MySQL - SQL Injection
SQL Injection
SQL injection is a code injection technique that can destroy your database. SQL injections are a common web hacking technique.
SQL injections are when attackers insert malicious SQL code into user-input fields, and this way can read, modify, or delete sensitive data in a database.
SQL injections usually occur when you ask a user for input, like username/userid, and instead of giving a name/id, the attacker inserts an SQL command that executes something in your database.
Look at the following example which creates a
SELECT statement by adding a variable
(txtUserId) to a select string. The variable is fetched from user input
(getRequestString):
Example
txtUserId = getRequestString("UserId");
txtSQL = "SELECT *
FROM Users WHERE UserId = " + txtUserId;
The rest of this chapter describes the potential dangers of using user input in SQL statements.
The next chapter show the most effective methods to prevent SQL injections, by using Prepared Statements.
SQL Injection Based on 1=1 is Always True
Look at the example above again. The original purpose of the SQL code was to select a user with a given user id.
If there is nothing to prevent a user from entering "wrong" input, the user can enter some "smart" input like this:
UserId:
Then, the SQL statement will look like this:
SELECT * FROM Users WHERE UserId = 105 OR 1=1;
The SQL above is valid and will return ALL rows from the "Users" table, since OR 1=1 is always TRUE.
Does the example above look dangerous? What if the "Users" table contains names and passwords?
A hacker might get access to all the user names and passwords in a database, by simply inserting 105 OR 1=1 into the input field.
SQL Injection Based on OR ""="" is Always True
Here is an example of a user login on a web site:
Username:
Password:
Example
uName = getRequestString("username");
uPass = getRequestString("userpassword");
sql = 'SELECT * FROM Users WHERE Name ="' + uName + '" AND Pass ="' + uPass +
'"'
Result
SELECT * FROM Users WHERE Name ="John Doe" AND Pass ="myPass"
A hacker might get access to user names and passwords in a database by simply inserting " OR ""=" into the user name or password text box:
User Name:
Password:
The SQL statement will now look like this:
Result
SELECT * FROM Users WHERE Name ="" or ""="" AND Pass ="" or ""=""
The SQL above is valid and will return ALL rows from the "Users" table, since OR ""="" is always TRUE.
SQL Injection From Batched SQL Statements
Batched SQL statements is a group of two or more SQL statements, separated by semicolons.
The SQL statement below will return all rows from the "Users" table, then delete the "Suppliers" table.
Example
SELECT * FROM Users; DROP TABLE Suppliers;
Look at the following example:
Example
txtUserId = getRequestString("UserId");
txtSQL = "SELECT *
FROM Users WHERE UserId = " + txtUserId;
And the following input:
User id:
The valid SQL statement would look like this:
Result
SELECT * FROM Users WHERE
UserId = 105; DROP TABLE Suppliers;
