Maintaining system security

SUID, SGID, and sticky bit clearing on writes

SCO OpenServer systems guarantee that the SUID, SGID and sticky bits are cleared on files that are written. This prevents users from substituting another program in the file to take advantage of its SUID or SGID bits (which they could not otherwise set).

NOTE: The clearing of SUID and SGID bits can be disabled if desired. Refer to ``Disabling C2 features'' for more information.

An SUID bit shows as an ``s'' in the permissions of a file. ``Bit clearing examples'' demonstrates bit clearing twice (user input is in boldface).

Bit clearing examples

   $ id
   uid=76(blf) gid=11(guru)
   $ ls -l myprogram
   -rwsrwsrwt   1 root  bin    10240 Jan 11 22:45 myprogram
   $ cat sneakyprog > myprogram
   $ ls -l myprogram
   -rwxrwxrwx   1 root  bin    10240 Mar 18 14:18 myprogram
   $ ls -l anotherprog
   -rws------   1 blf   guru  83706 Dec 15  1987 anotherprog
   $ strip anotherprog
   $ ls -l anotherprog
   -rwx------   1 blf   guru  17500 Mar 18 14:19 anotherprog

In the example, user blf (the id(C) utility was used to show the identity) first uses the cat utility to replace the contents of the file myprogram. The SUID bit is removed during this process. The second example demonstrates that the bit clearing is even done on files owned by the same user. When blf strips the file (removing the debugging information in a compiled binary file), the SUID bit is also removed. You should be aware that the clearing happens when files are replaced. Adjust any installation scripts to reset the proper modes. With this feature, you can place these bits on user programs without fear that the user can switch programs in the same file.

NOTE: SUID and SGID permission bits do not work on shell scripts.

The SUID, SGID, and sticky bits are not cleared on directories. The SUID bit has no meaning for directories, and both the SGID and sticky bits have a meaning for directories that warrant their remaining there.

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© 2003 Caldera International, Inc. All rights reserved.
SCO OpenServer Release 5.0.7 -- 11 February 2003