Revision as of 20:34, 5 May 2012

This is a language agnostic vulnerability, effecting any application which incorrectly uses string replacement for sanitization.

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.

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.