php-doc-en/chapters/security.xml

<?xml encoding="iso-8859-1"?>
<!-- $Revision: 1.30 $ -->
 <chapter id="security">
  <title>Security</title>

  <simpara>
   PHP is a powerful language and the interpreter, whether included
   in a web server as a module or executed as a separate
   <acronym>CGI</acronym> binary, is able to access files, execute
   commands and open network connections on the server.  These
   properties make anything run on a web server insecure by default.
   PHP is designed specifically to be a more secure language for
   writing CGI programs than Perl or C, and with correct selection of
   compile-time and runtime configuration options, and proper coding
   practices, it can give you exactly the combination of freedom and
   security you need.
  </simpara>
  <simpara>
   As there are many different ways of utilizing PHP, there are many
   configuration options controlling its behaviour.  A large
   selection of options guarantees you can use PHP for a lot of
   purposes, but it also means there are combinations of these
   options and server configurations that result in an insecure
   setup.
  </simpara>
  <simpara>
   The configuration flexibility of PHP is equally rivalled by the
   code flexibility. PHP can be used to build complete server
   applications, with all the power of a shell user, or it can be used
   for simple server-side includes with little risk in a tightly
   controlled environment. How you build that environment, and how
   secure it is, is largely up to the PHP developer.
  </simpara>
  <simpara>
   This chapter starts by explaining the different configuration
   option combinations and the situations they can be safely used. It
   then describes different considerations in coding for different
   levels of security, and ends with some general security advice.
  </simpara>

  <sect1 id="security.general">
   <title>General considerations</title>
   <simpara>
    A completely secure system is a virtual impossibility, so an
    approach often used in the security profession is one of balancing
    risk and usability. If every variable submitted by a user required
    two forms of biometric validation (such as a retinal scan and a
    fingerprint), you would have an extremely high level of
    accountability. It would also take half an hour to fill out a fairly
    complex form, which would tend to encourage users to find ways of
    bypassing the security.
   </simpara>
   <simpara>
    The best security is often inobtrusive enough to suit the
    requirements without the user being prevented from accomplishing
    their work, or over-burdening the code author with excessive
    complexity. Indeed, some security attacks are merely exploits of
    this kind of overly built security, which tends to erode over time.
   </simpara>
   <simpara>
    A phrase worth remembering: A system is only as good as the weakest
    link in a chain. If all transactions are heavily logged based on
    time, location, transaction type, etc. but the user is only
    verified based on a single cookie, the validity of tying the users
    to the transaction log is severely weakened.
   </simpara>
   <simpara>
    When testing, keep in mind that you will not be able to test all
    possibilities for even the simplest of pages. The input you
    may expect will be completely unrelated to the input given by
    a disgruntled employee, a cracker with months of time on their
    hands, or a housecat walking across the keyboard. This is why it's
    best to look at the code from a logical perspective, to discern
    where unexpected data can be introduced, and then follow how it is
    modified, reduced, or amplified.
   </simpara>
   <simpara>
    The Internet is filled with people trying to make a name for
    themselves by breaking your code, crashing your site, posting
    inappropriate content, and otherwise making your day interesting.
    It doesn't matter if you have a small or large site, you are
    a target by simply being online, by having a server that can be
    connected to. Many cracking programs do not discern by size, they
    simply trawl massive IP blocks looking for victims. Try not to
    become one.
   </simpara>
  </sect1>

  <sect1 id="security.cgi">
   <title>Installed as CGI binary</title>

   <sect2 id="security.cgi.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 CGI wrappers to create safe chroot and setuid
     environments for scripts.  This setup usually involves installing
     executable PHP binary to the web server cgi-bin directory.  CERT
     advisory <ulink url="&url.cert;">CA-96.11</ulink> recommends
     against placing any interpreters into cgi-bin.  Even if the PHP
     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 (?)  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, PHP 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:
       Action) 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, compile-time configuration option <link
       linkend="install.configure.enable-force-cgi-redirect">--enable-force-cgi-redirect</link>
       and runtime configuration directives <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>
   </sect2>

   <sect2 id="security.cgi.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 specify the option <link
     linkend="install.configure.enable-force-cgi-redirect">--enable-force-cgi-redirect</link>
     to the configure script.  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 AddHandler and
     Action directives (see below).
    </simpara>
   </sect2>
      
   <sect2 id="security.cgi.force-redirect">
    <title>Case 2: using --enable-force-cgi-redirect</title>
    <simpara>
     This compile-time option prevents anyone from calling PHP
     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">
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>
   </sect2>
      
   <sect2 id="security.cgi.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 consider 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 doc_root 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 CGI 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 user_dir is unset,
     only thing controlling the opened file name is
     <parameter>doc_root</parameter>.  Opening an 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
     doc_root (yes, a directory name starting with a tilde
     [<literal>~</literal>]).
    </simpara>      
    <simpara>
     If user_dir 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>
   </sect2>
      
   <sect2 id="security.cgi.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>
#!/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 php parser should be compiled with the <link
     linkend="install.configure.enable-discard-path">--enable-discard-path</link>
     configure option.
    </para>
   </sect2>
  
  </sect1>

  <sect1 id="security.apache">
   <title>Installed as an Apache module</title>
   <simpara>
    When PHP is used as an Apache module it inherits Apache's user
    permissions (typically those of the "nobody" user). This has several
    impacts on security and authorization. For example, if you are using
    PHP to access a database, unless that database has built-in access
    control, you will have to make the database accessable to the
    "nobody" user. This means a malicious script could access and modify
    the database, even without a username and password. It's entirely
    possible that a web spider could stumble across a database
    administrator's web page, and drop all of your databases. You can
    protect against this with Apache authorization, or you can design
    your own access model using LDAP, .htaccess files, etc. and include
    that code as part of your PHP scripts.
   </simpara>
   <simpara>
    Often, once security is established to the point where the PHP user
    (in this case, the apache user) has very little risk attached to it,
    it is discovered that PHP is now prevented from writing any files
    to user directories. Or perhaps it has been prevented from accessing
    or changing databases. It has equally been secured from writing
    good and bad files, or entering good and bad database transactions.
   </simpara>
   <simpara>
    A frequent security mistake made at this point is to allow apache
    root permissions, or to escalate apache's abilitites in some other
    way.
   </simpara>
   <simpara>
    Escalating the Apache user's permissions to root is extremely
    dangerous and may compromise the entire system, so sudo'ing,
    chroot'ing, or otherwise running as root should not be considered by
    those who are not security professionals.
   </simpara>
   <simpara>
    There are some simpler solutions. By using 
    <link linkend="ini.open-basedir">open_basedir</link> you can control and restrict what
    directories are allowed to be used for PHP. You can also set up
    apache-only areas, to restrict all web based activity to non-user,
    or non-system, files.
   </simpara>
  </sect1>

  <sect1 id="security.filesystem">
   <title>Filesystem Security</title>
   <simpara>
    PHP is subject to the security built into most server systems with
    respect to permissions on a file and directory basis. This allows
    you to control which files in the filesystem may be read. Care
    should be taken with any files which are world readable to ensure
    that they are safe for reading by all users who have access to that
    filesystem.
   </simpara>
   <simpara>
    Since PHP was designed to allow user level access to the filesystem,
    it's entirely possible to write a PHP script that will allow you
    to read system files such as /etc/password, modify your ethernet
    connections, send massive printer jobs out, etc. This has some
    obvious implications, in that you need to ensure that the files
    that you read from and write to are the appropriate ones.
   </simpara>
   <simpara>
    Consider the following script, where a user indicates that they'd
    like to delete a file in their home directory. This assumes a
    situation where a PHP web interface is regularly used for file
    management, so the Apache user is allowed to delete files in
    the user home directories.
   </simpara>
   <para>
    <example>
     <title>Poor variable checking leads to....</title>
     <programlisting role="php">
&lt;?php
// remove a file from the user's home directory
$username = $HTTP_POST_VARS['user_submitted_name'];
$homedir = "/home/$username";
$file_to_delete = "$userfile";
unlink ($homedir/$userfile);
echo "$file_to_delete has been deleted!";
?&gt;
     </programlisting>
    </example>
   Since the username is postable from a user form, they can submit
   a username and file belonging to someone else, and delete files.
   In this case, you'd want to use some other form of authentication.
   Consider what could happen if the variables submitted were
   "../etc/" and "passwd". The code would then effectively read:
    <example>
     <title>... A filesystem attack</title>
     <programlisting role="php">
&lt;?php
// removes a file from anywhere on the hard drive that
// the PHP user has access to. If PHP has root access:
$username = "../etc/";
$homedir = "/home/../etc/";
$file_to_delete = "passwd";
unlink ("/home/../etc/passwd");
echo "/home/../etc/passwd has been deleted!";
?&gt;
     </programlisting>
    </example>   
    There are two important measures you should take to prevent these
    issues.
    <itemizedlist>
     <listitem>
      <simpara>
       Only allow limited permissions to the PHP web user binary.
      </simpara>
     </listitem>
     <listitem>
      <simpara>
       Check all variables which are submitted.
      </simpara>
     </listitem>
    </itemizedlist>
    Here is an improved script:
    <example>
     <title>More secure file name checking</title>
     <programlisting role="php">
&lt;?php
// removes a file from the hard drive that
// the PHP user has access to. 
$username = $HTTP_SERVER_VARS['REMOTE_USER']; // using an authentication mechanisim

$homedir = "/home/$username";

$file_to_delete = basename("$userfile"); // strip paths
unlink ($homedir/$file_to_delete);

$fp = fopen("/home/logging/filedelete.log","+a"); //log the deletion
$logstring = "$username $homedir $file_to_delete";
fputs ($fp, $logstring);
fclose($fp);

echo "$file_to_delete has been deleted!";
?&gt;
     </programlisting>
    </example>
    However, even this is not without it's flaws. If your authentication
    system allowed users to create their own user logins, and a user
    chose the login "../etc/", the system is once again exposed. For
    this reason, you may prefer to write a more customized check:
    <example>
     <title>More secure file name checking</title>
     <programlisting role="php">
&lt;?php
$username = $HTTP_SERVER_VARS['REMOTE_USER']; // using an authentication mechanisim
$homedir = "/home/$username";

if (!ereg('^[^./][^/]*$', $userfile))
     die('bad filename'); //die, do not process
     
if (!ereg('^[^./][^/]*$', $username))
     die('bad username'); //die, do not process    
//etc...
?&gt;
     </programlisting>
    </example> 
   </para>
   <para>
    Depending on your operating system, there are a wide variety of files
    which you should be concerned about, including device entries (/dev/
    or COM1), configuration files (/etc/ files and the .ini files),
    well known file storage areas (/home/, My Documents), etc. For this
    reason, it's usually easier to create a policy where you forbid
    everything except for what you explicitly allow.
   </para>   
  </sect1>

  <sect1 id="security.errors">
   <title>Error Reporting</title>
   <para>
    With PHP security, there are two sides to error reporting. One is
    beneficial to increasing security, the other is detrimental.
   </para>
   <para>
    A standard attack tactic involves profiling a system by feeding
    it improper data, and checking for the kinds, and contexts, of the
    errors which are returned. This allows the system cracker to probe
    for information about the server, to determine possible weaknesses.
    For example, if an attacker had gleaned information about a page
    based on a prior form submission, they may attempt to override
    variables, or modify them:
    <example>
     <title>Attacking Variables with a custom HTML page</title>
     <programlisting role="php">
&lt;form method="post" action="attacktarget?username=badfoo&amp;password=badfoo"&gt;
&lt;input type="hidden" name="username" value="badfoo"&gt;
&lt;input type="hidden" name="password" value="badfoo"&gt;
&lt;/form&gt;
     </programlisting>
    </example> 
   </para>
   <para>
    The PHP errors which are normally returned can be quite helpful to a
    developer who is trying to debug a script, indicating such things
    as the function or file that failed, the PHP file it failed in,
    and the line number which the failure occured in. This is all
    information that can be exploited.  It is not uncommon for a php
    developer to use <function>show_source</function>, 
    <function>highlight_string</function>, or 
    <function>highlight_file</function> as a debugging measure, but in 
    a live site, this can expose hidden variables, unchecked syntax, 
    and other dangerous information. Especially dangerous is running
    code from known sources with built-in debugging handlers, or using
    common debugging techniques. If the attacker can determine what
    general technique you are using, they may try to brute-force a page,
    by sending various common debugging strings:
    <example>
     <title>Exploiting common debugging variables</title>
     <programlisting role="php">
&lt;form method="post" action="attacktarget?errors=Y&amp;showerrors=1"&amp;debug=1"&gt;
&lt;input type="hidden" name="errors" value="Y"&gt;
&lt;input type="hidden" name="showerrors" value="1"&gt;
&lt;input type="hidden" name="debug" value="1"&gt;
&lt;/form&gt;
     </programlisting>
    </example>     
   </para>
   <para>
    Regardless of the method of error handling, the ability to probe a
    system for errors leads to providing an attacker with more
    information.
   </para>
   <para>
    For example, the very style of a generic PHP error indicates a system
    is running PHP. If the attacker was looking at an .html page, and
    wanted to probe for the back-end (to look for known weaknesses in
    the system), by feeding it the wrong data they may be able to
    determine that a system was built with PHP.
   </para>
   <para>
    A function error can indicate whether a system may be running a
    specific database engine, or give clues as to how a web page or
    programmed or designed. This allows for deeper investigation into
    open database ports, or to look for specific bugs or weaknesses
    in a web page. By feeding different pieces of bad data, for example,
    an attacker can determine the order of authentication in a script,
    (from the line number errors) as well as probe for exploits that
    may be exploited in different locations in the script.
   </para>
   <para>
    A filesystem or general PHP error can indicate what permissions
    the webserver has, as well as the structure and organization of
    files on the web server. Developer written error code can aggravate
    this problem, leading to easy exploitation of formerly "hidden"
    information.
   </para>
   <para>
    There are three major solutions to this issue. The first is to
    scrutinize all functions, and attempt to compensate for the bulk
    of the errors. The second is to disable error reporting entirely
    on the running code. The third is to use PHP's custom error
    handling functions to create your own error handler. Depending
    on your security policy, you may find all three to be applicable
    to your situation.
   </para>
   <para>
    One way of catching this issue ahead of time is to make use of
    PHP's own <function>error_reporting</function>, to help you
    secure your code and find variable usage that may be dangerous.
    By testing your code, prior to deployment, with E_ALL, you can
    quickly find areas where your variables may be open to poisoning
    or modification in other ways. Once you are ready for deployment,
    by using E_NONE, you insulate your code from probing.
    <example>
     <title>Finding dangerous variables with E_ALL</title>
     <programlisting role="php">
&lt;?php
if ($username) {  // Not initialized or checked before usage
    $good_login = 1; 
}
if ($good_login == 1) { // If above test fails, not initialized or checked before usage
    fpassthru ("/highly/sensitive/data/index.html");
}
?&gt;
     </programlisting>
    </example>
   </para>
  </sect1>

  <sect1 id="security.registerglobals">
   <title>Using Register Globals</title>
   <para>
    One feature of PHP that can be used to enhance security is configuring PHP with 
    <link linkend="ini.register-globals">register_globals</link> = off.  
    By turning off the ability for any user-submitted variable to be injected 
    into PHP code, you can reduce the amount of variable
    poisoning a potential attacker may inflict. They will have
    to take the additional time to forge submissions, and your
    internal variables are effectively isolated from user
    submitted data.
   </para>
   <para>
    While it does slightly increase the amount of effort required
    to work with PHP, it has been argued that the benefits far
    outweigh the effort.
    <example>
     <title>Working without register_globals=off</title>
     <programlisting role="php">
&lt;?php
if ($username) {  // can be forged by a user in get/post/cookies
    $good_login = 1; 
}

if ($good_login == 1) { // can be forged by a user in get/post/cookies,
    fpassthru ("/highly/sensitive/data/index.html");
}
?&gt;
     </programlisting>
    </example>
    <example>
     <title>Working with register_globals = off</title>
     <programlisting role="php">
&lt;?php
if($HTTP_COOKIE_VARS['username']){ 
    // can only come from a cookie, forged or otherwise
    $good_login = 1;
    fpassthru ("/highly/sensitive/data/index.html");
}
?&gt;
     </programlisting>
    </example>
    By using this wisely, it's even possible to take preventative
    measures to warn when forging is being attempted. If you know
    ahead of time exactly where a variable should be coming from,
    you can check to see if submitted data is coming from an
    inappropriate kind of submission. While it doesn't guarantee
    that data has not been forged, it does require an attacker
    to guess the right kind of forging.
    <example>
     <title>Detecting simple variable poisoning</title>
     <programlisting role="php">
&lt;?php
if ($HTTP_COOKIE_VARS['username'] &amp;&amp;
    !$HTTP_POST_VARS['username'] &amp;&amp;
    !$HTTP_GET_VARS['username'] ) { 
    // Perform other checks to validate the user name...
    $good_login = 1;
    fpassthru ("/highly/sensitive/data/index.html");
} else {
   mail("admin@example.com", "Possible breakin attempt", $HTTP_SERVER_VARS['REMOTE_ADDR']);
   echo "Security violation, admin has been alerted.";
   exit;
}
?&gt;
     </programlisting>
    </example>
    Of course, simply turning on register globals does not mean code
    is secure. For every piece of data that is submitted, it
    should also be checked in other ways.
   </para>
  </sect1>

  
  <sect1 id="security.variables">
   <title>User Submitted Data</title>
   <para>
    The greatest weakness in many PHP programs is not inherent in the
    language itself, but merely an issue of code not being written with
    security in mind. For this reason, you should always take the time
    to consider the implications of a given piece of code, to ascertain
    the possible damage if an unexpected variable is submitted to it.
    <example>
     <title>Dangerous Variable Usage</title>
     <programlisting role="php">
&lt;?php
// remove a file from the user's home directory... or maybe
// somebody else's?
unlink ($evil_var);

// Write logging of their access... or maybe an /etc/password entry?
fputs ($fp, $evil_var);

// Execute something trivial.. or rm -rf *?
system ($evil_var);
exec ($evil_var);

?&gt;
     </programlisting>
    </example>
    You should always carefully examine your code to make sure that any
    variables being submitted from a web browser are being properly
    checked, and ask yourself the following questions:
    <itemizedlist>
     <listitem>
      <simpara>
       Will this script only affect the intended files?
      </simpara>
     </listitem>
     <listitem>
      <simpara>
       Can unusual or undesirable data be acted upon?
      </simpara>
     </listitem>
     <listitem>
     <simpara>
       Can this script be used in unintended ways?
      </simpara>
     </listitem>
     <listitem>
      <simpara>
       Can this be used in conjunction with other scripts in a negative
       manner?
      </simpara>
     </listitem> 
     <listitem>
      <simpara>
       Will any transactions be adequately logged?
      </simpara>
     </listitem>
    </itemizedlist>
    By adequately asking these questions while writing the script,
    rather than later, you prevent an unfortunate re-write when you
    need to increase your security. By starting out with this mindset,
    you won't guarantee the security of your system, but you can help
    improve it.
   </para>
   <para>
    You may also want to consider turning off register_globals,
    magic_quotes, or other convenience settings which may confuse
    you as to the validity, source, or value of a given variable.
    Working with PHP in error_reporting(E_ALL) mode can also help warn
    you about variables being used before they are checked or
    initialized (so you can prevent unusual data from being
    operated upon).
   </para>
  </sect1>
  
  <sect1 id="security.hiding">
   <title>Hiding PHP</title>
   <para>
    A few simple techniques can help to hide PHP, possibly slowing
    down an attacker who is attempting to disover weaknesses in your
    system. By setting expose_php = off in your php.ini file, you
    reduce the amount of information available to them.
   </para>
   <para>
    Another tactic is to configure web servers such as apache to
    parse different filetypes through PHP, either with an .htaccess 
    directive, or in the apache configuration file itself. You can
    then use misleading file extensions:
    <example>
     <title>Hiding PHP as another language</title>
     <programlisting role="php">
# Make PHP code look like other code types
AddType application/x-httpd-php .asp .py .pl
     </programlisting>
    </example>
    Or obscure it completely:
    <example>
     <title>Using unknown types for PHP extensions</title>
     <programlisting role="php">
# Make PHP code look like unknown types
AddType application/x-httpd-php .bop .foo .133t
     </programlisting>
    </example>
    Or hide it as html code, which has a slight performance hit because
    all html will be parsed through the PHP engine:
    <example>
     <title>Using html types for PHP extensions</title>
     <programlisting role="php">
# Make all PHP code look like html
AddType application/x-httpd-php .htm .html
     </programlisting>
    </example>
    For this to work effectively, you must rename your PHP files with
    the above extensions. While it is a form of security through
    obscurity, it's a minor preventative measure with few drawbacks.
   </para>
  </sect1>
  
  <sect1 id="security.current">
   <title>Keeping Current</title>
   <simpara>
    PHP, like any other large system, is under constant scrutiny and
    improvement. Each new version will often include both major and
    minor changes to enhance and repair security flaws, configuration
    mishaps, and other issues that will affect the overall security
    and stability of your system.
   </simpara>
   <simpara>
    Like other system-level scripting languages and programs, the best
    approach is to update often, and maintain awareness of the latest
    versions and their changes.
   </simpara>
  </sect1>
 </chapter>

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