File System Security in Unix

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File System Security in Unix

• Annie Calpe

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Overview

• Unix Basics
• File System Security
• - Account Security Passwords
• - File Permissions
• - Access Control Lists

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Unix Basics

• Developed in the late 1970s
• A multi-user environment
• Intended to be used only by a small number of
  people within the same company

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An Area of File System Security

• Account Security
• Concerned with keeping unauthorized users from
  gaining access into the system.
• A login feature is added for file security.

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Account Security Passwords

• One way hash encryption algorithm
• Uses a modified version of the Data Encryption
  Standard (DES)
• Uses a salt

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The UNIX File System

• Controls the way that information in files and
  directories is laid out on the disk
• The hierarchical organization of files
  directories is often represented with a tree
  structure

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Simple Unix File Structure
/(root)
u
tmp
bin
huey
cp
rm
myfile
file1
file2
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Another Area of File System Security

• File System Security
• Concerned with preventing unauthorized access
  to the systems data.
• Access of data is controlled through the use of
  file permissions.

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File Permissions

• a.k.a. mode bits or permission bits
• It controls which users can access what and how.
• They depend also on the permission bits
  associated with all directories above the file
  in the directory hierarchy.

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Why should you learn about permissions?

• May allow others to access, alter and even delete
  your files!
• The default setting is not acceptable where
  sensitive data is present.
• Many beginning users of Unix dont understand
  modes, thus compromising overall security.
• Permissions are a useful tool to enhance file
  system security!

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File Ownership Levels
User User who owns the file
Group Users in a group associated with the file
Other All other users world permissions
All Includes all of the above levels
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3 Levels of Access Protection
r read read a file read contents of directory
w write write data to a file change the contents of the directory
x execute run an executable program search the contents of a folder or subdirectory
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Format of Permissions

• _ _ _ _ _ _ _ _ _ _

file or directory?
User modes
Group modes
Other modes
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Additional Permissions

• 4th bit is available per set. These are only
  useful for executable files
• setuid (replaces users x by s)
• setgid (replaces groups x by s)
• sticky (replaces others x by t)

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Additional Permissionssetuid and setgid

• Used when another user must perform a task which
  only the owner of a file has the power to do
• setuid script which assumes the User ID of the
  program when run
• setgid script which assumes the Group ID of the
  program group when run

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Additional Permissions The sticky Bit

• When set, only the root or owner can
  unlink/rename files in a directory.
• Without it, anyone able to write to the directory
  can delete/rename files.
• It is commonly found on world-writable
  directories, such as /tmp.

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Checking Current Access Modes

• To check the file characteristics for all files
  within the current directory, type
• ls -l
• To check the file characteristics of a specific
  file or directory, type
• ls -l ltfilename or directorygt

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ls -l Example Output

• gt ls l

drwxr-xr-x 3 huey 512 Dec 27 1558 dir1
-rwx------ 1 huey 16384 Jun 1 1345 progfile1
-rwsr-sr-t 1 huey 24576 Jan 23 1635 progfile2
-rw-r--r-- 1 huey 40 Dec 29 1142 textfile1
-rw-rw-rw- 1 huey 1024 Mar 23 0819 textfile2
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Setting Permissions

• The chmod command is used to set or modify file
  and directory permissions
• 2 ways to specify permissions
• - symbolic mode
• - absolute mode

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Setting Permissions Symbolic Mode

• chmod ltwhogt op ltperm bitsgt ltfilenamegt

options
who u, g, o, a
op , -,
perm bits r, w, x, s, t
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Symbolic Mode - Example

• Apply to all rows in current directory
• gtchmod urwx,grx,or row
• Allow all users to read and search contents of
  directory
• gtchmod arx dir1

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Adding Permissions - Example

• gtls -l textfile1
• -rw-r--r-- (current permissions)
• gtchmod gw textfile1
• gtls -l textfile1
• -rw-rw-r-- (updated permissions)

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Subtracting Permissions

• gtls -l textfile1
• -rw-rw-r-- (current permissions)
• gtchmod g-w textfile1
• gtls -l textfile1
• -rw-r-r-- (updated permissions)

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Setting PermissionsAbsolute Mode

• chmod ltsum of permitted octal sgt ltfile(s)gt

Octal Permission
4000 2000 1000 0400 0200 0100 set user id set group id sticky bit Read by owner Write by owner Execute by owner
Octal Permission
0040 0020 0010 0004 0002 0001 Read by group Write by group Execute by group Read by other Write by other Execute by other
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Absolute Mode Example

• To let the owner read, write, and execute
• 0400 0200 0100 0700
• Yet, only allow group users and others to read
  and write to the file
• 0040 0020 0060 (group value)
• 0004 0002 0006 (others value)
• The octal value 0700 0060 0006
• gt chmod 766

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umask

• Used to turn off permissions
• umask ltvaluegt
• For a file determine the numeric value for the
  desired permissions and subtract it from 666 to
  get the umask value

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umask

• For a directory determine the numeric value for
  the desired permissions and subtract it from 777
  to get the umask value
• Used to set default permissions for newly created
  files within the directory, only during the
  current shell session.

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Access Mode Limitations

• Modes are defined to only user, group, and others
• i.e. Users cannot designate file access to
  specific users

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Access Control Lists (ACLs)

• Reduces complexity of managing permissions
• Stored as extended attributes
• Allows you to define lists that grant/deny access
  to a given file based on criteria that you
  provide
• Enabled ACLs will append a to the set of
  permissions.
• e.g. -rwxr--r--

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ACLs

• Can have separate access control specifications
• Can limit permissions granted to individually
  specified users or groups
• Can allow user and group permissions to be
  automatically specified upon file creation

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setfacl

• Used to modify(-m) or remove(-x) ACLs
• e.g. setfacl m duusernamerwx
• gstudentrwx
• mydir
• NOTE d is optional for setting default ACLs
• for a directory

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getfacl

• Lists ACLs on files and directories
• getfacl ltfilename or directorygt

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Summary

• Security is an important issue with the rise in
  popularity of Unix as a multi-user
  environment(due to its portability).
• The most secure way of protecting your files
  would be to not to store them in the system in
  the first place.
• Otherwise, having an understanding of permissions
  is a good start towards enhancing security.

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References

• FreeBSD Handbook
• Improving the Security of Your Unix System D.
  A. Curry
• OReilly Practical UNIX Internet Security S.
  Garfinkel G. Spafford
• UNIX Commands Western Michigan University
• Unix System Security A Guide for Users and
  System Administrators D. A. Curry
• http//acl.bestbits.at/