22.4.1 Accept/Reject Connections via libwrap.a

The TCP wrapper package is written and maintained by Wietse Venema at the Department of Mathematics and Computing Science, The Netherlands. It is available via anonymous FTP from:

ftp://ftp.cert.org/pub/tools/tcp_wrappers/
ftp://ftp.win.tue.nl/pub/security/

This package is used to screen incoming network connections and to accept or reject them on the basis of hostname, domain, or IP number. It is a powerful adjunct to security, and if you have not already done so, you should install it at your site.

Prior to V8.8 the only way sendmail could take advantage of this package was to be run from inetd(8) (see Section 36.7.11, -bs). Beginning with V8.8 sendmail, support for this package is built in.

If TCPWRAPPERS is defined in compiling (see Section 18.8.49, TCPWRAPPERS), sendmail will automatically use that package to verify and screen all incoming SMTP connections. If, as CERT recommends, you have ALL:ALL in your hosts.deny file, you will need to add this line to your hosts.allow file:

sendmail:ALL

Then, to selectively reject connection, you might add a line like this to your hosts.deny file:

sendmail:spam.host.domain

This causes the TCP wrapper package to tell sendmail to reject all SMTP connections from the spamming host spam.host.domain.

When mail comes in from spam.host.domain, sendmail will issue this SMTP message as a reply to all SMTP commands from that host:

550 Access denied

The only exception is the QUIT command (and beginning with V8.8.5, the HELO, EHLO, and NOOP commands), which allows the spamming host to disconnect.

Use of the TCP wrapper package imposes additional network traffic that may not be desirable. Both it and sendmail, for instance, may look up the same host with DNS. The wrapper software also sends identd(8) queries that a duplicate those used by sendmail. Finally, note that two files need to be opened and read for each connection. We recommend that you exclude support for this package (especially at high-volume sites) until you actually need it. At low- to medium-volume sites you may wish to include support for this package in sendmail but then to not implement that support (in hosts.allow and hosts.deny) until the need arises.

22.4.2 The F Command - File Form

The file form of the F configuration command (see Section 32.1.2, "The F Class Command") can be used to read sensitive information. That command looks like this in the configuration file:

FX/path pat

This form is used to read class macro entries from files. It can cause problems through a misunderstanding of the scanf(3) pattern pat. The /path is the name of the file, and the optional pat is a pattern to be used by scanf(3) (see Section 32.1.2.1, "scanf(3) variations").

To illustrate the risk of the pat, consider the following configuration file entry:

Fw/etc/myhostnames %[^#]

Normally, the F command reads only the first whitespace-delimited word from each line of the file. But if the optional pattern pat is specified, the F command instead reads one or more words from each line based on the nature of the pattern. The pattern is used by scanf(3) to extract words, and the specific pattern used here [^#] causes scanf(3) to read everything up to the first comment character (the #) from each line. This pat allows multiple hostnames to be conveniently listed on each line of the file. Now assume that a new administrator, who is not very familiar with sendmail, decides to add an F command to gather a list of UUCP hosts from the /etc/uucp/Systems file. Being a novice, the new administrator copies the existing entry for use with the new file:

FU/etc/uucp/Systems %[^#]

This is the same pattern that was correctly used for /etc/myhostnames. Unfortunately, the Systems file contains more than just host entries on each line:

linda Any ACU 2400 5551212  "" \d\n in:-\r-in: Uourhost word: MublyPeg
hoby Any ACU 2400 5551213  "" \d\n in:-\r-in: Uourhost word: FuMzz3.x

A part of each line (the last item in each) contains nonencrypted passwords. An unscrupulous user, noticing the mistaken [^#] in the configuration file, could run sendmail with a -d36.5 debugging switch and watch each password being processed. For example,

% /usr/lib/sendmail -d36.5 -bt < /dev/null
 ... some output deleted
STAB: hoby 1 entered
STAB: Any 1 entered
STAB: ACU 1 entered
STAB: 2400 1 entered
STAB: 5551213 1 entered
STAB: "" 1 type 1 val 0 0 200000 0
STAB: \d\n 1 entered
STAB: in:-\r-in: 1 entered
STAB: Uourhost 1 entered
STAB: word: 1 entered
STAB: FuMzz3.x 1 entered                          note
STAB: local 3 type 3 val 34d00 0 0 0
STAB: prog 3 type 3 val 34d80 0 0 0

Note the third line from the bottom, where the password for the UUCP login into the host hoby is printed. This example illustrates two rules about handling the configuration file:

• Avoid using the F command to read a file that is not already publicly readable. To do so can reveal sensitive information. Even if the scanf(3) option is correct, a core dump [10] can be examined for sensitive information from otherwise secured files.

  [10] Most versions of UNIX disallow core dumps of suid root programs.

• Avoid adding a new command to the configuration file by blindly copying and modifying another. Try to learn the rules governing the command first.

22.4.3 The F Command - Program Form

Another form of the F (File) configuration command is the program form, which looks like this:

FX|/path

Here, the | prefix to the /path tells sendmail that /path is the name of a program to run. The output produced by the program is appended to the class, here X.

To illustrate another potential security risk, consider a configuration file that is group writable, perhaps by a few administrators who share the job of postmaster. To break into root, the attacker only needs to assume the identity of one of those users and, under that identity, edit the configuration file. Consider the following bogus entry added by an attacker to that configuration file:

FX|/tmp/.sh

Consider further a change to the DefaultUser option (see Section 34.8.15, DefaultUser (g)(u)) that causes the default uid and gid to become those of root:

O DefaultUser=0:0

With these changes in place, the program (actually a shell script) called /tmp/.sh is run by sendmail to fill the class X with new values. All this seems harmless enough, but suppose /tmp/.sh does the unexpected:

#!/bin/sh
cp /bin/sh /tmp/.shell
chmod u+s /tmp/.shell

Here, the Bourne shell is copied to /tmp/.shell, and the suid root bit is set. Now, any user at all can run sendmail and become root:

% ls -l /tmp/.shell
/tmp/.shell not found
%  /usr/lib/sendmail -bt < /dev/null
% ls -l /tmp/.shell
-rwsr-xr-x  1 root       122880 Sep 24 13:20 /tmp/.shell

The program form of the F configuration command is clearly dangerous.

• The sendmail configuration file must never be writable by anyone other than root. It should also live in a directory, every path component of which is owned by and writable only by root. (We'll discuss this latter point in greater detail soon.) If the configuration file is created with the m4 technique, then care must be taken to ensure that only root can install it.

22.4.4 The P= of Delivery Agents

Just as the program form of the F command can pose a security risk if the configuration file is poorly protected, so can the M delivery agent definition. Specifically, the P= equate for a delivery agent (see Section 30.4.9, P=) can be modified to run a bogus program that gives away root privilege. Consider the following modification to the local delivery agent:

Mlocal, P=/bin/mail, F=rlsDFMmnP, S=10, R=20, A=mail -d $u
           
        becomes
           
Mlocal, P=/tmp/mail, U=0, F=SrlsDFMmnP, S=10, R=20, A=mail -d $u
                           
                     note   note

Here, local mail should be delivered with the /bin/mail program, but instead it is delivered with a bogus frontend, /tmp/mail. If /tmp/mail is carefully crafted, users will never notice that the mail has been diverted. The S flag in the F= equate (see Section 30.8.40, F=S) causes sendmail to retain its default identity when executing the bogus /tmp/mail. The U=0 equate (see Section 30.4.13, U=) causes that default to become the identity of root.

• Delivery agent P= equates must be protected by protecting the configuration file. As an additional precaution, never use relative pathnames in the P= equate.

• The F=S and U=0 are especially dangerous. They should never appear in your configuration file unless you have deliberately placed them there and are 100 percent certain of their effect.

22.4.5 The S Option and the Statistics File

When sendmail attempts to record its delivery agent statistics (see Section 26.2.1, "The sendmail.st File"), it checks for the existence and write permissions of the file specified by the StatusFile (S) option (see Section 34.8.66, StatusFile (S)). The sendmail program does not care where that file lives or what permissions it has - only that it exists.

A security problem can arise if one is tempted to locate the statistics file in a spool or temporary area. Consider the following location, for example:

OS/usr/tmp/sendmail.st

Here the administrator sets the StatusFile (S) option to locate the statistics file in the /usr/tmp directory. The intention is that the file can be easily created by anyone who wishes to gather statistics for a while, then removed. Unfortunately, the /usr/tmp directory is usually world-writable.

Thus any unhappy or malicious user can bring the system to its knees:

% cd /usr/tmp
% ln -s /vmunix sendmail.st

Here, sendmail clobbers the disk copy of the kernel. Nothing bad may happen at first, [11] but the machine will require manual intervention to boot in the future. [12] Clearly, precautions must be taken. For example, any file that sendmail writes to (such as the StatusFile (S) option statistics file or the aliases database files) must be writable only by root and live in a directory, every path component of which is writable only by root.

[11] Programs that need kernel symbols, such as ps(1), will cease to work or will produce garbage output.

[12] The savvy administrator can still boot off the network or from a CD-ROM and quickly install a new kernel.