SQL injection is a 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 in fact 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

renders this SQL statement by the parent language:

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):

This input renders the final SQL statement as follows:

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:

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

will drop (delete) the "users" table from the database, since the SQL would be rendered as follows:

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^[1]. 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. A tool called Absinthe can automate these attacks once the location of the vulnerability and the target information has been established.

Conditional Responses

One type of blind sql injection forces the database to evaluate a logical statement on an ordinary application screen.

will result in a normal page while

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.^[2]

Conditional Errors

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,

the division by zero will only be evaluated and result in an error if user Ralph exists.

Time Delays

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, user input must be escaped or filtered or parameterized statements must be used.

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 JDBC API:

PreparedStatement prep = conn.prepareStatement("SELECT * FROM USERS WHERE USERNAME=? AND PASSWORD=?");
prep.setString(1, username);
prep.setString(2, password); 

The same goes for C#:

using (SqlCommand myCommand = new SqlCommand("select * from Users where UserName=@username and Password=@password", myConnection))
                {                    
                    myCommand.Parameters.AddWithValue("@username", user);
                    myCommand.Parameters.AddWithValue("@password", pass);

                    myConnection.Open();
                    SqlDataReader myReader = myCommand.ExecuteReader())
                    ...................
                }

In PHP, it's usual to just escape the parameters before sending the SQL query:

$query = sprintf("SELECT * FROM Users where UserName='%s' and Password='%s'", 
                  mysql_real_escape_string($Username), 
                  mysql_real_escape_string($Password));
mysql_query($query);

For PHP version 5 and MySQL version 4.1 and above, however, you can use a vendor specific extension like msqli for "true" prepared statement queries: ^[3]

$db = new mysqli("localhost", "user", "pass", "database");
$stmt = $db -> prepare("SELECT priv FROM testUsers WHERE username=? AND password=?");
$stmt -> bind_param("ss", $user, $pass);
$stmt -> execute(); 

The mysql_real_escape_string adds backslashes (\) to escaped characters like single quotes ' and double quotes ". Though, you should read this

The magic_quotes_gpc PHP configuration directive affects Get, Post and Cookie values. If turned on, value (It's "PHP!") will automagically become (It\'s \"PHP!\").

In ColdFusion, the CFQUERYPARAM statement is useful in conjunction with the CFQUERY statement to nullify the effect of SQL code passed within the CFQUERYPARAM value as part of the SQL clause.^{[5] [6]}. An example is below.

Enforcing the Use of Parameterized Statements

There are two ways to ensure an application is not vulnerable to SQL injection: using code reviews (which is a manual process), and enforcing the use of parameterized statements. Enforcing the use of parameterized statements means that SQL statements with embedded user input are rejected at runtime. Currently only the H2 Database Engine supports this feature.

Real-world examples

• On October 26, 2005, Unknown Heise readers replaced a page owned by the German TV station ARD which advertised a pro-RIAA sitcom with Goatse using SQL injection^[7]

• On November 01, 2005, A high school student used a SQL injection to break into the site of a Taiwanese information security magazine from the Tech Target group and steal customer's information.^[8]

• On January 13, 2006, Russian hackers broke into a Rhode Island government web site and allegedly stole credit card data from individuals who have done business online with state agencies.^[9]

• On March 29, 2006, Susam Pal discovered a SQL injection flaw in an official Indian government tourism site.^[10]

• On March 2, 2007, Sebastian Bauer discovered a SQL injection flaw in the knorr.de login page.^[11]

• On June 29, 2007, Hacker Defaces Microsoft U.K. Web Page using SQL injection. ^[12][13]. U.K. website The Register quoted a Microsoft spokesperson acknowledging the problem.

• On August 12, 2007, The United Nations web site was defaced using SQL injection.^[14]

• On January 2008, tens of thousands of PCs were infected by an automated SQL injection attack that exploited a vulnerability in Microsoft SQL Server. ^[15]

• On April 13, 2008, Sexual and Violent Offender Registry of Oklahoma shuts down site for 'routine maintenance' after being informed that 10,597 social security numbers from sex offenders had been downloaded by SQL injection ^[16]

• In May 2008, a server farm inside China used automated queries to Google's search engine to identify SQL server websites which were vulnerable to the attack of an automated SQL injection tool. ^[15][17]

• In May 2008, discussion groups covering identity theft problems faced by Lifelock's president exploited a SQL Injection vulnerability in Lifelock's server that would result in yearly membership for $0.00. ^[18]

• In July 2008, Kaspersky's Malaysian site was hacked by Turkish hacker going by the handle of "m0sted", who claimed to have used SQL injection. ^[19]

• In 2008, at least April through August, a sweep of attacks began exploiting the SQL injection vulnerabilities of Microsoft's IIS web server and SQL Server database server. The attack doesn't require guessing the name of a table or column, and corrupts all text columns in all tables in a single request. ^[20] An HTML string is appended to each value that references a malware JavaScript file. When that database value is later displayed to a website visitor, the script attempts several approaches at gaining control over a visitor's system. The number of exploited web pages is estimated at 500,000^[21]

• In August 2008, Subeta, a virtual pet site, had to rollback their database to a backup version, due to an attack using a SQL Injection. ^[22] On February 2007 another SQL injection attack was used to steal the passwords of all users. ^[23]

In popular culture

In the xkcd comic strip "Exploits of a Mom", a school loses year's worth of database records due to a student named "Robert'); DROP TABLE students;--" ^[24]

References

External links

• SQL Injection WASC Threat Classification Entry, by the Web Application Security Consortium
• SQL Injections on PHP, GNU-licensed online book chapter
• Advanced SQL injection on SQL server / ASP pages, 2002 second part
• SQL Server vulnerabilities

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