What is SQL injection in PHP, how to prevent SQL injection, and what will happen if you don't prevent it (detailed explanation of SQL injection)

Table of contents

What happens if you don't prevent SQL injection

What is SQL injection

SQL injection is a common security vulnerability that allows an attacker to perform unauthorized database operations by injecting malicious SQL code in the input of the application. When an application does not properly validate and filter user input, an attacker could exploit this vulnerability to perform unexpected database queries, modify or delete operations, or even obtain sensitive data.

SQL injection typically occurs in applications that use dynamically constructed SQL queries, such as when user-supplied data is used to construct query statements. Attackers can perform malicious operations by interfering with the application's query logic by inserting special SQL characters or statements in user input.

Below is a simple example illustrating how SQL injection works. Suppose there is a login form where the user authenticates by entering a username and password:

$username = $_POST['username'];
$password = $_POST['password'];

$query = "SELECT * FROM users WHERE username='$username' AND password='$password'";

The above code directly splices the user input into the SQL query statement. If the input is not validated and filtered, the attacker can bypass authentication by entering special characters:

Username: ' OR 1=1 --
Password: any password

In this case, since the input is inserted directly into the query statement, the query becomes:

SELECT * FROM users WHERE username='' OR 1=1 --' AND password='任意密码'

Since 1=1this is always true, an attacker will successfully bypass authentication and log into an account even if they do not provide a valid username and password.

SQL injection attacks can lead to serious security issues, including data leakage, data destruction, and privilege escalation attacks. Therefore, it is very important to write safe SQL queries and to validate and filter user input. So how to prevent it, here is an example

How to prevent SQL injection

In PHP, there are several common methods that can help prevent SQL injection attacks:

Use Prepared Statements: A prepared statement is a pre-prepared template before executing a SQL query. It uses placeholders (such as question marks or named placeholders) to represent parameters in the query, and then separates the parameter values from the query. This prevents malicious users from corrupting queries by injecting malicious SQL code. Both PDO (PHP Data Objects) and mysqli extensions in PHP support prepared statements.

Sample code using prepared statements is as follows:
```
$stmt = $pdo->prepare("SELECT * FROM users WHERE username = ?");
$stmt->execute([$username]);
```
Use parameterized queries (Parameterized Queries): Parameterized queries are a method of separating query parameters from query statements. It does this by binding parameters to placeholders in the query. Parameterized queries prevent SQL injection attacks by preventing situations where user input is interpreted as SQL code.

The sample code using parameterized query is as follows:
```
$stmt = $pdo->prepare("SELECT * FROM users WHERE username = :username");
$stmt->bindParam(':username', $username);
$stmt->execute();
```
Input Validation and Filtering: Validating and filtering user input before receiving it is an important security measure. Input can be validated using filter functions provided by PHP such as filter_var(), to check that it conforms to the expected format and type. You can also use mysqli_real_escape_string()functions or PDO's predefined parameter types (such as PDO::PARAM_STR) to filter the input to ensure that special characters in the input will not be interpreted as SQL codes.

Sample code for input validation and filtering is as follows:
```
$username = filter_var($_POST['username'], FILTER_SANITIZE_STRING);
$username = $pdo->quote($_POST['username']);
```
Principle of Least Privilege: Assigning the least amount of privileges to database users is an effective security practice. Make sure database users only have privileges to perform necessary operations to reduce the scope of a potential attack.
Error message handling: In a production environment, do not display verbose error messages to users, as they may contain sensitive database information. Error messages can be logged to a log file for developer troubleshooting.

These methods are not absolute security measures, so when writing secure SQL queries, you still need to be vigilant and consider other security measures, such as input verification, authentication, and access control, and of course the most basic prevention of SQL injection The method is our commonly used token token. Finally, let me explain to you what happens if you don't prevent it.

What happens if you don't prevent SQL injection

If the database is not protected against SQL injection, it can cause the following problems:

Data Leakage: SQL injection attacks can lead to the leakage of sensitive data in the database. Attackers can bypass authentication by injecting malicious SQL code, access and obtain data in the database, including user passwords, personal information, and other sensitive data.
Data Destruction: SQL injection attacks can also cause data in the database to be destroyed or tampered with. Attackers can modify, delete or destroy data in the database by injecting malicious SQL code, thereby compromising the integrity and availability of the application.
Privilege escalation attack: Through SQL injection, an attacker can use the privileges of the database to perform malicious operations. If the database user has high privileges, the attacker can use the injected SQL code to perform dangerous operations, such as creating new users, deleting data tables, or executing system commands.
Denial of Service (DoS) Attacks: Malicious SQL injection attacks can overload database servers, rendering applications inoperable. An attacker can inject a large amount of malicious SQL code, causing the database server to run out of resources and unable to process normal requests.

To sum up, failure to prevent SQL injection may lead to security risks such as data leakage, data destruction, privilege escalation attacks, and denial of service attacks. Therefore, preventing SQL injection is a very important security measure for any application involving a database. By using prepared statements, parameterized queries, input validation and filtering, etc., you can effectively prevent SQL injection attacks and improve the security of applications.