Messages in this thread | | | From | (Måns Rullgård) | Subject | Re: File Permissions are incorrect. Security flaw in Linux | Date | Wed, 01 Oct 2003 15:08:28 +0200 |
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"Lisa R. Nelson" <lisanels@cableone.net> writes:
> [1.] One line summary of the problem: > A low level user can delete a file owned by root and belonging to group > root even if the files permissions are 744. This is not in agreement > with Unix, and is a major security issue. > > [2.] Full description of the problem/report: > Permissions on a file basis take precedence over directory > permissions (for most cases), but in Linux they do not. In order to > secure a file, you have to secure the directory which effects all files > within it. > As user 'lisa', I do all my work on my server. One task is to move > pictures from my digital camera to my server picture directory that is > wide open to everyone. All users can create sub-folders and put > pictures in there. But every hour I have a cron job run that changes > the ownership to root, and sets the permissions to 644 on all files in > that directory structure. Thinking the files could no longer be altered > by anyone but root (as would be the case in unix), and found anyone > could delete them. That's when I discovered this major bug.
This is not a bug. Deleting a file is in effect a modification to the directory containing the file, not to the file itself. If the file has a hard link in another directory, it will still remain there, unmodified. If you want only the owner of a file to be able to delete it, set the sticky bit on the directory containing it, like this:
chmod 1777 /the/dir
This is commonly used on /tmp. It is also the all Unix systems I've been using work.
-- Måns Rullgård mru@users.sf.net
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