The following code is vulnerable to SQL injection because the user input is concatenated directly into the query:

String query = "SELECT * FROM products WHERE category = '"+ input + "'";

Statement statement = connection.createStatement();

ResultSet resultSet = statement.executeQuery(query);

This code can be easily rewritten in a way that prevents the user input from interfering with the query structure:

PreparedStatement statement = connection.prepareStatement("SELECT * FROM products WHERE category = ?");

statement.setString(1, input);

ResultSet resultSet = statement.executeQuery();

Parameterized queries can be used for any situation where untrusted input appears as data within the query, including the WHERE clause and values in an INSERT or UPDATE statement. They can't be used to handle untrusted input in other parts of the query, such as table or column names, or the ORDER BY clause. Application functionality that places untrusted data into those parts of the query will need to take a different approach, such as white-listing permitted input values, or using different logic to deliver the required behavior.

For a parameterized query to be effective in preventing SQL injection, the string that is used in the query must always be a hard-coded constant, and must never contain any variable data from any origin. Do not be tempted to decide case-by-case whether an item of data is trusted, and continue using string concatenation within the query for cases that are considered safe. It is all too easy to make mistakes about the possible origin of data, or for changes in other code to violate assumptions about what data is tainted.

https://insecure-website.com/products?category=Gifts

This causes the application to make an SQL query to retrieve details of the relevant products from the database:

SELECT * FROM products WHERE category = 'Gifts' AND released = 1

This SQL query asks the database to return:

• all details (*)
• from the products table
• where the category is Gifts
• and released is 1.

The restriction released = 1 is being used to hide products that are not released. For unreleased products, presumably released = 0.

The application doesn't implement any defenses against SQL injection attacks, so an attacker can construct an attack like:

https://insecure-website.com/products?category=Gifts'--

This results in the SQL query:

SELECT * FROM products WHERE category = 'Gifts'--' AND released = 1

The key thing here is that the double-dash sequence -- is a comment indicator in SQL, and means that the rest of the query is interpreted as a comment. This effectively removes the remainder of the query, so it no longer includes AND released = 1. This means that all products are displayed, including unreleased products.

Going further, an attacker can cause the application to display all the products in any category, including categories that they don't know about:

https://insecure-website.com/products?category=Gifts'+OR+1=1--

This results in the SQL query:

SELECT * FROM products WHERE category = 'Gifts' OR 1=1--' AND released = 1

The modified query will return all items where either the category is Gifts, or 1 is equal to 1. Since 1=1 is always true, the query will return all items.

Se pensate che una cosa simile possa accadere solo al cinema, vi ricrederete: questa scena viene girata ogni giorno decine, se non centinaia di volte, a causa di una delle tecniche di hacking del web più diffuse: la SQL injection. L’introduzione romanzata non esagera riguardo alla semplicità con cui un aggressore può oltrepassare i login messi a protezione delle nostre applicazioni web: è davvero questione di secondi.