SQL Injection Prevention Cheat Sheet

Introduction

This cheat sheet will help you prevent SQL injection flaws in your applications. It will define what SQL injection is, explain where those flaws occur, and provide four options for defending against SQL injection attacks. SQL Injection attacks are common because:

1. SQL Injection vulnerabilities are very common.
2. The application's database is a frequent target for attackers because it typically contains sensitive or critical data.

What Is a SQL Injection Attack?

Attackers can use SQL injection on an application if it has dynamic database queries that use string concatenation and user-supplied input. To avoid SQL injection flaws, developers need to:

1. Stop writing dynamic queries with string concatenation.
2. Prevent malicious SQL input from being included in executed queries.

There are simple techniques for preventing SQL injection vulnerabilities, and they can be used with practically any kind of programming language and any type of database. While XML databases can have similar problems (e.g., XPath and XQuery injection), these techniques can be used to protect them as well.

Anatomy of a Typical SQL Injection Vulnerability

A common SQL injection flaw in Java is shown below. Because its unvalidated "customerName" parameter is simply appended to the query, an attacker can enter SQL code into that query and the application would take the attacker's code and execute it on the database.

String query = "SELECT account_balance FROM user_data WHERE user_name = "
             + request.getParameter("customerName");
try {
    Statement statement = connection.createStatement( ... );
    ResultSet results = statement.executeQuery( query );
}

...

Primary Defenses

• Option 1: Use of Prepared Statements (with Parameterized Queries)
• Option 2: Use of Properly Constructed Stored Procedures
• Option 3: Allow-list Input Validation
• Option 4: STRONGLY DISCOURAGED: Escaping All User Supplied Input

Defense Option 1: Prepared Statements (with Parameterized Queries)

When developers are taught how to write database queries, they should be told to use prepared statements with variable binding (aka parameterized queries). Prepared statements are simple to write and easier to understand than dynamic queries, and parameterized queries force the developer to define all SQL code first and pass in each parameter to the query later.

If database queries use this coding style, the database will always distinguish between code and data, regardless of what user input is supplied. Also, prepared statements ensure that an attacker cannot change the intent of a query, even if SQL commands are inserted by an attacker.

Safe Java Prepared Statement Example

In the safe Java example below, if an attacker were to enter the userID as tom' or '1'='1, the parameterized query would look for a username that literally matches the entire string tom' or '1'='1. Thus, the database would be protected against injections of malicious SQL code.

The following code example uses a PreparedStatement, Java's implementation of a parameterized query, to execute the same database query.

// This should REALLY be validated too
String custname = request.getParameter("customerName");
// Perform input validation to detect attacks
String query = "SELECT account_balance FROM user_data WHERE user_name = ? ";
PreparedStatement pstmt = connection.prepareStatement( query );
pstmt.setString( 1, custname);
ResultSet results = pstmt.executeQuery( );

Safe C# .NET Prepared Statement Example

In .NET, the creation and execution of the query doesn't change. Just pass the parameters to the query using the Parameters.Add() call as shown below.

String query = "SELECT account_balance FROM user_data WHERE user_name = ?";
try {
  OleDbCommand command = new OleDbCommand(query, connection);
  command.Parameters.Add(new OleDbParameter("customerName", CustomerName Name.Text));
  OleDbDataReader reader = command.ExecuteReader();
  // …
} catch (OleDbException se) {
  // error handling
}

While we have shown examples in Java and .NET, practically all other languages (including Cold Fusion and Classic ASP) support parameterized query interfaces. Even SQL abstraction layers, like the Hibernate Query Language (HQL) with the same type of injection problems (called HQL Injection) support parameterized queries as well:

Hibernate Query Language (HQL) Prepared Statement (Named Parameters) Example

// This is an unsafe HQL statement
Query unsafeHQLQuery = session.createQuery("from Inventory where productID='"+userSuppliedParameter+"'");
// Here is a safe version of the same query using named parameters
Query safeHQLQuery = session.createQuery("from Inventory where productID=:productid");
safeHQLQuery.setParameter