22.5.1 Dangerous Write Permissions

The sendmail program, of necessity, needs to trust its configuration file. To aid in the detection of risks, it checks the permissions of its configuration file when first reading that file. If the file is writable by group or world, sendmail logs the following message: [13]

[13] This is done only when not in rule-testing mode to prevent spurious warnings when you already know you are using a weak configuration file with -C.

configfile: WARNING: dangerous write permissions

If sendmail is being started as a daemon or is being used to initialize the aliases database, it will print the same message to its standard error output.

22.5.2 Permissions for :include:

The sendmail program doesn't always run as root. When delivering mail, it often changes its identity into that of a nonprivileged user. When delivering to a :include: mailing list, for example, it can change its identity to that of the owner of the list. This too can pose security risks if permissions are not appropriate. Consider the following aliases file entry:

newprogs: :include:/usr/local/lists/proglist

Here, notification of new programs are mailed to the alias newprogs. The list of recipients is taken from the following file:

-rw-rw-r-  2 bin  prog   704 Sep 21 14:46 /usr/local/lists/proglist

Because this file is owned by bin, sendmail changes its identity to bin when delivering to the list of recipients. Unfortunately, the file is also writable by the group prog. Anyone in the group prog can add a recipient to that list, including one of the form

|/tmp/x.sh

This tells sendmail to deliver a copy of the message by running the program (a shell script) /tmp/x.sh. The sendmail program (which is still running as bin) executes that program as bin. Further, suppose the program /tmp/x.sh contains the following:

#!/bin/sh
cp /bin/sh /tmp/sh
chmod u+s /tmp/sh
cat - > /dev/null
exit 0

This causes bin first to make a copy of the Bourne shell in /tmp (a copy that will be owned by bin), then to set the suid bit on that copy (the u+s):

-rwsr-xr-x  1 bin    64668 Sep 22 07:38 /tmp/sh

The script then throws away the incoming mail message and exits with a zero value to keep sendmail unsuspecting. Through this process, an ordinary user in the group prog has created an suid shell that allows anyone to become the semiprivileged user bin. From the preceding discussion (see Section 22.5, "Permissions") you can see the trouble that can cause!

• Mailing lists (:include:) must live in a directory, all the components of which are writable only by root. The lists themselves must be writable only by the owner. If the owner is an ordinary user, group write permissions can be enabled, provided that the user is advised of the risks.

• Mailing list (:include:) files may safely be owned by root. When sendmail processes a root-owned mailing list, it changes itself to run as the user and group specified by the DefaultUser option (see Section 34.8.15). That option defaults to daemon but should be set to nobody and nogroup. The DefaultUser option should never be set to root.

22.5.3 Permissions for ~/.forward Files

The ~/.forward file can pose a security risk to individual users. There is a higher degree of risk if the user is root or one of the semiprivileged users (such as bin). Because the ~/.forward file is like an individual mailing list (:include:) for the user, risk can be encountered if that file is writable by anyone but the user. Consider the following for example:

drwxr-xr-x 50 george guest        3072 Sep 27 09:19 /home/george/
-rw-rw-r-  1 george guest          62 Sep 17 09:49 /home/george/.forward

Here, the user george's ~/.forward file is writable by the group guest. Anyone in group guest can edit george's ~/.forward file, possibly placing something like this into it:

\george
|"cp /bin/sh /home/george/.x; chmod u+s /home/george/.x"

Now all the attacker has to do is send george mail to create an suid george shell. Then, by executing /home/george/.x, the attacker becomes george.

• The semiprivileged users such as bin, and root in particular, should never have ~/.forward files. Instead, they should forward their mail by means of the aliases file directly.

• User ~/.forward files must be writable only by the owning user. Similarly, user home directories must live in a directory that is owned and writable only by root, and must themselves be owned and writable only by the user.

Some users, such as the pseudo-user uucp, have home directories that must be world-writable for software to work properly. If that software is not needed (if a machine, for example, doesn't run UUCP software), that home directory should be removed. If the directory must exist and must be world-writable, you should create a protected ~/.forward file there before someone else does. The best protection is to create a nonempty directory called ~/.forward, owned by root, and set its permissions to 000:

# cd ~uucp
# rm -f .forward
# mkdir .forward
# touch .forward/uucp
# chown root .forward .forward/uucp
# chmod 000 .forward .forward/uucp
# chmod +t ~uucp
# chown root ~uucp

Even though the ~uucp directory is world-writable (so anyone can remove anything from it), no one but root can remove the ~/.forward directory because it is not empty. The mode of 000 protects the file .forward/uucp from being removed. The mode of +t prevents users from renaming files or directories that they do not own. Finally, root is made to own the ~uucp directory so that uucp will be unable to clear the +t bit. Even with this protection, mail for uucp should be routed to a real user with the aliases(5) file.

Note that all critical dot files in a world-writable home directory must be protected from creation by others. Each of .forward, .rhosts, .login, .cshrc, .profile, and .logout should be a nonempty, root-owned directory with mode 000. World-writable home directories must be owned by root instead of by the user, and they must have the +t (sticky bit) set.

When processing a user's ~/.forward file, sendmail requires that the file be owned by the user or by root. If ownership is correct, it then examines the ~/.forward file's permissions. If that file is world-writable, sendmail ignores (and logs) attempts to run programs and to write directly to files. If the UnsafeGroupWrites option (see Section 34.8.73, UnsafeGroupWrites) is true, sendmail also checks for group write permissions and, if it is found, similarly ignores attempts to run programs and to write to files.

22.5.4 Recommended Permissions

Table 22.1 shows the recommended ownerships and permissions for all the files and directories in the sendmail system. The path components will vary depending on the vendor version of sendmail you are running. For example, where we show the /usr/lib/sendmail directory, your site may use /usr/etc/sendmail, or even /usr/lib/mail/sendmail.

[14] The sendmail program sometimes lives in /usr/sbin or in some other directory. If so, adjust this path accordingly.

[15] The sendmail program may need to be sgid kmem for the load average to be checked on some systems.

[16] CERT (the Computing Emergency Response Team) recommends that the mqueue directory be mode 0700 to prevent potential security breaches.