| <?xml version="1.0" encoding="UTF-8" ?> |
| <!DOCTYPE manualpage SYSTEM "./style/manualpage.dtd"> |
| <?xml-stylesheet type="text/xsl" href="./style/manual.en.xsl"?> |
| <!-- $Revision: 1.15 $ --> |
| |
| <!-- |
| Copyright 2002-2004 The Apache Software Foundation |
| |
| Licensed under the Apache License, Version 2.0 (the "License"); |
| you may not use this file except in compliance with the License. |
| You may obtain a copy of the License at |
| |
| http://www.apache.org/licenses/LICENSE-2.0 |
| |
| Unless required by applicable law or agreed to in writing, software |
| distributed under the License is distributed on an "AS IS" BASIS, |
| WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| See the License for the specific language governing permissions and |
| limitations under the License. |
| --> |
| |
| <manualpage metafile="suexec.xml.meta"> |
| |
| <title>suEXEC Support</title> |
| |
| <summary> |
| <p>The <strong>suEXEC</strong> feature provides |
| Apache users the ability |
| to run <strong>CGI</strong> and <strong>SSI</strong> programs |
| under user IDs different from the user ID of the calling |
| web-server. Normally, when a CGI or SSI program executes, it |
| runs as the same user who is running the web server.</p> |
| |
| <p>Used properly, this feature can reduce |
| considerably the security risks involved with allowing users to |
| develop and run private CGI or SSI programs. However, if suEXEC |
| is improperly configured, it can cause any number of problems |
| and possibly create new holes in your computer's security. If |
| you aren't familiar with managing <em>setuid root</em> programs |
| and the security issues they present, we highly recommend that |
| you not consider using suEXEC.</p> |
| </summary> |
| |
| <section id="before"><title>Before we begin</title> |
| |
| <p>Before jumping head-first into this document, |
| you should be aware of the assumptions made on the part of the |
| Apache Group and this document.</p> |
| |
| <p>First, it is assumed that you are using a UNIX |
| derivative operating system that is capable of |
| <strong>setuid</strong> and <strong>setgid</strong> operations. |
| All command examples are given in this regard. Other platforms, |
| if they are capable of supporting suEXEC, may differ in their |
| configuration.</p> |
| |
| <p>Second, it is assumed you are familiar with |
| some basic concepts of your computer's security and its |
| administration. This involves an understanding of |
| <strong>setuid/setgid</strong> operations and the various |
| effects they may have on your system and its level of |
| security.</p> |
| |
| <p>Third, it is assumed that you are using an |
| <strong>unmodified</strong> version of suEXEC code. All code |
| for suEXEC has been carefully scrutinized and tested by the |
| developers as well as numerous beta testers. Every precaution |
| has been taken to ensure a simple yet solidly safe base of |
| code. Altering this code can cause unexpected problems and new |
| security risks. It is <strong>highly</strong> recommended you |
| not alter the suEXEC code unless you are well versed in the |
| particulars of security programming and are willing to share |
| your work with the Apache Group for consideration.</p> |
| |
| <p>Fourth, and last, it has been the decision of |
| the Apache Group to <strong>NOT</strong> make suEXEC part of |
| the default installation of Apache. To this end, suEXEC |
| configuration requires of the administrator careful attention |
| to details. After due consideration has been given to the |
| various settings for suEXEC, the administrator may install |
| suEXEC through normal installation methods. The values for |
| these settings need to be carefully determined and specified by |
| the administrator to properly maintain system security during |
| the use of suEXEC functionality. It is through this detailed |
| process that the Apache Group hopes to limit suEXEC |
| installation only to those who are careful and determined |
| enough to use it.</p> |
| |
| <p>Still with us? Yes? Good. Let's move on!</p> |
| </section> |
| |
| <section id="model"><title>suEXEC Security Model</title> |
| |
| <p>Before we begin configuring and installing |
| suEXEC, we will first discuss the security model you are about |
| to implement. By doing so, you may better understand what |
| exactly is going on inside suEXEC and what precautions are |
| taken to ensure your system's security.</p> |
| |
| <p><strong>suEXEC</strong> is based on a setuid |
| "wrapper" program that is called by the main Apache web server. |
| This wrapper is called when an HTTP request is made for a CGI |
| or SSI program that the administrator has designated to run as |
| a userid other than that of the main server. When such a |
| request is made, Apache provides the suEXEC wrapper with the |
| program's name and the user and group IDs under which the |
| program is to execute.</p> |
| |
| <p>The wrapper then employs the following process |
| to determine success or failure -- if any one of these |
| conditions fail, the program logs the failure and exits with an |
| error, otherwise it will continue:</p> |
| |
| <ol> |
| <li> |
| <strong>Is the user executing this wrapper a valid user of |
| this system?</strong> |
| |
| <p class="indent"> |
| This is to ensure that the user executing the wrapper is |
| truly a user of the system. |
| </p> |
| </li> |
| |
| <li> |
| <strong>Was the wrapper called with the proper number of |
| arguments?</strong> |
| |
| <p class="indent"> |
| The wrapper will only execute if it is given the proper |
| number of arguments. The proper argument format is known |
| to the Apache web server. If the wrapper is not receiving |
| the proper number of arguments, it is either being |
| hacked, or there is something wrong with the suEXEC |
| portion of your Apache binary. |
| </p> |
| </li> |
| |
| <li> |
| <strong>Is this valid user allowed to run the |
| wrapper?</strong> |
| |
| <p class="indent"> |
| Is this user the user allowed to run this wrapper? Only |
| one user (the Apache user) is allowed to execute this |
| program. |
| </p> |
| </li> |
| |
| <li> |
| <strong>Does the target CGI or SSI program have an unsafe |
| hierarchical reference?</strong> |
| |
| <p class="indent"> |
| Does the target CGI or SSI program's path contain a leading |
| '/' or have a '..' backreference? These are not allowed; the |
| target CGI/SSI program must reside within suEXEC's document |
| root (see <code>--with-suexec-docroot=<em>DIR</em></code> |
| below). |
| </p> |
| </li> |
| |
| <li> |
| <strong>Is the target user name valid?</strong> |
| |
| <p class="indent"> |
| Does the target user exist? |
| </p> |
| </li> |
| |
| <li> |
| <strong>Is the target group name valid?</strong> |
| |
| <p class="indent"> |
| Does the target group exist? |
| </p> |
| </li> |
| |
| <li> |
| <strong>Is the target user <em>NOT</em> superuser?</strong> |
| |
| |
| <p class="indent"> |
| Presently, suEXEC does not allow <code><em>root</em></code> |
| to execute CGI/SSI programs. |
| </p> |
| </li> |
| |
| <li> |
| <strong>Is the target userid <em>ABOVE</em> the minimum ID |
| number?</strong> |
| |
| <p class="indent"> |
| The minimum user ID number is specified during |
| configuration. This allows you to set the lowest possible |
| userid that will be allowed to execute CGI/SSI programs. |
| This is useful to block out "system" accounts. |
| </p> |
| </li> |
| |
| <li> |
| <strong>Is the target group <em>NOT</em> the superuser |
| group?</strong> |
| |
| <p class="indent"> |
| Presently, suEXEC does not allow the <code><em>root</em></code> |
| group to execute CGI/SSI programs. |
| </p> |
| </li> |
| |
| <li> |
| <strong>Is the target groupid <em>ABOVE</em> the minimum ID |
| number?</strong> |
| |
| <p class="indent"> |
| The minimum group ID number is specified during |
| configuration. This allows you to set the lowest possible |
| groupid that will be allowed to execute CGI/SSI programs. |
| This is useful to block out "system" groups. |
| </p> |
| </li> |
| |
| <li> |
| <strong>Can the wrapper successfully become the target user |
| and group?</strong> |
| |
| <p class="indent"> |
| Here is where the program becomes the target user and |
| group via setuid and setgid calls. The group access list |
| is also initialized with all of the groups of which the |
| user is a member. |
| </p> |
| </li> |
| |
| <li> |
| <strong>Can we change directory to the one in which the target |
| CGI/SSI program resides?</strong> |
| |
| <p class="indent"> |
| If it doesn't exist, it can't very well contain files. If we |
| can't change directory to it, it might aswell not exist. |
| </p> |
| </li> |
| |
| <li> |
| <strong>Is the directory within the Apache |
| webspace?</strong> |
| |
| <p class="indent"> |
| If the request is for a regular portion of the server, is |
| the requested directory within suEXEC's document root? If |
| the request is for a UserDir, is the requested directory |
| within the directory configured as suEXEC's userdir (see |
| <a href="#install">suEXEC's configuration options</a>)? |
| </p> |
| </li> |
| |
| <li> |
| <strong>Is the directory <em>NOT</em> writable by anyone |
| else?</strong> |
| |
| <p class="indent"> |
| We don't want to open up the directory to others; only |
| the owner user may be able to alter this directories |
| contents. |
| </p> |
| </li> |
| |
| <li> |
| <strong>Does the target CGI/SSI program exist?</strong> |
| |
| <p class="indent"> |
| If it doesn't exists, it can't very well be executed. |
| </p> |
| </li> |
| |
| <li> |
| <strong>Is the target CGI/SSI program <em>NOT</em> writable |
| by anyone else?</strong> |
| |
| <p class="indent"> |
| We don't want to give anyone other than the owner the |
| ability to change the CGI/SSI program. |
| </p> |
| </li> |
| |
| <li> |
| <strong>Is the target CGI/SSI program <em>NOT</em> setuid or |
| setgid?</strong> |
| |
| <p class="indent"> |
| We do not want to execute programs that will then change |
| our UID/GID again. |
| </p> |
| </li> |
| |
| <li> |
| <strong>Is the target user/group the same as the program's |
| user/group?</strong> |
| |
| <p class="indent"> |
| Is the user the owner of the file? |
| </p> |
| </li> |
| |
| <li> |
| <strong>Can we successfully clean the process environment |
| to ensure safe operations?</strong> |
| |
| <p class="indent"> |
| suEXEC cleans the process' environment by establishing a |
| safe execution PATH (defined during configuration), as |
| well as only passing through those variables whose names |
| are listed in the safe environment list (also created |
| during configuration). |
| </p> |
| </li> |
| |
| <li> |
| <strong>Can we successfully become the target CGI/SSI program |
| and execute?</strong> |
| |
| <p class="indent"> |
| Here is where suEXEC ends and the target CGI/SSI program begins. |
| </p> |
| </li> |
| </ol> |
| |
| <p>This is the standard operation of the |
| suEXEC wrapper's security model. It is somewhat stringent and |
| can impose new limitations and guidelines for CGI/SSI design, |
| but it was developed carefully step-by-step with security in |
| mind.</p> |
| |
| <p>For more information as to how this security |
| model can limit your possibilities in regards to server |
| configuration, as well as what security risks can be avoided |
| with a proper suEXEC setup, see the <a |
| href="#jabberwock">"Beware the Jabberwock"</a> section of this |
| document.</p> |
| </section> |
| |
| <section id="install"><title>Configuring & Installing |
| suEXEC</title> |
| |
| <p>Here's where we begin the fun.</p> |
| |
| <p><strong>suEXEC configuration |
| options</strong><br /> |
| </p> |
| |
| <dl> |
| <dt><code>--enable-suexec</code></dt> |
| |
| <dd>This option enables the suEXEC feature which is never |
| installed or activated by default. At least one |
| <code>--with-suexec-xxxxx</code> option has to be provided |
| together with the <code>--enable-suexec</code> option to let |
| APACI accept your request for using the suEXEC feature.</dd> |
| |
| <dt><code>--with-suexec-bin=<em>PATH</em></code></dt> |
| |
| <dd>The path to the <code>suexec</code> binary must be hard-coded |
| in the server for security reasons. Use this option to override |
| the default path. <em>e.g.</em> |
| <code>--with-suexec-bin=/usr/sbin/suexec</code></dd> |
| |
| <dt><code>--with-suexec-caller=<em>UID</em></code></dt> |
| |
| <dd>The <a href="mod/mpm_common.html#user">username</a> under which |
| Apache normally runs. This is the only user allowed to |
| execute this program.</dd> |
| |
| <dt><code>--with-suexec-userdir=<em>DIR</em></code></dt> |
| |
| <dd>Define to be the subdirectory under users' home |
| directories where suEXEC access should be allowed. All |
| executables under this directory will be executable by suEXEC |
| as the user so they should be "safe" programs. If you are |
| using a "simple" UserDir directive (ie. one without a "*" in |
| it) this should be set to the same value. suEXEC will not |
| work properly in cases where the UserDir directive points to |
| a location that is not the same as the user's home directory |
| as referenced in the passwd file. Default value is |
| "public_html".<br /> |
| If you have virtual hosts with a different UserDir for each, |
| you will need to define them to all reside in one parent |
| directory; then name that parent directory here. <strong>If |
| this is not defined properly, "~userdir" cgi requests will |
| not work!</strong></dd> |
| |
| <dt><code>--with-suexec-docroot=<em>DIR</em></code></dt> |
| |
| <dd>Define as the DocumentRoot set for Apache. This will be |
| the only hierarchy (aside from UserDirs) that can be used for |
| suEXEC behavior. The default directory is the <code>--datadir</code> |
| value with the suffix "/htdocs", <em>e.g.</em> if you configure |
| with "<code>--datadir=/home/apache</code>" the directory |
| "/home/apache/htdocs" is used as document root for the suEXEC |
| wrapper.</dd> |
| |
| <dt><code>--with-suexec-uidmin=<em>UID</em></code></dt> |
| |
| <dd>Define this as the lowest UID allowed to be a target user |
| for suEXEC. For most systems, 500 or 100 is common. Default |
| value is 100.</dd> |
| |
| <dt><code>--with-suexec-gidmin=<em>GID</em></code></dt> |
| |
| <dd>Define this as the lowest GID allowed to be a target |
| group for suEXEC. For most systems, 100 is common and |
| therefore used as default value.</dd> |
| |
| <dt><code>--with-suexec-logfile=<em>FILE</em></code></dt> |
| |
| <dd>This defines the filename to which all suEXEC |
| transactions and errors are logged (useful for auditing and |
| debugging purposes). By default the logfile is named |
| "suexec_log" and located in your standard logfile directory |
| (<code>--logfiledir</code>).</dd> |
| |
| <dt><code>--with-suexec-safepath=<em>PATH</em></code></dt> |
| |
| <dd>Define a safe PATH environment to pass to CGI |
| executables. Default value is |
| "/usr/local/bin:/usr/bin:/bin".</dd> |
| </dl> |
| |
| <p><strong>Compiling and installing the suEXEC |
| wrapper</strong><br /> |
| If you have enabled the suEXEC feature with the |
| <code>--enable-suexec</code> option the <code>suexec</code> binary |
| (together with Apache itself) is automatically built if you execute |
| the <code>make</code> command.<br /> |
| After all components have been built you can execute the |
| command <code>make install</code> to install them. The binary image |
| <code>suexec</code> is installed in the directory defined by the |
| <code>--sbindir</code> option. The default location is |
| "/usr/local/apache2/sbin/suexec".<br /> |
| Please note that you need <strong><em>root |
| privileges</em></strong> for the installation step. In order |
| for the wrapper to set the user ID, it must be installed as |
| owner <code><em>root</em></code> and must have the setuserid |
| execution bit set for file modes.</p> |
| |
| <p><strong>Setting paranoid permissions</strong><br /> |
| Although the suEXEC wrapper will check to ensure that its |
| caller is the correct user as specified with the |
| <code>--with-suexec-caller</code> configure option, there is |
| always the possibility that a system or library call suEXEC uses |
| before this check may be exploitable on your system. To counter |
| this, and because it is best-practise in general, you should use |
| filesystem permissions to ensure that only the group Apache |
| runs as may execute suEXEC.</p> |
| |
| <p>If for example, your web-server is configured to run as:</p> |
| |
| <example> |
| User www<br /> |
| Group webgroup<br /> |
| </example> |
| |
| <p>and <code>suexec</code> is installed at |
| "/usr/local/apache2/sbin/suexec", you should run:</p> |
| |
| <example> |
| chgrp webgroup /usr/local/apache2/bin/suexec<br /> |
| chmod 4750 /usr/local/apache2/bin/suexec<br /> |
| </example> |
| |
| <p>This will ensure that only the group Apache runs as can even |
| execute the suEXEC wrapper.</p> |
| </section> |
| |
| <section id="enable"><title>Enabling & Disabling |
| suEXEC</title> |
| |
| <p>Upon startup of Apache, it looks for the file |
| <code>suexec</code> in the directory defined by the |
| <code>--sbindir</code> option (default is |
| "/usr/local/apache/sbin/suexec"). If Apache finds a properly |
| configured suEXEC wrapper, it will print the following message |
| to the error log:</p> |
| |
| <example> |
| [notice] suEXEC mechanism enabled (wrapper: <em>/path/to/suexec</em>) |
| </example> |
| |
| <p>If you don't see this message at server startup, the server is |
| most likely not finding the wrapper program where it expects |
| it, or the executable is not installed <em>setuid root</em>.</p> |
| |
| <p>If you want to enable the suEXEC mechanism for the first time |
| and an Apache server is already running you must kill and |
| restart Apache. Restarting it with a simple HUP or USR1 signal |
| will not be enough. </p> |
| <p>If you want to disable suEXEC you should kill and restart |
| Apache after you have removed the <code>suexec</code> file. </p> |
| </section> |
| |
| <section id="usage"><title>Using suEXEC</title> |
| |
| <p>Requests for CGI programs will call the suEXEC wrapper only if |
| they are for a virtual host containing a <directive |
| module="mod_suexec">SuexecUserGroup</directive> directive or if |
| they are processed by <module>mod_userdir</module>.</p> |
| |
| <p><strong>Virtual Hosts:</strong><br /> One way to use the suEXEC |
| wrapper is through the <directive |
| module="mod_suexec">SuexecUserGroup</directive> directive in |
| <directive module="core">VirtualHost</directive> definitions. By |
| setting this directive to values different from the main server |
| user ID, all requests for CGI resources will be executed as the |
| <em>User</em> and <em>Group</em> defined for that <directive |
| module="core" type="section">VirtualHost</directive>. If this |
| directive is not specified for a <directive module="core" |
| type="section">VirtualHost</directive> then the main server userid |
| is assumed.</p> |
| |
| <p><strong>User directories:</strong><br /> Requests that are |
| processed by <module>mod_userdir</module> will call the suEXEC |
| wrapper to execute CGI programs under the userid of the requested |
| user directory. The only requirement needed for this feature to |
| work is for CGI execution to be enabled for the user and that the |
| script must meet the scrutiny of the <a href="#model">security |
| checks</a> above. See also the |
| <code>--with-suexec-userdir</code> <a href="#install">compile |
| time option</a>.</p> </section> |
| |
| <section id="debug"><title>Debugging suEXEC</title> |
| |
| <p>The suEXEC wrapper will write log information |
| to the file defined with the <code>--with-suexec-logfile</code> |
| option as indicated above. If you feel you have configured and |
| installed the wrapper properly, have a look at this log and the |
| error_log for the server to see where you may have gone astray.</p> |
| |
| </section> |
| |
| <section id="jabberwock"><title>Beware the Jabberwock: |
| Warnings & Examples</title> |
| |
| <p><strong>NOTE!</strong> This section may not be |
| complete. For the latest revision of this section of the |
| documentation, see the Apache Group's <a |
| href="http://httpd.apache.org/docs-2.1/suexec.html">Online |
| Documentation</a> version.</p> |
| |
| <p>There are a few points of interest regarding |
| the wrapper that can cause limitations on server setup. Please |
| review these before submitting any "bugs" regarding suEXEC.</p> |
| |
| <ul> |
| <li><strong>suEXEC Points Of Interest</strong></li> |
| |
| <li> |
| Hierarchy limitations |
| |
| <p class="indent"> |
| For security and efficiency reasons, all suEXEC requests |
| must remain within either a top-level document root for |
| virtual host requests, or one top-level personal document |
| root for userdir requests. For example, if you have four |
| VirtualHosts configured, you would need to structure all |
| of your VHosts' document roots off of one main Apache |
| document hierarchy to take advantage of suEXEC for |
| VirtualHosts. (Example forthcoming.) |
| </p> |
| </li> |
| |
| <li> |
| suEXEC's PATH environment variable |
| |
| <p class="indent"> |
| This can be a dangerous thing to change. Make certain |
| every path you include in this define is a |
| <strong>trusted</strong> directory. You don't want to |
| open people up to having someone from across the world |
| running a trojan horse on them. |
| </p> |
| </li> |
| |
| <li> |
| Altering the suEXEC code |
| |
| <p class="indent"> |
| Again, this can cause <strong>Big Trouble</strong> if you |
| try this without knowing what you are doing. Stay away |
| from it if at all possible. |
| </p> |
| </li> |
| </ul> |
| |
| </section> |
| |
| </manualpage> |