Preventing Cross-Site Scripting






Preventing Cross-Site Scripting

Problem

You want to securely display user-entered data on an HTML page. For example, you want to allow users to add comments to a blog post without worrying that HTML or JavaScript in a comment will cause problems.

Solution

Pass user input through htmlentities( ) before displaying it, as in Figure.

Escaping HTML

<?php
print 'The comment was: ';
print htmlentities($_POST['comment']);
?>

Discussion

PHP has a pair of functions to escape HTML entities. The most basic is htmlspecialchars( ), which escapes four characters: < > " and &. Depending on optional parameters, it can also translate ' instead of or in addition to ". For more complex encoding, use htmlentities( ); it expands on htmlspecialchars( ) to encode any character that has an HTML entity. Figure shows htmlspecialchars( ) in action.

Escaping HTML entities

<?php
$html = "<a href='fletch.html'>Stew's favorite movie.</a>\n";
print htmlspecialchars($html);                // double-quotes
print htmlspecialchars($html, ENT_QUOTES);    // single- and double-quotes
print htmlspecialchars($html, ENT_NOQUOTES);  // neither

Figure prints:

&lt;a href=&quot;fletch.html&quot;&gt;Stew's favorite movie.&lt;/a&gt;
&lt;a href=&quot;fletch.html&quot;&gt;Stew&#039;s favorite movie.&lt;/a&gt;
&lt;a href="fletch.html"&gt;Stew's favorite movie.&lt;/a&gt;

By default, both htmlentities( ) and htmlspecialchars( ) use the ISO-8859-1 character set. To use a different character set, pass the character set as a third argument. For example, to use UTF-8, call htmlentities($string, ENT_QUOTES, 'UTF-8').

See Also

Recipes 18.4 and 19.13; documentation on htmlentities( ) at http://www.php.net/htmlentities and htmlspecialchars( ) at http://www.php.net/htmlspecialchars.



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