PHP Security and Encryption Avoiding Cross-Site Scripting

PHP Security and Encryption

Avoiding Cross-Site Scripting

Problem

You need to safely avoid cross-site scripting (XSS) attacks in your PHP applications.

Solution

Escape all HTML output with htmlentities(), being sure to indicate the correct character encoding:

       /* Note the character encoding. */

       header('Content-Type: text/html; charset=UTF-8');

       /* Initialize an array for escaped data. */

       $html = array();

       /* Escape the filtered data. */

       $html['username'] = htmlentities($clean['username'], ENT_QUOTES, 'UTF-8');

       echo "<p>Welcome back, {$html['username']}.</p>";

Discussion

The htmlentities() function replaces each character with its HTML entity, if it has one. For example, > is replaced with &gt;. Although the immediate effect is that the data is modified, the purpose of the escaping is to preserve the data in a different context. Whenever a browser renders &gt; as HTML, it appears on the screen as >.

XSS attacks try to take advantage of a situation where data provided by a third party is included in the HTML without being escaped properly. A clever attacker can provide code that can be very dangerous to your users when interpreted by their browsers. By using htmlentities(), you can be sure that such third-party data is displayed properly and not interpreted.