Revision as of 20:33, 5 May 2012

Overview

Unsafe use of string replacement functions to sanitize user input is extremely common. Because string replace (str_replace in PHP) functions only do a single replacement, it is necessary to loop over them until all unsafe characters or strings are removed if you are replacing more than a single character. This is a language agnostic vulnerability, effecting any application which incorrectly uses string replacement for sanitization.

Example

A trivial example:

 
<?php
$filepath = $_GET['file'];
 
$safe_filepath = str_replace('../', '', $filepath);
 
echo("Safe filepath is '" . $safe_filepath . "'<br />");
include($safe_filepath);
?>
 

First an attacker may try a simple directory transversal attack, using '../' to escape. The result:

 Safe filepath is 'etc/passwd'

No dice, the dangerous string ('../') is dutifully sanitized by str_replace. But, our attacker isn't going to give up yet, now armed with the knowledge that '../' is being filtered out, he may try:

 test.php?file=....//....//....//....//....//....//....//....//....//....//....//....//....//etc/passwd

The result:

 Safe filepath is '../../../../../../../../../../../etc/passwd'
 [contents of /etc/passwd]

Even if '../' is replaced twice, it can be easily bypassed by using ......///. No matter how many times the replacement is made, the attacker simply needs to nest another layer.

Other examples of unsafe uses of string replacement include:

 
str_replace('<?', '', $source);

 Bypassed by '<<??'

 
str_replace(array('<script', '<img'), '', $source);
 

 Bypassed by '<<imgscript>'

 
str_replace('file://', '', $source);
 

 Bypassed by 'file:/file:///'

Defense

If one must use str_replace for sensitization, it must be done recursively (thought this implementation uses a loop, for efficiency reasons):

 
function safe_str_replace($search, $replace, $subject)
{
        while(strstr($subject, $search) !== FALSE) {
                $subject = str_replace($search, $replace, $subject);
        }
 
        return $subject;
}
 

However, the use of whitelists with 'positive' regex matching (i.e. does the input match /[a-z]+/) is more effective.