### smrsh-1.0 README file

    README  smrsh - sendmail restricted shell.

    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/etc directory for the remainder of this document.

    As root, install smrsh in /usr/etc directory, with mode 511.

    host.domain# mv smrsh /usr/etc
    host.domain# chmod 511 /usr/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.  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/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/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/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