SQL injection is a code injection technique that exploits a security vulnerability occurring in the database layer of an application. The vulnerability is present when user input is either incorrectly filtered for string literal escape characters embedded in SQL statements or user input is not strongly typed and thereby unexpectedly executed. It is an instance of a more general class of vulnerabilities that can occur whenever one programming or scripting language is embedded inside another.
Forms of SQL injection vulnerabilities
Incorrectly filtered escape characters
This form of SQL injection occurs when user input is not filtered for escape characters and is then passed into a SQL statement. This results in the potential manipulation of the statements performed on the database by the end user of the application.
The following line of code illustrates this vulnerability:
statement = "SELECT * FROM users WHERE name = '" + userName + "';"
This SQL code is designed to pull up the records of a specified username from its table of users. However, if the "userName" variable is crafted in a specific way by a malicious user, the SQL statement may do more than the code author intended. For example, setting the "userName" variable as
a' or 't'='t
renders this SQL statement by the parent language:
SELECT * FROM users WHERE name = 'a' OR 't'='t';
If this code were to be used in an authentication procedure then this example could be used to force the selection of a valid username because the evaluation of 't'='t' is always true.
While most SQL Server implementations allow multiple statements to be executed with one call, some SQL APIs such as php's mysql_query do not allow this for security reasons. This prevents hackers from injecting entirely separate queries, but doesn't stop them from modifying queries. The following value of "userName" in the statement below would cause the deletion of the "users" table as well as the selection of all data from the "data" table (in essence revealing the information of every user):
a';DROP TABLE users; SELECT * FROM data WHERE name LIKE '%
This input renders the final SQL statement as follows:
SELECT * FROM Users WHERE name = 'a';DROP TABLE users; SELECT * FROM DATA WHERE name LIKE '%';Incorrect type handling
This form of SQL injection occurs when a user supplied field is not strongly typed or is not checked for type constraints. This could take place when a numeric field is to be used in a SQL statement, but the programmer makes no checks to validate that the user supplied input is numeric. For example:
statement := "SELECT * FROM data WHERE id = " + a_variable + ";"
It is clear from this statement that the author intended a_variable to be a number correlating to the "id" field. However, if it is in fact a string then the end user may manipulate the statement as they choose, thereby bypassing the need for escape characters. For example, setting a_variable to
1;DROP TABLE users
will drop (delete) the "users" table from the database, since the SQL would be rendered as follows:
SELECT * FROM DATA WHERE id=1;DROP TABLE users;
The magic string is a simple string of SQL used primarily at login pages. The magic string is
When used at a login page, you will be logged in as the user on top of the SQL table.
Vulnerabilities inside the database server
Sometimes vulnerabilities can exist within the database server software itself, as was the case with the MySQL server's
mysql_real_escape_string() function. This would allow an attacker to perform a successful SQL injection attack based on bad Unicode characters even if the user's input is being escaped.
Blind SQL Injection
Blind SQL Injection is used when a web application is vulnerable to SQL injection but the results of the injection are not visible to the attacker. The page with the vulnerability may not be one that displays data but will display differently depending on the results of a logical statement injected into the legitimate SQL statement called for that page. This type of attack can become time-intensive because a new statement must be crafted for each bit recovered. There are several tools that can automate these attacks once the location of the vulnerability and the target information has been established.
One type of blind SQL injection forces the database to evaluate a logical statement on an ordinary application screen.
SELECT booktitle FROM booklist WHERE bookId = 'OOk14cd' AND 1=1
will result in a normal page while
SELECT booktitle FROM booklist WHERE bookId = 'OOk14cd' AND 1=2
will likely give a different result if the page is vulnerable to a SQL injection. An injection like this will prove that a blind SQL injection is possible, leaving the attacker to devise statements that evaluate to true or false depending on the contents of a field in another table.
This type of blind SQL injection causes a SQL error by forcing the database to evaluate a statement that causes an error if the WHERE statement is true. For example,
SELECT 1/0 FROM users WHERE username='Ralph'
the division by zero will only be evaluated and result in an error if user Ralph exists.
Time Delays are a type of blind SQL injection that cause the SQL engine to execute a long running query or a time delay statement depending on the logic injected. The attacker can then measure the time the page takes to load to determine if the injected statement is true.
Preventing SQL Injection
To protect against SQL injection, user input must not directly be embedded in SQL statements. Instead, parameterized statements must be used (preferred), or user input must be carefully escaped or filtered.
Using Parameterized Statements
In some programming languages such as Java and .NET parameterized statements can be used that work with parameters (sometimes called placeholders or bind variables) instead of embedding user input in the statement. In many cases, the SQL statement is fixed. The user input is then assigned (bound) to a parameter. This is an example using Java and the JDBCAPI:
PreparedStatement prep = conn.prepareStatement("SELECT * FROM USERS WHERE USERNAME=? AND PASSWORD=?"); prep.setString(1, username); prep.setString(2, password);