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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 UserDir, is the requested directory
254 within the directory configured as suEXEC's userdir (see
255 <a href="#install">suEXEC's configuration options</a>)?
260 <strong>Is the directory <em>NOT</em> writable by anyone
264 We don't want to open up the directory to others; only
265 the owner user may be able to alter this directories
271 <strong>Does the target CGI/SSI program exist?</strong>
274 If it doesn't exists, it can't very well be executed.
279 <strong>Is the target CGI/SSI program <em>NOT</em> writable
280 by anyone else?</strong>
283 We don't want to give anyone other than the owner the
284 ability to change the CGI/SSI program.
289 <strong>Is the target CGI/SSI program <em>NOT</em> setuid or
293 We do not want to execute programs that will then change
299 <strong>Is the target user/group the same as the program's
303 Is the user the owner of the file?
308 <strong>Can we successfully clean the process environment
309 to ensure safe operations?</strong>
312 suEXEC cleans the process' environment by establishing a
313 safe execution PATH (defined during configuration), as
314 well as only passing through those variables whose names
315 are listed in the safe environment list (also created
316 during configuration).
321 <strong>Can we successfully become the target CGI/SSI program
322 and execute?</strong>
325 Here is where suEXEC ends and the target CGI/SSI program begins.
330 <p>This is the standard operation of the
331 suEXEC wrapper's security model. It is somewhat stringent and
332 can impose new limitations and guidelines for CGI/SSI design,
333 but it was developed carefully step-by-step with security in
336 <p>For more information as to how this security
337 model can limit your possibilities in regards to server
338 configuration, as well as what security risks can be avoided
339 with a proper suEXEC setup, see the <a
340 href="#jabberwock">"Beware the Jabberwock"</a> section of this
344 <section id="install"><title>Configuring & Installing
347 <p>Here's where we begin the fun.</p>
349 <p><strong>suEXEC configuration
350 options</strong><br />
354 <dt><code>--enable-suexec</code></dt>
356 <dd>This option enables the suEXEC feature which is never
357 installed or activated by default. At least one
358 <code>--with-suexec-xxxxx</code> option has to be provided
359 together with the <code>--enable-suexec</code> option to let
360 APACI accept your request for using the suEXEC feature.</dd>
362 <dt><code>--with-suexec-bin=<em>PATH</em></code></dt>
364 <dd>The path to the <code>suexec</code> binary must be hard-coded
365 in the server for security reasons. Use this option to override
366 the default path. <em>e.g.</em>
367 <code>--with-suexec-bin=/usr/sbin/suexec</code></dd>
369 <dt><code>--with-suexec-caller=<em>UID</em></code></dt>
371 <dd>The <a href="mod/mpm_common.html#user">username</a> under which
372 Apache normally runs. This is the only user allowed to
373 execute this program.</dd>
375 <dt><code>--with-suexec-userdir=<em>DIR</em></code></dt>
377 <dd>Define to be the subdirectory under users' home
378 directories where suEXEC access should be allowed. All
379 executables under this directory will be executable by suEXEC
380 as the user so they should be "safe" programs. If you are
381 using a "simple" UserDir directive (ie. one without a "*" in
382 it) this should be set to the same value. suEXEC will not
383 work properly in cases where the UserDir directive points to
384 a location that is not the same as the user's home directory
385 as referenced in the passwd file. Default value is
387 If you have virtual hosts with a different UserDir for each,
388 you will need to define them to all reside in one parent
389 directory; then name that parent directory here. <strong>If
390 this is not defined properly, "~userdir" cgi requests will
391 not work!</strong></dd>
393 <dt><code>--with-suexec-docroot=<em>DIR</em></code></dt>
395 <dd>Define as the DocumentRoot set for Apache. This will be
396 the only hierarchy (aside from UserDirs) that can be used for
397 suEXEC behavior. The default directory is the <code>--datadir</code>
398 value with the suffix "/htdocs", <em>e.g.</em> if you configure
399 with "<code>--datadir=/home/apache</code>" the directory
400 "/home/apache/htdocs" is used as document root for the suEXEC
403 <dt><code>--with-suexec-uidmin=<em>UID</em></code></dt>
405 <dd>Define this as the lowest UID allowed to be a target user
406 for suEXEC. For most systems, 500 or 100 is common. Default
409 <dt><code>--with-suexec-gidmin=<em>GID</em></code></dt>
411 <dd>Define this as the lowest GID allowed to be a target
412 group for suEXEC. For most systems, 100 is common and
413 therefore used as default value.</dd>
415 <dt><code>--with-suexec-logfile=<em>FILE</em></code></dt>
417 <dd>This defines the filename to which all suEXEC
418 transactions and errors are logged (useful for auditing and
419 debugging purposes). By default the logfile is named
420 "suexec_log" and located in your standard logfile directory
421 (<code>--logfiledir</code>).</dd>
423 <dt><code>--with-suexec-safepath=<em>PATH</em></code></dt>
425 <dd>Define a safe PATH environment to pass to CGI
426 executables. Default value is
427 "/usr/local/bin:/usr/bin:/bin".</dd>
430 <p><strong>Compiling and installing the suEXEC
431 wrapper</strong><br />
432 If you have enabled the suEXEC feature with the
433 <code>--enable-suexec</code> option the <code>suexec</code> binary
434 (together with Apache itself) is automatically built if you execute
435 the <code>make</code> command.<br />
436 After all components have been built you can execute the
437 command <code>make install</code> to install them. The binary image
438 <code>suexec</code> is installed in the directory defined by the
439 <code>--sbindir</code> option. The default location is
440 "/usr/local/apache2/bin/suexec".<br />
441 Please note that you need <strong><em>root
442 privileges</em></strong> for the installation step. In order
443 for the wrapper to set the user ID, it must be installed as
444 owner <code><em>root</em></code> and must have the setuserid
445 execution bit set for file modes.</p>
447 <p><strong>Setting paranoid permissions</strong><br />
448 Although the suEXEC wrapper will check to ensure that its
449 caller is the correct user as specified with the
450 <code>--with-suexec-caller</code> <program>configure</program>
452 always the possibility that a system or library call suEXEC uses
453 before this check may be exploitable on your system. To counter
454 this, and because it is best-practise in general, you should use
455 filesystem permissions to ensure that only the group Apache
456 runs as may execute suEXEC.</p>
458 <p>If for example, your web server is configured to run as:</p>
465 <p>and <program>suexec</program> is installed at
466 "/usr/local/apache2/bin/suexec", you should run:</p>
469 chgrp webgroup /usr/local/apache2/bin/suexec<br />
470 chmod 4750 /usr/local/apache2/bin/suexec<br />
473 <p>This will ensure that only the group Apache runs as can even
474 execute the suEXEC wrapper.</p>
477 <section id="enable"><title>Enabling & Disabling
480 <p>Upon startup of Apache, it looks for the file
481 <program>suexec</program> in the directory defined by the
482 <code>--sbindir</code> option (default is
483 "/usr/local/apache/sbin/suexec"). If Apache finds a properly
484 configured suEXEC wrapper, it will print the following message
485 to the error log:</p>
488 [notice] suEXEC mechanism enabled (wrapper: <var>/path/to/suexec</var>)
491 <p>If you don't see this message at server startup, the server is
492 most likely not finding the wrapper program where it expects
493 it, or the executable is not installed <em>setuid root</em>.</p>
495 <p>If you want to enable the suEXEC mechanism for the first time
496 and an Apache server is already running you must kill and
497 restart Apache. Restarting it with a simple HUP or USR1 signal
498 will not be enough. </p>
499 <p>If you want to disable suEXEC you should kill and restart
500 Apache after you have removed the <program>suexec</program> file.</p>
503 <section id="usage"><title>Using suEXEC</title>
505 <p>Requests for CGI programs will call the suEXEC wrapper only if
506 they are for a virtual host containing a <directive
507 module="mod_suexec">SuexecUserGroup</directive> directive or if
508 they are processed by <module>mod_userdir</module>.</p>
510 <p><strong>Virtual Hosts:</strong><br /> One way to use the suEXEC
511 wrapper is through the <directive
512 module="mod_suexec">SuexecUserGroup</directive> directive in
513 <directive module="core">VirtualHost</directive> definitions. By
514 setting this directive to values different from the main server
515 user ID, all requests for CGI resources will be executed as the
516 <em>User</em> and <em>Group</em> defined for that <directive
517 module="core" type="section">VirtualHost</directive>. If this
518 directive is not specified for a <directive module="core"
519 type="section">VirtualHost</directive> then the main server userid
522 <p><strong>User directories:</strong><br /> Requests that are
523 processed by <module>mod_userdir</module> will call the suEXEC
524 wrapper to execute CGI programs under the userid of the requested
525 user directory. The only requirement needed for this feature to
526 work is for CGI execution to be enabled for the user and that the
527 script must meet the scrutiny of the <a href="#model">security
528 checks</a> above. See also the
529 <code>--with-suexec-userdir</code> <a href="#install">compile
530 time option</a>.</p> </section>
532 <section id="debug"><title>Debugging suEXEC</title>
534 <p>The suEXEC wrapper will write log information
535 to the file defined with the <code>--with-suexec-logfile</code>
536 option as indicated above. If you feel you have configured and
537 installed the wrapper properly, have a look at this log and the
538 error_log for the server to see where you may have gone astray.</p>
542 <section id="jabberwock"><title>Beware the Jabberwock:
543 Warnings & Examples</title>
545 <p><strong>NOTE!</strong> This section may not be
546 complete. For the latest revision of this section of the
547 documentation, see the Apache Group's <a
548 href="http://httpd.apache.org/docs/&httpd.docs;/suexec.html">Online
549 Documentation</a> version.</p>
551 <p>There are a few points of interest regarding
552 the wrapper that can cause limitations on server setup. Please
553 review these before submitting any "bugs" regarding suEXEC.</p>
556 <li><strong>suEXEC Points Of Interest</strong></li>
559 Hierarchy limitations
562 For security and efficiency reasons, all suEXEC requests
563 must remain within either a top-level document root for
564 virtual host requests, or one top-level personal document
565 root for userdir requests. For example, if you have four
566 VirtualHosts configured, you would need to structure all
567 of your VHosts' document roots off of one main Apache
568 document hierarchy to take advantage of suEXEC for
569 VirtualHosts. (Example forthcoming.)
574 suEXEC's PATH environment variable
577 This can be a dangerous thing to change. Make certain
578 every path you include in this define is a
579 <strong>trusted</strong> directory. You don't want to
580 open people up to having someone from across the world
581 running a trojan horse on them.
586 Altering the suEXEC code
589 Again, this can cause <strong>Big Trouble</strong> if you
590 try this without knowing what you are doing. Stay away
591 from it if at all possible.