Explain the mechanics of SQL injection attacks and how they can be used to compromise databases.

SQL injection is a type of web application vulnerability that allows attackers to interfere with the queries that an application makes to its database. It exploits the fact that many web applications construct SQL queries by concatenating user-supplied data with SQL commands. When input validation is absent or insufficient, attackers can insert malicious SQL code into the application's input fields, allowing them to execute unauthorized database commands. This can lead to a variety of serious security breaches, including data theft, data modification, denial-of-service, and even complete control of the database server.

The fundamental mechanic of SQL injection involves manipulating the structure of a SQL query by injecting malicious code within the input parameters. Consider a simple scenario where a web application accepts a username input and then constructs a SQL query like: "SELECT FROM users WHERE username = '" + user_input + "'"; If the application does not properly validate the user input, an attacker can insert malicious SQL code within the input field. For example, instead of entering a valid username, an attacker might enter the string "admin' OR '1'='1". The resulting SQL query would become: "SELECT FROM users WHERE username = 'admin' OR '1'='1'". Since '1'='1' is always true, this query effectively bypasses the username check and returns all the rows in the users table. This would give the attacker access to every username and password stored in the database.

Another example of a simple SQL injection could involve an attacker trying to get access to data from another table by using a query like "SELECT FROM products WHERE productID = 1". Here, an attacker could inject data such as "1 UNION SELECT username, password FROM users". The resulting SQL query, if not properly handled, might then retrieve all user usernames and passwords along with the existing results for the product table. The results of this query might then be displayed on the web page allowing the attacker to view the data.

SQL injection attacks can be broadly classified into several types based on the attack's intent. One common type is error-based injection where an attacker attempts to force the database to display error messages by providing invalid SQL commands. The attacker then analyzes these error messages to understand the database structure, table names, and column names, which helps to refine the attack. Another type is union-based injection, where attackers use the UNION operator to combine the results of a legitimate query with a query of their choosing. Using the earlier example, attackers might try to retrieve additional table data along with the usual output by using the UNION operator. Blind SQL injection is a more advanced technique where the database does not reveal any errors or output data directly. In this case, the attacker relies on observing the server's behavior to infer information. For example, an attacker might construct SQL queries that cause different delays in the server response based on boolean conditions, using this technique they can extract information about the database and its structure without directly seeing the data. Another technique might involve injecting time-based statements, such as `WAITFOR DELAY '0:0:10'`, and then observing how long the server takes to respond to this injected statement. If the injected statement is successful, the delay is present, which indicates the success of the attack.

By successfully exploiting these types of SQL injection vulnerabilities, attackers can gain unauthorized access to sensitive information stored in the database, such as user credentials, personal data, or financial information. They can also modify or delete existing data, leading to significant business disruption. Attackers may even use SQL injection to gain full administrative control of the database server, allowing them to execute arbitrary commands on the system and even to launch further attacks. For example, attackers might be able to modify user roles and permissions to grant themselves administrative access to the application or even to access underlying system files, depending on the privileges of the database user that is being used by the application. The attacker could also execute commands on the operating system of the database server using specific SQL functions or stored procedures, which can be configured in certain database systems. This can include installing malware, creating backdoors, or destroying sensitive files and data.

In summary, SQL injection attacks involve the injection of malicious SQL code into application inputs to manipulate database queries. The attacks can lead to significant breaches, including data theft, modification, denial of service, and full control over the database server. It’s critical to implement proper input validation, use parameterized queries, and adopt strong database access controls to prevent SQL injection vulnerabilities.