User Submitted Data

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.

<?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);

?>

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:

• Will this script only affect the intended files?
• Can unusual or undesirable data be acted upon?
• Can this script be used in unintended ways?
• Can this be used in conjunction with other scripts in a negative manner?
• Will any transactions be adequately logged?

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.

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.

Features like register_globals and magic_quotes (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.

Enable error_reporting(E_ALL) to help detect uninitialized variables and validate input. Use strict types (declare(strict_types=1), introduced in PHP 7) to enforce type safety, prevent unintended type conversions, and improving overall security.