Unsafe string replacement

Introduction

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 [File Inclusion] 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.

Defense

If one must use str_replace for sensitization (which is strongly advised against), the secure way of doing so would be to loop until no more dangerous strings are found in the source string. The example program implemented in this fashion would look like this:

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

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