Introduction

SQL injection is a code injection technique that allows an attacker to execute malicious SQL statements on a database. SQL injection can compromise the security, integrity, and availability of the data stored in the database and the application that interacts with it.

Blind SQL injection is a type of SQL injection attack that occurs when the application does not return any data or error messages from the database to the attacker. This makes it difficult for the attacker to see the results of the injected SQL query or exploit the vulnerability directly.

However, blind SQL injection is still possible and dangerous, as the attacker can infer information from the database by observing changes in the behavior or output of the application. For example, the attacker can measure the time difference between sending a request and receiving a response or trigger different responses conditionally, depending on an injected condition.

There are two main types of blind SQL injection attacks: content-based and time-based. Content-based blind SQL injection relies on the application displaying different content depending on whether the injected condition is true or false. Time-based blind SQL injection relies on the database pausing for a specified time before returning results, indicating successful execution of the injected query.

Some examples of blind SQL injection attacks are:

• Injecting is a condition that causes the application to display an error message or a blank page if it is true and normal content if it is false.
• Injecting a condition that causes the database to sleep for a certain number of seconds if it is true and returns results immediately if it is false.
• Injecting a condition that causes the database to perform a DNS lookup or an HTTP request to an external server controlled by the attacker if it is true and do nothing if it is false.

The impact of blind SQL injection attacks can be severe, as the attacker can retrieve sensitive data from the database, such as user credentials, personal information, financial records, etc. The attacker can also manipulate data in the database, such as inserting, updating, deleting, or dropping records or tables. Sometimes, the attacker can execute arbitrary commands on the server or create a reverse shell.

To prevent blind SQL injection attacks, following secure coding practices and implementing effective countermeasures is essential. Some best practices are input validation, prepared statements, allowlisting, and regular security testing.

How to detect blind SQL injection attacks

Detecting blind SQL injection attacks can be challenging, as they do not produce any visible errors or output from the database. However, there are some common symptoms and indicators that can help to identify blind SQL injection vulnerabilities in web applications.

Common symptoms of blind SQL injection attacks

Some common symptoms of blind SQL injection attacks are:

• The application takes longer than usual to respond to certain requests.
• The application displays different content or behavior depending on certain input values.
• The application generates unexpected network traffic or requests to external servers.
• The application logs show anomalous or malicious SQL queries or errors.

Tools and techniques for detecting blind SQL injection attacks

Various tools and techniques can help to detect blind SQL injection attacks in web applications. Some of them are:

• Manual testing involves using a web browser or a proxy tool to intercept and modify HTTP requests and responses and observe how the application reacts to different input values. Manual testing can help to discover subtle changes in content or timing that indicate a blind SQL injection vulnerability. However, manual testing can be time-consuming and tedious, especially for complex or large applications.
• Automated scanning: Automated scanning involves using a software tool that crawls and analyzes web applications for vulnerabilities, including blind SQL injection. Automated scanning can help to identify potential blind SQL injection vulnerabilities quickly and efficiently by generating and sending various payloads and measuring their effects. However, automated scanning can generate false positives or negatives and may not cover all possible scenarios or parameters.
• Static analysis: Static analysis involves examining the source code of the web application for vulnerabilities, including blind SQL injection. Static analysis can help to detect insecure coding practices or flaws in logic that may lead to blind SQL injection vulnerabilities. However, static analysis can also be difficult or impractical, especially for large or obfuscated code bases.

How to exploit blind SQL injection attacks

Exploiting blind SQL injection attacks can be difficult, requiring more time and effort than normal SQL injection attacks. However, some general steps and strategies can help exploit blind SQL injection vulnerabilities in web applications.

Building a blind SQL injection attack chain

To exploit a blind SQL injection vulnerability, an attacker needs to build an attack chain that consists of three main components:

• A vulnerable parameter: A vulnerable parameter is an input field or value that is directly concatenated or interpolated into a SQL query without proper sanitization or validation. This allows an attacker to inject arbitrary SQL commands into the query.
• A testable condition: A testable condition is an expression that evaluates to either true or false depending on an injected value or subquery. This allows an attacker to ask the database yes or no questions and determine the answer based on the application’s response.
• A feedback mechanism: A feedback mechanism is a way of observing the result of the injected condition. This can be either content-based or time-based, depending on how the application or the database behaves differently when the condition is true or false.

For example, suppose an application has a web page that displays the details of a product based on its ID. The URL of the web page is:

https://example.com/product.php?id=1

The application uses the following SQL query to fetch the product details from the database:

SELECT name, price, description FROM products WHERE id = 1

An attacker can exploit this blind SQL injection vulnerability by using the following attack chain:

• A vulnerable parameter: The id parameter is vulnerable to SQL injection, as it is directly inserted into the SQL query without any sanitization or validation.
• A testable condition: The attacker can inject a condition that compares the first character of the name of the product with a given letter, such as:

https://example.com/product.php?id=1 AND SUBSTRING(name, 1, 1) = 'A'

This condition will return true if the first character of the product’s name is A and false otherwise.

• A feedback mechanism: The attacker can use a content-based feedback mechanism, as the application displays a “Product not found” message if the query does not return any results and normal content if it does. The attacker can then infer whether the injected condition is true or false by observing the web page’s content.

Extracting data from a database using blind SQL injection

To extract data from a database using blind SQL injection, an attacker needs to use a technique called boolean-based blind SQL injection. This technique uses a series of testable conditions to enumerate each data piece one bit or character at a time.

For example, suppose there is a table called Users with the columns Username and Password and a user called Administrator. The attacker can extract the password for this user by using the following steps:

• Determine the length of the password: The attacker can use a condition that compares the length of the password with a given number, such as:

https://example.com/product.php?id=1 AND LENGTH( (SELECT Password FROM Users WHERE Username = 'Administrator')) = 8

This condition will return true if the password length is 8 and false otherwise. The attacker can then use a binary search algorithm to find the exact length of the password in a few iterations.

• Determine each character of the password: The attacker can use a condition that compares each character of the password with a given letter, such as:

https://example.com/product.php?id=1 AND SUBSTRING( (SELECT Password FROM Users WHERE Username = 'Administrator'), 1, 1) = 'A'

This condition will return true if the first character of the password is A and false otherwise. The attacker can then use a binary search algorithm to find each character of the password in a few iterations.

The attacker can eventually extract the entire password from the database by repeating these steps for each password character.

How to defend against blind SQL injection attacks

Defending against blind SQL injection attacks requires implementing various security measures and best practices at different levels of the web application and its environment. Some of them are:

Secure coding practices

Secure coding practices involve following guidelines and standards that help to prevent or mitigate common vulnerabilities, including blind SQL injection. Some examples of secure coding practices are:

• Avoiding dynamic SQL queries: Dynamic SQL queries are constructed by concatenating or interpolating user input values into an SQL query string. Dynamic SQL queries are prone to blind SQL injection attacks, allowing attackers to inject arbitrary SQL commands into the query. Instead of using dynamic SQL queries, it is recommended to use prepared statements or stored procedures, which separate the query structure from the user input values and prevent SQL injection attacks.

• Escaping or encoding user input: Escaping or encoding user input is a technique that transforms the user input into a different format or representation that is safe and compatible with the SQL query. For example, escaping or encoding special characters, such as single quotes (‘), double quotes (“), backticks (`), semicolons (;), etc., into their corresponding HTML entities, such as'”, `;, etc. Escaping or encoding user input can help prevent blind SQL injection attacks by preventing the user input from breaking out of the query context and injecting SQL commands. However, escaping or encoding user input is not a foolproof solution, as it can be bypassed by an attacker who can craft complex or obfuscated payloads that can evade the escaping or encoding rules. Therefore, escaping or encoding user input should be used with other techniques, such as prepared statements and allowlisting.
• Validating user input: Validating user input is checking and filtering the user input before passing it to the SQL query. Validating user input can help prevent blind SQL injection attacks by ensuring that the input conforms to the expected format and type of the parameter. User input can be validated on the application’s client and server sides. Some examples of validating user input techniques are:
  1. Allowlisting: Allowlisting is a technique that allows only specific values or characters to be accepted as valid input. For example, suppose an input parameter is expected to be an integer number between 1 and 10. In that case, the validating user input can reject any input that is not a number or outside the range.
  2. Blocklisting: Blocklisting is a technique that blocks specific values or characters known as malicious or dangerous from being accepted as valid input. For example, suppose an input parameter is expected to be a string. In that case, the validating user input can reject any input that contains SQL keywords or operators, such as SELECT, UNION, OR, AND, etc.

Database security

Database security involves applying various security measures and best practices to protect the database from unauthorized access or manipulation. Database security can help to prevent blind SQL injection attacks by limiting the attack surface and reducing the impact of a successful attack. Some examples of database security are:

• Using strong passwords: Using strong passwords is a basic but essential security measure that helps to prevent unauthorized access to the database. Strong passwords are long, complex, random, and unique. They should not contain personal or predictable information, such as names, dates, words, etc. They should also be changed regularly and stored securely.
• Restricting privileges: Restricting privileges is a security measure that involves granting only the minimum necessary permissions and roles to the database users and applications. Restricting privileges can help to prevent blind SQL injection attacks by limiting what an attacker can do with the database if they manage to inject SQL commands. For example, restricting privileges can prevent an attacker from dropping tables, executing commands, accessing sensitive data, etc.
• Encrypting data: Encrypting data is a security measure that involves transforming the data stored in the database into an unreadable format using a secret key or algorithm. Encrypting data can help prevent blind SQL injection attacks by protecting the confidentiality and integrity of the data in case of a data breach. For example, encrypting data can prevent an attacker from reading or modifying sensitive data, such as passwords, credit card numbers, personal information, etc.

WAFs and other security controls

WAFs (Web Application Firewalls) and other security controls are software or hardware devices that monitor and filter the incoming and outgoing traffic between the web application and its environment. WAFs and other security controls can help to prevent blind SQL injection attacks by detecting and blocking malicious requests or responses that contain SQL injection payloads or indicators. Some examples of WAFs and other security controls are:

• ModSecurity: ModSecurity is an open-source WAF that provides various features and modules for web application security, such as real-time monitoring, logging, auditing, access control, anomaly detection, and attack prevention. ModSecurity can be integrated with web servers like Apache, Nginx, and IIS. ModSecurity can be configured with various rules and policies to detect and block SQL injection attacks, such as the OWASP Core Rule Set (CRS).

• Cloudflare: Cloudflare is a cloud-based service that provides various features and solutions for web performance and security, such as CDN, DNS, SSL, DDoS protection, firewall, and more. Cloudflare can protect web applications from SQL injection attacks by filtering and blocking malicious requests at the network edge before they reach the origin server. Cloudflare can also provide analytics and insights on web traffic and threats.
• Snort: Snort is an open-source network intrusion detection and prevention system (NIDS/NIPS) that monitors and analyzes network traffic for signs of malicious activity, such as SQL injection attacks. Snort can be deployed on various platforms, such as Windows, Linux, and Mac OS X. Snort can be configured with various rules and signatures to detect and block SQL injection attacks, such as the Emerging Threats (ET) ruleset.

Conclusion

Blind SQL injection is a code injection technique that allows an attacker to execute malicious SQL statements on a database without receiving any direct feedback from the database. Blind SQL injection can compromise the security, integrity, and availability of the data stored in the database and the application that interacts with it.

Blind SQL injection occurs when the application does not return any data or error messages from the database to the attacker. There are two main types of blind SQL injection attacks: content-based and time-based. Content-based blind SQL injection relies on the application displaying different content depending on whether the injected condition is true or false. Time-based blind SQL injection relies on the database pausing for a specified time before returning results, indicating successful execution of the injected query.

To detect blind SQL injection attacks, it is important to look for common symptoms and indicators that suggest a blind SQL injection vulnerability in the web application. Some common symptoms are delayed responses, different content or behavior, unexpected network traffic or requests, and anomalous or malicious SQL queries or errors. To detect blind SQL injection attacks, it is also useful to use various tools and techniques that can help to find and exploit blind SQL injection vulnerabilities in web applications. Some are manual testing, automated scanning, static analysis, etc.

To prevent blind SQL injection attacks, following secure coding practices and implementing effective countermeasures at different levels of the web application and its environment is essential. Some are input validation, prepared statements, allowlisting, database security, WAFs, and other security controls.