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Scan For World- and Group-Writable Directories Hack #3 - Security

Security isn't a noun, it's a verb; not a product, but a process. Today, learn the hacks involved in reducing the risks involved in offering services on a Unix-based system. This the first part of chapter one in Network Security Hacks, by Andrew Lockhart (ISBN 0-596-00643-8, O'Reilly & Associates, 2004).

TABLE OF CONTENTS:
  1. Unix Host Security: Hacks 1-10
  2. Secure Mount Points Hack #1
  3. Scan for SUID and SGID Programs Hack #2
  4. Scan For World- and Group-Writable Directories Hack #3
  5. Create Flexible Permissions Hierarchies w/ith POSIX ACLs Hack #4
  6. Protect Your Logs from Tampering Hack #5
  7. Delegate Administrative Roles Hack #6
  8. Automate Cryptographic Signature Verification Hack #7
  9. Check for Listening Services Hack #8
  10. Prevent Services from Binding to an Interface Hack #9
  11. Restrict Services with Sandboxed Environments Hack #10
By: O'Reilly Media
Rating: starstarstarstarstar / 37
May 04, 2004

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Quickly scan for directories with loose permissions.


World- and group-writable directories present a problem: if the users of a system have not set their umask properly, they will inadvertently create insecure files, completely unaware of the implications. With this in mind, it seems it would be good to scan for directories with loose permissions. Much like “Scan for SUID and SGID Programs” [Hack #2], this can be accomplished by running the find command:

# find / -type d \( -perm -g+w -o -perm -o+w \) -exec ls -lad {} \;

Any directories that are listed in the output should have the sticky bit set, which is denoted by a t in the directory’s permission bits. A world-writable directory with the sticky bit set ensures that even though anyone may create files in the directory, they may not delete or modify another user’s files. If you see a directory in the output that does not contain a sticky bit, consider whether it really needs to be world-writable or whether the use of groups or ACLs [Hack #4] will work better for your situation. If you really do need the directory to be world-writable, set the sticky bit on it using chmod +t.

To get a list of the directories that don’t have their sticky bit set, run this:

# find / -type d \( -perm -g+w -o -perm -o+w \) \
-not -perm -a+t -exec ls -lad {} \;

If you’re using a system that creates a unique group for each user (e.g., you create a user andrew, which in turn creates a group andrew as the primary group), you may want to modify the commands to not scan for group-writable directories. (Otherwise, you will get a lot of output that really isn’t pertinent.) To do this, run the command without the -perm -g+w portion.

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