2 README smrsh - sendmail restricted shell.
4 This README file is provided as a courtesy of the CERT Coordination Center,
5 Software Engineering Institute, Carnegie Mellon University. This file is
6 intended as a supplement to the CERT advisory CA-93:16.sendmail.vulnerability,
7 and to the software, smrsh.c, written by Eric Allman.
11 The smrsh(8) program is intended as a replacement for /bin/sh in the
12 program mailer definition of sendmail(8). This README file describes
13 the steps needed to compile and install smrsh.
15 smrsh is a restricted shell utility that provides the ability to
16 specify, through a configuration, an explicit list of executable
17 programs. When used in conjunction with sendmail, smrsh effectively
18 limits sendmail's scope of program execution to only those programs
19 specified in smrsh's configuration.
21 smrsh has been written with portability in mind, and uses traditional
22 Unix library utilities. As such, smrsh should compile on most
25 smrsh should build on most systems with the enclosed Build script:
29 To compile smrsh.c by hand, use the following command:
31 host.domain% cc -o smrsh smrsh.c
33 For machines that provide dynamic linking, it is advisable to compile
34 smrsh without dynamic linking. As an example with the Sun Microsystems
35 compiler, you should compile with the -Bstatic option.
37 host.domain% cc -Bstatic -o smrsh smrsh.c
39 host.domain% sh Build LDOPTS=-Bstatic
41 With gcc, the GNU C compiler, use the -static option.
43 host.domain% cc -static -o smrsh smrsh.c
45 host.domain% sh Build LDOPTS=-static
49 As root, install smrsh in /usr/libexec. Using the Build script:
51 host.domain# sh Build install
53 For manual installation: install smrsh in the /usr/libexec
54 directory, with mode 511.
56 host.domain# mv smrsh /usr/libexec
57 host.domain# chmod 511 /usr/libexec/smrsh
61 Next, determine the list of commands that smrsh should allow sendmail
62 to run. This list of allowable commands can be determined by:
64 1. examining your /etc/mail/aliases file, to indicate what commands
65 are being used by the system.
67 2. surveying your host's .forward files, to determine what
68 commands users have specified.
70 See the man page for aliases(5) if you are unfamiliar with the format of
71 these specifications. Additionally, you should include in the list,
72 popular commands such as /usr/ucb/vacation.
74 You should NOT include interpreter programs such as sh(1), csh(1),
75 perl(1), uudecode(1) or the stream editor sed(1) in your list of
78 If your platform doesn't have a default SMRSH_CMDDIR setting, you will
79 next need to create the directory /usr/adm/sm.bin and populate
80 it with the programs that your site feels are allowable for sendmail
81 to execute. This directory is explicitly specified in the source
82 code for smrsh, so changing this directory must be accompanied with
86 You will have to be root to make these modifications.
88 After creating the /usr/adm/sm.bin directory, either copy the programs
89 to the directory, or establish links to the allowable programs from
90 /usr/adm/sm.bin. Change the file permissions, so that these programs
91 can not be modified by non-root users. If you use links, you should
92 ensure that the target programs are not modifiable.
94 To allow the popular vacation(1) program by creating a link in the
95 /usr/adm/sm.bin directory, you should:
97 host.domain# cd /usr/adm/sm.bin
98 host.domain# ln -s /usr/ucb/vacation vacation
103 After populating the /usr/adm/sm.bin directory, you can now configure
104 sendmail to use the restricted shell. Save the current sendmail.cf
105 file prior to modifying it, as a prudent precaution.
107 Typically, the program mailer is defined by a single line in the
108 sendmail configuration file, sendmail.cf. This file is traditionally
109 found in the /etc, /usr/lib or /etc/mail directories, depending on
112 If you are unsure of the location of the actual sendmail configuration
113 file, a search of the strings(1) output of the sendmail binary, will
116 In order to configure sendmail to use smrsh, you must modify the Mprog
117 definition in the sendmail.cf file, by replacing the /bin/sh specification
118 with /usr/libexec/smrsh.
122 In most Sun Microsystems' sendmail.cf files, the line is:
123 Mprog, P=/bin/sh, F=lsDFMeuP, S=10, R=20, A=sh -c $u
125 which should be changed to:
126 Mprog, P=/usr/libexec/smrsh, F=lsDFMeuP, S=10, R=20, A=sh -c $u
129 A more generic line may be:
130 Mprog, P=/bin/sh, F=lsDFM, A=sh -c $u
132 and should be changed to;
133 Mprog, P=/usr/libexec/smrsh, F=lsDFM, A=sh -c $u
136 After modifying the Mprog definition in the sendmail.cf file, if a frozen
137 configuration file is being used, it is essential to create a new one.
138 You can determine if you need a frozen configuration by discovering
139 if a sendmail.fc file currently exists in either the /etc/, /usr/lib,
140 or /etc/mail directories. The specific location can be determined using
141 a search of the strings(1) output of the sendmail binary.
143 In order to create a new frozen configuration, if it is required:
144 host.domain# /usr/lib/sendmail -bz
146 Now re-start the sendmail process. An example of how to do this on
147 a typical system follows:
149 host.domain# cat /var/run/sendmail.pid
151 /usr/sbin/sendmail -bd -q30m
152 host.domain# /bin/kill -15 130
153 host.domain# /usr/sbin/sendmail -bd -q30m
156 $Revision: 8.8 $, Last updated $Date: 2001/01/24 00:05:58 $