Php/docs/security.filesystem.nullbytes
As PHP uses the underlying C functions for filesystem related operations, it may handle null bytes in a quite unexpected way. As null bytes denote the end of a string in C, strings containing them won't be considered entirely but rather only until a null byte occurs. The following example shows a vulnerable code that demonstrates this problem:
Example #1 Script vulnerable to null bytes
<?php$file = $_GET['file']; // "../../etc/passwd\0"if (file_exists('/home/wwwrun/'.$file.'.php')) { // file_exists will return true as the file /home/wwwrun/../../etc/passwd exists include '/home/wwwrun/'.$file.'.php'; // the file /etc/passwd will be included}?>
Therefore, any tainted string that is used in a filesystem operation should always be validated properly. Here is a better version of the previous example:
Example #2 Correctly validating the input
<?php$file = $_GET['file']; // Whitelisting possible valuesswitch ($file) { case 'main': case 'foo': case 'bar': include '/home/wwwrun/include/'.$file.'.php'; break; default: include '/home/wwwrun/include/main.php';}?>
