鬼仔's Blog

Mysql charset Truncation vulnerability

By http://www.80sec.com/

We found that there is a interesting feature in mysql database,when you are using utf8,gbk or other charsets.This feature may make your application unsecure.

Stefen Esser shows some attack manners of mysql in his paper[1], in which he issues the SQL Column Truncation vulnerability.

The application is a forum where new users can register
The administrator’s name is known e.g. ‘admin’
MySQL is used in the default mode
There is no application restriction on the length of new user names
The database column username is limited to 16 characters

Although the application restrict the length of the username, we can bypass it in the following example:

<?php
$user=$_REQUEST['user'];
mysql_connect(”localhost”,  “root”, “”) or
die(”Could not connect: ” .  mysql_error());
mysql_select_db(”test”);
mysql_query(”SET names  utf8″);
$result = mysql_query(”SELECT * from test_user where  user=’$user’”);
if(trim($user)==” or strlen($user)>20 ){
die(”Input  user Invalid”);
}
if(@mysql_fetch_array($result, MYSQL_NUM))  {
die(”already exist”);
}
else {
$sql=”insert test_user values  (’$user’)”;
mysql_query($sql);
echo “$user register  OK!”;
}
mysql_free_result($result);
?>

Read the code here:

$result = mysql_query("SELECT * from test_user where  user='$user'");

If the attacker input a username ‘admin z’, and the sql will be like this:

SELECT * FROM user WHERE username='admin z'

And the application will check the length of username with the following code:

if(trim($user)=='' or strlen($user)>20 ){
die("Input user  Invalid");
}

The attack will failed because the length of the username ‘admin z’ is greater then 20.

But it will not end here, attacker can input username ‘admin0xc1zzz’, and the sql will be like this:

SELECT * FROM user WHERE username='admin0xc1zzz'

This pass the application’s logic,when the insert commond executes:

insert test_user values ('admin0xc1zzz')

because the table is created in charset utf8,the 0xc1 is not a valid utf8 character,it will be striped,also all of the next characters will be striped too.Then the attacker got a user “admin”;

As you see,when mysql works at utf8,the invalid data will be striped ,but the webapplication doesn’t know this,it works at binaray.The difference between webapplication and database make a vulnerability.

Reference:

[1] http://www.suspekt.org/2008/08/18/mysql-and-sql-column-truncation-vulnerabilities/

Mysql charset Truncation vulnerability:http://www.80sec.com/mysql-charset-truncation-vulnerability.html