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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>

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 <acronym>PHP</acronym> 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>

</chapter>

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