1 <?xml version="1.0" encoding="UTF-8" ?>
2 <!DOCTYPE manualpage SYSTEM "./style/manualpage.dtd">
3 <?xml-stylesheet type="text/xsl" href="./style/manual.en.xsl"?>
4 <!-- $LastChangedRevision$ -->
7 Licensed to the Apache Software Foundation (ASF) under one or more
8 contributor license agreements. See the NOTICE file distributed with
9 this work for additional information regarding copyright ownership.
10 The ASF licenses this file to You under the Apache License, Version 2.0
11 (the "License"); you may not use this file except in compliance with
12 the License. You may obtain a copy of the License at
14 http://www.apache.org/licenses/LICENSE-2.0
16 Unless required by applicable law or agreed to in writing, software
17 distributed under the License is distributed on an "AS IS" BASIS,
18 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
19 See the License for the specific language governing permissions and
20 limitations under the License.
23 <manualpage metafile="suexec.xml.meta">
25 <title>suEXEC Support</title>
28 <p>The <strong>suEXEC</strong> feature provides
29 Apache users the ability
30 to run <strong>CGI</strong> and <strong>SSI</strong> programs
31 under user IDs different from the user ID of the calling
32 web server. Normally, when a CGI or SSI program executes, it
33 runs as the same user who is running the web server.</p>
35 <p>Used properly, this feature can reduce
36 considerably the security risks involved with allowing users to
37 develop and run private CGI or SSI programs. However, if suEXEC
38 is improperly configured, it can cause any number of problems
39 and possibly create new holes in your computer's security. If
40 you aren't familiar with managing <em>setuid root</em> programs
41 and the security issues they present, we highly recommend that
42 you not consider using suEXEC.</p>
45 <section id="before"><title>Before we begin</title>
47 <p>Before jumping head-first into this document,
48 you should be aware of the assumptions made on the part of the
49 Apache Group and this document.</p>
51 <p>First, it is assumed that you are using a UNIX
52 derivative operating system that is capable of
53 <strong>setuid</strong> and <strong>setgid</strong> operations.
54 All command examples are given in this regard. Other platforms,
55 if they are capable of supporting suEXEC, may differ in their
58 <p>Second, it is assumed you are familiar with
59 some basic concepts of your computer's security and its
60 administration. This involves an understanding of
61 <strong>setuid/setgid</strong> operations and the various
62 effects they may have on your system and its level of
65 <p>Third, it is assumed that you are using an
66 <strong>unmodified</strong> version of suEXEC code. All code
67 for suEXEC has been carefully scrutinized and tested by the
68 developers as well as numerous beta testers. Every precaution
69 has been taken to ensure a simple yet solidly safe base of
70 code. Altering this code can cause unexpected problems and new
71 security risks. It is <strong>highly</strong> recommended you
72 not alter the suEXEC code unless you are well versed in the
73 particulars of security programming and are willing to share
74 your work with the Apache Group for consideration.</p>
76 <p>Fourth, and last, it has been the decision of
77 the Apache Group to <strong>NOT</strong> make suEXEC part of
78 the default installation of Apache. To this end, suEXEC
79 configuration requires of the administrator careful attention
80 to details. After due consideration has been given to the
81 various settings for suEXEC, the administrator may install
82 suEXEC through normal installation methods. The values for
83 these settings need to be carefully determined and specified by
84 the administrator to properly maintain system security during
85 the use of suEXEC functionality. It is through this detailed
86 process that the Apache Group hopes to limit suEXEC
87 installation only to those who are careful and determined
90 <p>Still with us? Yes? Good. Let's move on!</p>
93 <section id="model"><title>suEXEC Security Model</title>
95 <p>Before we begin configuring and installing
96 suEXEC, we will first discuss the security model you are about
97 to implement. By doing so, you may better understand what
98 exactly is going on inside suEXEC and what precautions are
99 taken to ensure your system's security.</p>
101 <p><strong>suEXEC</strong> is based on a setuid
102 "wrapper" program that is called by the main Apache web server.
103 This wrapper is called when an HTTP request is made for a CGI
104 or SSI program that the administrator has designated to run as
105 a userid other than that of the main server. When such a
106 request is made, Apache provides the suEXEC wrapper with the
107 program's name and the user and group IDs under which the
108 program is to execute.</p>
110 <p>The wrapper then employs the following process
111 to determine success or failure -- if any one of these
112 conditions fail, the program logs the failure and exits with an
113 error, otherwise it will continue:</p>
117 <strong>Is the user executing this wrapper a valid user of
118 this system?</strong>
121 This is to ensure that the user executing the wrapper is
122 truly a user of the system.
127 <strong>Was the wrapper called with the proper number of
131 The wrapper will only execute if it is given the proper
132 number of arguments. The proper argument format is known
133 to the Apache web server. If the wrapper is not receiving
134 the proper number of arguments, it is either being
135 hacked, or there is something wrong with the suEXEC
136 portion of your Apache binary.
141 <strong>Is this valid user allowed to run the
145 Is this user the user allowed to run this wrapper? Only
146 one user (the Apache user) is allowed to execute this
152 <strong>Does the target CGI or SSI program have an unsafe
153 hierarchical reference?</strong>
156 Does the target CGI or SSI program's path contain a leading
157 '/' or have a '..' backreference? These are not allowed; the
158 target CGI/SSI program must reside within suEXEC's document
159 root (see <code>--with-suexec-docroot=<em>DIR</em></code>
165 <strong>Is the target user name valid?</strong>
168 Does the target user exist?
173 <strong>Is the target group name valid?</strong>
176 Does the target group exist?
181 <strong>Is the target user <em>NOT</em> superuser?</strong>
185 Presently, suEXEC does not allow <code><em>root</em></code>
186 to execute CGI/SSI programs.
191 <strong>Is the target userid <em>ABOVE</em> the minimum ID
195 The minimum user ID number is specified during
196 configuration. This allows you to set the lowest possible
197 userid that will be allowed to execute CGI/SSI programs.
198 This is useful to block out "system" accounts.
203 <strong>Is the target group <em>NOT</em> the superuser
207 Presently, suEXEC does not allow the <code><em>root</em></code>
208 group to execute CGI/SSI programs.
213 <strong>Is the target groupid <em>ABOVE</em> the minimum ID
217 The minimum group ID number is specified during
218 configuration. This allows you to set the lowest possible
219 groupid that will be allowed to execute CGI/SSI programs.
220 This is useful to block out "system" groups.
225 <strong>Can the wrapper successfully become the target user
229 Here is where the program becomes the target user and
230 group via setuid and setgid calls. The group access list
231 is also initialized with all of the groups of which the
237 <strong>Can we change directory to the one in which the target
238 CGI/SSI program resides?</strong>
241 If it doesn't exist, it can't very well contain files. If we
242 can't change directory to it, it might aswell not exist.
247 <strong>Is the directory within the Apache
251 If the request is for a regular portion of the server, is
252 the requested directory within suEXEC's document root? If
253 the request is for a <directive module="mod_userdir"
254 >UserDir</directive>, is the requested directory
255 within the directory configured as suEXEC's userdir (see
256 <a href="#install">suEXEC's configuration options</a>)?
261 <strong>Is the directory <em>NOT</em> writable by anyone
265 We don't want to open up the directory to others; only
266 the owner user may be able to alter this directories
272 <strong>Does the target CGI/SSI program exist?</strong>
275 If it doesn't exists, it can't very well be executed.
280 <strong>Is the target CGI/SSI program <em>NOT</em> writable
281 by anyone else?</strong>
284 We don't want to give anyone other than the owner the
285 ability to change the CGI/SSI program.
290 <strong>Is the target CGI/SSI program <em>NOT</em> setuid or
294 We do not want to execute programs that will then change
300 <strong>Is the target user/group the same as the program's
304 Is the user the owner of the file?
309 <strong>Can we successfully clean the process environment
310 to ensure safe operations?</strong>
313 suEXEC cleans the process' environment by establishing a
314 safe execution PATH (defined during configuration), as
315 well as only passing through those variables whose names
316 are listed in the safe environment list (also created
317 during configuration).
322 <strong>Can we successfully become the target CGI/SSI program
323 and execute?</strong>
326 Here is where suEXEC ends and the target CGI/SSI program begins.
331 <p>This is the standard operation of the
332 suEXEC wrapper's security model. It is somewhat stringent and
333 can impose new limitations and guidelines for CGI/SSI design,
334 but it was developed carefully step-by-step with security in
337 <p>For more information as to how this security
338 model can limit your possibilities in regards to server
339 configuration, as well as what security risks can be avoided
340 with a proper suEXEC setup, see the <a
341 href="#jabberwock">"Beware the Jabberwock"</a> section of this
345 <section id="install"><title>Configuring & Installing
348 <p>Here's where we begin the fun.</p>
350 <p><strong>suEXEC configuration
351 options</strong><br />
355 <dt><code>--enable-suexec</code></dt>
357 <dd>This option enables the suEXEC feature which is never
358 installed or activated by default. At least one
359 <code>--with-suexec-xxxxx</code> option has to be provided
360 together with the <code>--enable-suexec</code> option to let
361 APACI accept your request for using the suEXEC feature.</dd>
363 <dt><code>--with-suexec-bin=<em>PATH</em></code></dt>
365 <dd>The path to the <code>suexec</code> binary must be hard-coded
366 in the server for security reasons. Use this option to override
367 the default path. <em>e.g.</em>
368 <code>--with-suexec-bin=/usr/sbin/suexec</code></dd>
370 <dt><code>--with-suexec-caller=<em>UID</em></code></dt>
372 <dd>The <a href="mod/mpm_common.html#user">username</a> under which
373 Apache normally runs. This is the only user allowed to
374 execute this program.</dd>
376 <dt><code>--with-suexec-userdir=<em>DIR</em></code></dt>
378 <dd>Define to be the subdirectory under users' home
379 directories where suEXEC access should be allowed. All
380 executables under this directory will be executable by suEXEC
381 as the user so they should be "safe" programs. If you are
382 using a "simple" <directive module="mod_userdir">UserDir</directive>
383 directive (ie. one without a "*" in it) this should be set to the same
384 value. suEXEC will not work properly in cases where the <directive
385 module="mod_userdir">UserDir</directive> directive points to
386 a location that is not the same as the user's home directory
387 as referenced in the <code>passwd</code> file. Default value is
388 "<code>public_html</code>".<br />
389 If you have virtual hosts with a different <directive
390 module="mod_userdir">UserDir</directive> for each,
391 you will need to define them to all reside in one parent
392 directory; then name that parent directory here. <strong>If
393 this is not defined properly, "~userdir" cgi requests will
394 not work!</strong></dd>
396 <dt><code>--with-suexec-docroot=<em>DIR</em></code></dt>
398 <dd>Define as the DocumentRoot set for Apache. This will be
399 the only hierarchy (aside from <directive module="mod_userdir"
400 >UserDir</directive>s) that can be used for suEXEC behavior. The
401 default directory is the <code>--datadir</code> value with the suffix
402 "<code>/htdocs</code>", <em>e.g.</em> if you configure with
403 "<code>--datadir=/home/apache</code>" the directory
404 "<code>/home/apache/htdocs</code>" is used as document root for the
407 <dt><code>--with-suexec-uidmin=<em>UID</em></code></dt>
409 <dd>Define this as the lowest UID allowed to be a target user
410 for suEXEC. For most systems, 500 or 100 is common. Default
413 <dt><code>--with-suexec-gidmin=<em>GID</em></code></dt>
415 <dd>Define this as the lowest GID allowed to be a target
416 group for suEXEC. For most systems, 100 is common and
417 therefore used as default value.</dd>
419 <dt><code>--with-suexec-logfile=<em>FILE</em></code></dt>
421 <dd>This defines the filename to which all suEXEC
422 transactions and errors are logged (useful for auditing and
423 debugging purposes). By default the logfile is named
424 "<code>suexec_log</code>" and located in your standard logfile
425 directory (<code>--logfiledir</code>).</dd>
427 <dt><code>--with-suexec-safepath=<em>PATH</em></code></dt>
429 <dd>Define a safe PATH environment to pass to CGI
430 executables. Default value is
431 "<code>/usr/local/bin:/usr/bin:/bin</code>".</dd>
435 <title>Compiling and installing the suEXEC wrapper</title>
437 <p>If you have enabled the suEXEC feature with the
438 <code>--enable-suexec</code> option the <code>suexec</code> binary
439 (together with Apache itself) is automatically built if you execute
440 the <code>make</code> command.</p>
442 <p>After all components have been built you can execute the
443 command <code>make install</code> to install them. The binary image
444 <code>suexec</code> is installed in the directory defined by the
445 <code>--sbindir</code> option. The default location is
446 "/usr/local/apache2/bin/suexec".</p>
448 <p>Please note that you need <strong><em>root
449 privileges</em></strong> for the installation step. In order
450 for the wrapper to set the user ID, it must be installed as
451 owner <code><em>root</em></code> and must have the setuserid
452 execution bit set for file modes.</p>
456 <title>Setting paranoid permissions</title>
458 <p>Although the suEXEC wrapper will check to ensure that its
459 caller is the correct user as specified with the
460 <code>--with-suexec-caller</code> <program>configure</program>
462 always the possibility that a system or library call suEXEC uses
463 before this check may be exploitable on your system. To counter
464 this, and because it is best-practise in general, you should use
465 filesystem permissions to ensure that only the group Apache
466 runs as may execute suEXEC.</p>
468 <p>If for example, your web server is configured to run as:</p>
475 <p>and <program>suexec</program> is installed at
476 "/usr/local/apache2/bin/suexec", you should run:</p>
479 chgrp webgroup /usr/local/apache2/bin/suexec<br />
480 chmod 4750 /usr/local/apache2/bin/suexec<br />
483 <p>This will ensure that only the group Apache runs as can even
484 execute the suEXEC wrapper.</p>
489 <section id="enable"><title>Enabling & Disabling
492 <p>Upon startup of Apache, it looks for the file
493 <program>suexec</program> in the directory defined by the
494 <code>--sbindir</code> option (default is
495 "/usr/local/apache/sbin/suexec"). If Apache finds a properly
496 configured suEXEC wrapper, it will print the following message
497 to the error log:</p>
500 [notice] suEXEC mechanism enabled (wrapper: <var>/path/to/suexec</var>)
503 <p>If you don't see this message at server startup, the server is
504 most likely not finding the wrapper program where it expects
505 it, or the executable is not installed <em>setuid root</em>.</p>
507 <p>If you want to enable the suEXEC mechanism for the first time
508 and an Apache server is already running you must kill and
509 restart Apache. Restarting it with a simple HUP or USR1 signal
510 will not be enough. </p>
511 <p>If you want to disable suEXEC you should kill and restart
512 Apache after you have removed the <program>suexec</program> file.</p>
515 <section id="usage"><title>Using suEXEC</title>
517 <p>Requests for CGI programs will call the suEXEC wrapper only if
518 they are for a virtual host containing a <directive
519 module="mod_suexec">SuexecUserGroup</directive> directive or if
520 they are processed by <module>mod_userdir</module>.</p>
522 <p><strong>Virtual Hosts:</strong><br /> One way to use the suEXEC
523 wrapper is through the <directive
524 module="mod_suexec">SuexecUserGroup</directive> directive in
525 <directive module="core">VirtualHost</directive> definitions. By
526 setting this directive to values different from the main server
527 user ID, all requests for CGI resources will be executed as the
528 <em>User</em> and <em>Group</em> defined for that <directive
529 module="core" type="section">VirtualHost</directive>. If this
530 directive is not specified for a <directive module="core"
531 type="section">VirtualHost</directive> then the main server userid
534 <p><strong>User directories:</strong><br /> Requests that are
535 processed by <module>mod_userdir</module> will call the suEXEC
536 wrapper to execute CGI programs under the userid of the requested
537 user directory. The only requirement needed for this feature to
538 work is for CGI execution to be enabled for the user and that the
539 script must meet the scrutiny of the <a href="#model">security
540 checks</a> above. See also the
541 <code>--with-suexec-userdir</code> <a href="#install">compile
542 time option</a>.</p> </section>
544 <section id="debug"><title>Debugging suEXEC</title>
546 <p>The suEXEC wrapper will write log information
547 to the file defined with the <code>--with-suexec-logfile</code>
548 option as indicated above. If you feel you have configured and
549 installed the wrapper properly, have a look at this log and the
550 error_log for the server to see where you may have gone astray.</p>
554 <section id="jabberwock"><title>Beware the Jabberwock:
555 Warnings & Examples</title>
557 <p><strong>NOTE!</strong> This section may not be
558 complete. For the latest revision of this section of the
559 documentation, see the Apache Group's <a
560 href="http://httpd.apache.org/docs/&httpd.docs;/suexec.html">Online
561 Documentation</a> version.</p>
563 <p>There are a few points of interest regarding
564 the wrapper that can cause limitations on server setup. Please
565 review these before submitting any "bugs" regarding suEXEC.</p>
568 <li><strong>suEXEC Points Of Interest</strong></li>
571 Hierarchy limitations
574 For security and efficiency reasons, all suEXEC requests
575 must remain within either a top-level document root for
576 virtual host requests, or one top-level personal document
577 root for userdir requests. For example, if you have four
578 VirtualHosts configured, you would need to structure all
579 of your VHosts' document roots off of one main Apache
580 document hierarchy to take advantage of suEXEC for
581 VirtualHosts. (Example forthcoming.)
586 suEXEC's PATH environment variable
589 This can be a dangerous thing to change. Make certain
590 every path you include in this define is a
591 <strong>trusted</strong> directory. You don't want to
592 open people up to having someone from across the world
593 running a trojan horse on them.
598 Altering the suEXEC code
601 Again, this can cause <strong>Big Trouble</strong> if you
602 try this without knowing what you are doing. Stay away
603 from it if at all possible.