Sharing Files Richard Newman based on Smith “Elementary Information Security”

1. Sharing FilesRichard Newmanbased on Smith “Elementary Information Security”

2. File Sharing Policies Generic • Isolation • Share everything Tailored • Privacy • Shared Reading • Shared Updating

3. Tailored File Sharing Policy Considerations Which objects are involved • By owner • By project • By type Which users are granted new rights • Individual • Group • Role • Role relative to X Default access • Deny or allow? What access rights to enforce • R, W, X, ... • Conditions?

4. File Sharing on Windows Default: Isolation Per folder file sharing • Chose people to share with • Permission level Permission levels • Owner – full access, owns folder • Co-owner – full access, doesn't own folder • Contributor (not in Windows 7) – create files, modify own files • Reader – read files in folder

5. User Groups Unix, “professional” Windows Implementation of group access • File has group owner attribute, and group access rights • Group named on ACL with access rights Implementation of groups • Special file – edited • System file accessed through utilities • Built-in groups • Administrators, wheel group, per-user group • Associate user ID with group upon login • Allows more precise audit

6. Least Privilege Administrator access • Log in as admin • Log in as user with admin rights (in group) • Log in as regular user, but ability to authenticate as admin for specific programs, actiions (sudo, etc.) • Switch mode as needed (setuid) • UAC – User Account Control - “permission to continue”

7. Posix File Permissions Three classes of entity relative to a file • User (Owner) – one uid per file • Group – one gid per file • Other (World) • Of course, there is always root.... Access permission bits – rwx • r – read file or directory listing • w – write/modify file or create/rename/delete files in directory • x – execute file or use directory in path Changing permissions • chmod • chown • chgrp

8. Rights Ambiguity Conflicting rights bits • e.g, group allowed to write but user not allowed to write • e.g. other allowed to read but group not allowed to read • Any user attempting access belongs to “all” and zero, one, or two other categories How are conflicting rights bits treated • OpenVMS - “OR” bits of all applicable classes • Unix – use bits of most specific applicable class – explicit denial • Root • User • Group • Other

9. Access Control Lists (ACLs) Need for ACLs • Policy with read access for group A, read/write access for group B • Can't do it in Unix rwx bits Multics • Pioneered ACLs • Subject matched to a.b.c with wildcards • a is user name, b is project name, c is compartment • rew rights Modern systems • Posix.1e • Apple Macintosh OS-X • MS Windows • NFSv4

10. Posix.1e ACLs Classes • User – user::rwx • Group – group::rwx • World – other::rwx • Specific user – user:user-name:rwx • Specific group – group:group-name:rwx • Mask applied to specific users/groups – mask::rwx

11. Windows/Mac ACLs Per file/directory • Owner rights • Everyone rights • Specific user rights • Selection of no access/read only/read-write Special considerations: Must have admin rights on Mac

12. Apple OS/X ACLs Entities • Owner • Everyone • Specific individuals • Groups Access rights • No access • Read-only • Read-write • “OR” of rights apply to requester Groups • Created under “System Preferences -> User Accounts” • Requires admin access

13. Windows ACLs Entities • Owner • Everyone • Specific individuals • Groups Access rights • “Full Control” • Modify • Read & Execute • Read • Write • Special Permissions • Allow and deny flags • Apply deny first, then check for allow