README smrsh - sendmail restricted shell.
@(#)README 8.2 11/11/1995
This README file is provided as a courtesy of the CERT Coordination Center,
Software Engineering Institute, Carnegie Mellon University. This file is
intended as a supplement to the CERT advisory CA-93:16.sendmail.vulnerability,
and to the software, smrsh.c, written by Eric Allman.
The smrsh(8) program is intended as a replacement for /bin/sh in the
program mailer definition of sendmail(8). This README file describes
the steps needed to compile and install smrsh.
smrsh is a restricted shell utility that provides the ability to
specify, through a configuration, an explicit list of executable
programs. When used in conjunction with sendmail, smrsh effectively
limits sendmail's scope of program execution to only those programs
specified in smrsh's configuration.
smrsh has been written with portability in mind, and uses traditional
Unix library utilities. As such, smrsh should compile on most
Unix C compilers.
To compile smrsh.c, use the following command:
host.domain% cc -o smrsh smrsh.c
For machines that provide dynamic linking, it is advisable to compile
smrsh without dynamic linking. As an example with the Sun Microsystems
compiler, you should compile with the -Bstatic option.
host.domain% cc -Bstatic -o smrsh smrsh.c
Choose a directory that smrsh will reside in. We will use the traditional
/usr/local/etc directory for the remainder of this document.
As root, install smrsh in /usr/local/etc directory, with mode 511.
host.domain# mv smrsh /usr/local/etc
host.domain# chmod 511 /usr/local/etc/smrsh
Next, determine the list of commands that smrsh should allow sendmail
to run. This list of allowable commands can be determined by:
1. examining your /etc/aliases file, to indicate what commands
are being used by the system.
2. surveying your host's .forward files, to determine what
commands users have specified.
See the man page for aliases(5) if you are unfamiliar with the format of
these specifications. Additionally, you should include in the list,
popular commands such as /usr/ucb/vacation.
You should NOT include interpreter programs such as sh(1), csh(1),
perl(1), uudecode(1) or the stream editor sed(1) in your list of
acceptable commands.
You will next need to create the directory /usr/adm/sm.bin and populate
it with the programs that your site feels are allowable for sendmail
to execute. This directory is explicitly specified in the source
code for smrsh, so changing this directory must be accompanied with
a change in smrsh.c.
You will have to be root to make these modifications.
After creating the /usr/adm/sm.bin directory, either copy the programs
to the directory, or establish links to the allowable programs from
/usr/adm/sm.bin. Change the file permissions, so that these programs
can not be modified by non-root users. If you use links, you should
ensure that the target programs are not modifiable.
To allow the popular vacation(1) program by creating a link in the
/usr/adm/sm.bin directory, you should:
host.domain# cd /usr/adm/sm.bin
host.domain# ln -s /usr/ucb/vacation vacation
After populating the /usr/adm/sm.bin directory, you can now configure
sendmail to use the restricted shell. Save the current sendmail.cf
file prior to modifying it, as a prudent precaution.
Typically, the program mailer is defined by a single line in the
sendmail configuration file, sendmail.cf. This file is traditionally
found in the /etc, /usr/lib or /etc/mail directories, depending on
the UNIX vendor.
If you are unsure of the location of the actual sendmail configuration
file, a search of the strings(1) output of the sendmail binary, will
help to locate it.
In order to configure sendmail to use smrsh, you must modify the Mprog
definition in the sendmail.cf file, by replacing the /bin/sh specification
with /usr/local/etc/smrsh.
As an example:
In most Sun Microsystems' sendmail.cf files, the line is:
Mprog, P=/bin/sh, F=lsDFMeuP, S=10, R=20, A=sh -c $u
which should be changed to:
Mprog, P=/usr/local/etc/smrsh, F=lsDFMeuP, S=10, R=20, A=sh -c $u
^^^^^^^^^^^^^^^^^^^^
A more generic line may be:
Mprog, P=/bin/sh, F=lsDFM, A=sh -c $u
and should be changed to;
Mprog, P=/usr/local/etc/smrsh, F=lsDFM, A=sh -c $u
After modifying the Mprog definition in the sendmail.cf file, if a frozen
configuration file is being used, it is essential to create a new one.
You can determine if you need a frozen configuration by discovering
if a sendmail.fc file currently exists in either the /etc/, /usr/lib,
or /etc/mail directories. The specific location can be determined using
a search of the strings(1) output of the sendmail binary.
In order to create a new frozen configuration, if it is required:
host.domain# /usr/lib/sendmail -bz
Now re-start the sendmail process. An example of how to do this on
a typical system follows:
host.domain# /usr/bin/ps aux | /usr/bin/grep sendmail
root 130 0.0 0.0 168 0 ? IW Oct 2 0:10 /usr/lib/sendmail -bd -q
host.domain# /bin/kill -9 130
host.domain# /usr/lib/sendmail -bd -q30m
