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<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<!-- splitted from ./index.xml, last change in rev 1.66 -->
<chapter xml:id="security.cgi-bin" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
<title>Installed as CGI binary</title>
<sect1 xml:id="security.cgi-bin.attacks">
<title>Possible attacks</title>
<simpara>
Using PHP as a <acronym>CGI</acronym> binary is an option for
setups that for some reason do not wish to integrate PHP as a
module into server software (like Apache), or will use PHP with
different kinds of <acronym>CGI</acronym> wrappers to create safe
<command>chroot</command> and <command>setuid</command>
environments for scripts. This setup usually involves installing
executable <command>php</command> binary to the web server <filename class="directory">cgi-bin</filename> directory.
CERT advisory <link xlink:href="&url.cert;">CA-96.11</link> recommends
against placing any interpreters into <filename class="directory">cgi-bin</filename>.
Even if the <command>php</command> binary can be used as a standalone interpreter,
PHP is designed to prevent the attacks this setup makes possible:
</simpara>
<itemizedlist>
<listitem>
<simpara>
Accessing system files: <filename
role="url">http://my.host/cgi-bin/php?/etc/passwd</filename>
</simpara>
<simpara>
The query information in a URL after the question mark (<literal>?</literal>) is
passed as command line arguments to the interpreter by the CGI
interface. Usually interpreters open and execute the file
specified as the first argument on the command line.
</simpara>
<simpara>
When invoked as a CGI binary, <command>php</command> refuses to interpret the
command line arguments.
</simpara>
</listitem>
<listitem>
<simpara>
Accessing any web document on server: <filename
role="url">http://my.host/cgi-bin/php/secret/doc.html</filename>
</simpara>
<simpara>
The path information part of the URL after the PHP binary name,
<filename role="uri">/secret/doc.html</filename> is
conventionally used to specify the name of the file to be
opened and interpreted by the <acronym>CGI</acronym> program.
Usually some web server configuration directives (Apache:
<literal>Action</literal>) are used to redirect requests to documents like
<filename
role="url">http://my.host/secret/script.php</filename> to the
PHP interpreter. With this setup, the web server first checks
the access permissions to the directory <filename
role="uri">/secret</filename>, and after that creates the
redirected request <filename
role="url">http://my.host/cgi-bin/php/secret/script.php</filename>.
Unfortunately, if the request is originally given in this form,
no access checks are made by web server for file <filename
role="uri">/secret/script.php</filename>, but only for the
<filename role="uri">/cgi-bin/php</filename> file. This way
any user able to access <filename
role="uri">/cgi-bin/php</filename> is able to access any
protected document on the web server.
</simpara>
<simpara>
In PHP, runtime configuration directives <link
linkend="ini.cgi.force-redirect">cgi.force_redirect</link>, <link
linkend="ini.doc-root">doc_root</link> and <link
linkend="ini.user-dir">user_dir</link> can be used to prevent
this attack, if the server document tree has any directories
with access restrictions. See below for full the explanation
of the different combinations.
</simpara>
</listitem>
</itemizedlist>
</sect1>
<sect1 xml:id="security.cgi-bin.default">
<title>Case 1: only public files served</title>
<simpara>
If your server does not have any content that is not restricted
by password or IP based access control, there is no need for
these configuration options. If your web server does not allow
you to do redirects, or the server does not have a way to
communicate to the PHP binary that the request is a safely
redirected request, you can enable the
<link linkend="ini.cgi.force-redirect">cgi.force_redirect</link>
ini directive. You still have to make sure your PHP
scripts do not rely on one or another way of calling the script,
neither by directly <filename
role="php">http://my.host/cgi-bin/php/dir/script.php</filename>
nor by redirection <filename
role="php">http://my.host/dir/script.php</filename>.
</simpara>
<simpara>
Redirection can be configured in Apache by using <literal>AddHandler</literal> and
<literal>Action</literal> directives (see below).
</simpara>
</sect1>
<sect1 xml:id="security.cgi-bin.force-redirect">
<title>Case 2: using <literal>cgi.force_redirect</literal></title>
<simpara>
The configuration directive <link
linkend="ini.cgi.force-redirect">cgi.force_redirect</link>
prevents anyone from calling <command>php</command>
directly with a URL like <filename
role="php">http://my.host/cgi-bin/php/secretdir/script.php</filename>.
Instead, PHP will only parse in this mode if it has gone through
a web server redirect rule.
</simpara>
<simpara>
Usually the redirection in the Apache configuration is done with
the following directives:
</simpara>
<programlisting role="apache-conf">
<![CDATA[
Action php-script /cgi-bin/php
AddHandler php-script .php
]]>
</programlisting>
<simpara>
This option has only been tested with the Apache web server, and
relies on Apache to set the non-standard CGI environment variable
<envar>REDIRECT_STATUS</envar> on redirected requests. If your
web server does not support any way of telling if the request is
direct or redirected, you cannot use this option and you must use
one of the other ways of running the CGI version documented
here.
</simpara>
</sect1>
<sect1 xml:id="security.cgi-bin.doc-root">
<title>Case 3: setting doc_root or user_dir</title>
<simpara>
To include active content, like scripts and executables, in the
web server document directories is sometimes considered an insecure
practice. If, because of some configuration mistake, the scripts
are not executed but displayed as regular HTML documents, this
may result in leakage of intellectual property or security
information like passwords. Therefore many sysadmins will prefer
setting up another directory structure for scripts that are
accessible only through the PHP CGI, and therefore always
interpreted and not displayed as such.
</simpara>
<simpara>
Also if the method for making sure the requests are not
redirected, as described in the previous section, is not
available, it is necessary to set up a
script <link linkend="ini.doc-root">doc_root</link> that is
different from web document root.
</simpara>
<simpara>
You can set the PHP script document root by the configuration
directive <link linkend="ini.doc-root">doc_root</link> in the
<link linkend="configuration.file">configuration file</link>, or
you can set the environment variable
<envar>PHP_DOCUMENT_ROOT</envar>. If it is set, the <acronym>CGI</acronym>
version of PHP will always construct the file name to open with this
<parameter>doc_root</parameter> and the path information in the
request, so you can be sure no script is executed outside this
directory (except for <parameter>user_dir</parameter>
below).
</simpara>
<simpara>
Another option usable here is <link
linkend="ini.user-dir">user_dir</link>. When <parameter>user_dir</parameter> is
unset, only thing controlling the opened file name is
<parameter>doc_root</parameter>. Opening a URL like <filename
role="url">http://my.host/~user/doc.php</filename> does not
result in opening a file under users home directory, but a file
called <filename role="uri">~user/doc.php</filename> under
<parameter>doc_root</parameter> (yes, a directory name starting with a tilde
[<literal>~</literal>]).
</simpara>
<simpara>
If <parameter>user_dir</parameter> is set to for example <filename
role="dir">public_php</filename>, a request like <filename
role="url">http://my.host/~user/doc.php</filename> will open a
file called <filename>doc.php</filename> under the directory
named <filename role="dir">public_php</filename> under the home
directory of the user. If the home of the user is <filename
role="dir">/home/user</filename>, the file executed is
<filename>/home/user/public_php/doc.php</filename>.
</simpara>
<simpara>
<parameter>user_dir</parameter> expansion happens regardless of
the <parameter>doc_root</parameter> setting, so you can control
the document root and user directory access
separately.
</simpara>
</sect1>
<sect1 xml:id="security.cgi-bin.shell">
<title>Case 4: PHP parser outside of web tree</title>
<para>
A very secure option is to put the PHP parser binary somewhere
outside of the web tree of files. In <filename
role="dir">/usr/local/bin</filename>, for example. The only real
downside to this option is that you will now have to put a line
similar to:
<informalexample>
<programlisting>
<![CDATA[
#!/usr/local/bin/php
]]>
</programlisting>
</informalexample>
as the first line of any file containing PHP tags. You will also
need to make the file executable. That is, treat it exactly as
you would treat any other CGI script written in Perl or sh or any
other common scripting language which uses the
<literal>#!</literal> shell-escape mechanism for launching
itself.
</para>
<para>
To get PHP to handle <envar>PATH_INFO</envar> and
<envar>PATH_TRANSLATED</envar> information correctly with this
setup, the <link linkend="ini.cgi.discard-path">cgi.discard_path</link>
ini directive has to be enabled.
</para>
</sect1>
</chapter>
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