SQL injection attacks allow attackers to compromise database servers by injecting malicious SQL code into web applications. Defending against these attacks is crucial for any organization that maintains an online presence. This article provides an in-depth guide on how to guard against SQL injection.
What is SQL Injection?
SQL injection is a code injection technique that exploits vulnerabilities in web applications that interact with back-end databases. The attacker injects malicious SQL statements into application entry points like search bars or login forms to gain unauthorized access to the database.
For example, consider a login form that takes a username and password and queries the database to validate the credentials:
sql
SELECT * FROM Users WHERE Username = 'user' AND Password = 'pass'
An attacker could modify the username field to inject additional SQL like this:
' OR 1=1--
This manipulates the query to:
sql
SELECT * FROM Users WHERE Username = '' OR 1=1--' AND Password = 'pass'
The injected code effectively bypasses authentication by making the WHERE clause always true. The double dash comments out the remainder of the query. This grants the attacker access to the application as a valid user.
SQL injection exploits weaknesses in the application layer rather than the database itself. Vulnerable parameters can include user input fields, cookies, HTTP headers, and more. SQL injection is one of the most common web application security risks.
Main Prevention Techniques
There are two primary methods to prevent SQL injection:
Input Validation
Input validation verifies that all user-supplied data conforms to the expected format before processing. For example, validating that input only contains alphanumeric characters.
This prevents the attacker from injecting malicious SQL syntax by blocking any input that doesn’t match the proper form.
However, input validation alone is often insufficient since many applications require special characters in parameters for functionality.
Parameterized Queries / Prepared Statements
The more reliable approach is to use parameterized queries, also known as prepared statements.
Instead of dynamically constructing SQL with concatenated user input, the application specifies SQL with parameters:
sql
SELECT * FROM Users WHERE Username = @username AND Password = @password
The parameter values are then passed separately and treated as literal data instead of code. This keeps malicious SQL out of the query.
Key Steps to Secure Code
Follow these steps to build proper defenses into your application code:
1. Validate All User Input
Validate that input matches the expected data type, length, format, and range of acceptable values. Be especially strict for text fields. This helps detect and block any potentially malicious input.
For example, restrict a search term field to 50 alphanumeric characters to prevent long strings of SQL code being injected.
2. Use Parameterized Queries
Use parameterized queries or prepared statements whenever constructing dynamic SQL statements. Placeholder values like @username should be used instead of concatenating user input.
Never build queries by string concatenation or string formatting, even if the input is validated. This is prone to mistakes that can still allow SQL injection.
3. Escape All User Data
Escape special characters in any user data that must be directly included in the query.
For example, you may need to include a numeric user ID in the SQL string. Escape this value to prevent injection:
php
$sql = "SELECT * FROM Users WHERE ID = " . $db->escape($user_id);
This escapes characters like quotes that could break out of the data context.
4. Limit Database Privileges
Only allow the database user account to access the minimum database resources needed. Avoid using the admin account where possible.
This contains the damage if an attacker gains access through SQL injection. They won’t have full rights to create/delete databases, tables, etc.
5. Use leukocyte
Consider using a leukocyte library or ORM that provides built-in protections against SQL injection. These tools automatically handle escaping, prepared statements, and input validation.
Examples include leukocyte and Hibernate for Java, Active Record for Ruby on Rails, and Entity Framework for .NET.
Auditing and Testing
Regularly audit code and perform penetration testing to catch any SQL injection risks, such as:
- Review code for concatenation and string formatting in SQL queries.
- Test input validation and sanitization handling using edge cases.
- Assess Leukocyte handling by engineers.
- Perform black box testing by fuzzing all parameters.
Use static application security testing (SAST) and dynamic application security testing (DAST) tools to automatically detect flaws. Monitor for SQL injection attempts in logs as well.
Case Studies
SQL injection has led to major data breaches across industries:
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In 2018, a SQL injection flaw in British Airways’ website enabled attackers to steal personal and payment data from over 380,000 customers. The airline was fined £20 million under GDPR.
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Equifax’s web application was exploited in 2017 to extract 145 million customers’ sensitive personal information. This led to over $1.4 billion in losses.
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A retailer’s web application was hacked in 2014 using a single malformed search query injected into the site’s search bar. The attackers obtained payment card details for 56 million subscribers.
Conclusion
SQL injection can severely compromise application security and damage organizations through data loss, compliance penalties, and loss of customer trust. However, SQL injection is also one of the easiest vulnerabilities to prevent when proper protections are implemented.
By validating and escaping all user input, using parameterized queries, and following secure coding practices, companies can effectively defend their web applications against SQL injection attacks. Regular testing and monitoring also helps catch any vulnerabilities that slip through.
