variables.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
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<chapter xml:id="security.variables" xmlns="http://docbook.org/ns/docbook">
 <title>User Submitted Data</title>
 <para>
  The greatest weakness in many <acronym>PHP</acronym> 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">
<![CDATA[
<?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/passwd entry?
fwrite ($fp, $evil_var);

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

?>
]]>
   </programlisting>
  </example>
 </para>
 <para>
  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>
 </para>
 <para>
  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>
  Improve security by disabling convenience settings that obscure input
  data's origin, validity, or integrity. Implicit variable creation and
  unchecked input can lead to vulnerabilities like injection attacks and
  data manipulation.
 </para>
 <para>
  Features like <literal>register_globals</literal> and
  <literal>magic_quotes</literal> (both removed in PHP 5.4.0) once contributed
  to these risks by automatically creating variables from user input and
  escaping data inconsistently. While no longer in PHP, similar risks persist
  if input handling is mismanaged.
 </para>
 <para>
  Enable <link linkend="function.error-reporting">error_reporting(E_ALL)</link> to
  help detect uninitialized variables and validate input. Use strict types
  (<link linkend="language.types.declarations.strict">declare(strict_types=1)</link>,
  introduced in PHP 7) to enforce type safety, prevent unintended type conversions,
  and improving overall security.
 </para>
</chapter>

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