personal computer, there is often a single user, so indeed there are less security concern. Yet many services run as special accounts and have right to access only their files. That's a good way to prevent security holes and viruses, etc... – Levans Aug 08 '13 at 07:25
  • Yeah, so in that sense that's why I referred to it as "directing traffic". It's more to protect files from services, because on my Mac I notice a lot of special user names and groups (staff, wheel, etc.) – Gary Aug 08 '13 at 07:30