SQL injection is a type of cyber attack that allows attackers to insert malicious code into a web application's SQL statements. This can enable attackers to gain unauthorized access to sensitive data, to manipulate or delete data, or to execute arbitrary commands on the underlying database.
SQL injection attacks are possible because many web applications do not properly validate user-supplied input before using it in SQL statements. This allows attackers to craft input that contains malicious SQL code, which is then executed by the database.
To illustrate how SQL injection works, let's consider the following example in Java:
String username = request.getParameter("username"); String password = request.getParameter("password"); String query = "SELECT * FROM users WHERE username='" + username + "' AND password='" + password + "'"; Statement stmt = connection.createStatement(); ResultSet rs = stmt.executeQuery(query);
This code is intended to authenticate a user by checking their username and password against the values stored in a users table in the database. However, this code is vulnerable to SQL injection because it does not properly validate the username and password parameters before using them in the SQL query.
An attacker could exploit this vulnerability by crafting a username or password value that contains malicious SQL code. For example, the attacker could enter the following value for the username parameter:
' OR '1'='1
This value would be concatenated into the SQL query as follows:
The resulting query would always evaluate to true, effectively bypassing the authentication check and allowing the attacker to gain unauthorized access to the application.
To prevent SQL injection attacks, it is important to properly validate user-supplied input before using it in SQL statements. This can be done in a number of ways, such as using prepared statements with parameterized queries, or using input validation routines to strip out any potentially malicious characters.
For example, the vulnerable code from the previous example could be rewritten as follows to prevent SQL injection:
String username = request.getParameter("username"); String password = request.getParameter("password"); String query = "SELECT * FROM users WHERE username=? AND password=?"; PreparedStatement pstmt = connection.prepareStatement(query); pstmt.setString(1, username); pstmt.setString(2, password); ResultSet rs = pstmt.executeQuery();
In this version of the code, the username and password parameters are passed to the PreparedStatement as placeholder values (represented by the ? characters), instead of being directly concatenated into the SQL query. This prevents an attacker from injecting malicious code into the query, because the placeholder values are treated as literal strings by the database.
By using prepared statements and parameterized queries, web applications can protect against SQL injection attacks and maintain the security and integrity of their data.
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