Hacks From Pax: PHP Web Application Security
By: Pax Dickinson

Today on Hacks From Pax we'll be discussing PHP web application security. PHP is a great language for rapidly developing web applications, and is very friendly to beginning programmers, but some of its design can make it difficult to write web apps that are properly secure. We'll discuss some of the main security "gotchas" when developing PHP web applications, from proper user input sanitization to avoiding SQL injection vulnerabilities.

Many PHP application vulnerabilities are caused by not properly initializing variables. This is an example of how PHP, by not requiring the developer to initialize a variable before using it, sacrifices security for ease of use. For example, the following code is easily exploitable.

   if (user_auth()) {
    $access = true;
  }
    if ($access) {
    do_sensitive_things();
  }

This could be exploited by tacking an ?access=true to the end of the url, and the if ($access) test would be passed despite the user_auth() function returning false. This hole could be closed easily by adding a $access = false; at the top of the script, but not all security holes are this easy to spot.

Thankfully, PHP now defaults the register_globals option to off. This setting would pass the access variable sent by the url to the script as $_GET[access] rather than just $access. This closes off many of these types of vulnerabilities, but when writing PHP code, especially code for distribution, you should never assume that this option will be set correctly, and always initialize your PHP variables. Users in a shared hosting environment may not have the ability to set these options to their most secure setting.

• Always initialize PHP variables before using them.
• Always set register_globals to off, but never write code that assumes this setting.
• You can use the ini_get() function to determine if register_globals is set at runtime.

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http://www.linuxsecurity.com/content/view/120043/49/

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