Mitigating Fragmented SQL Injection Attacks: Effective Solutions

This blog post breaks down Fragmented SQL Injection, a method hackers use to bypass authentication by manipulating two different input fields at the same time. Our security expert explains why single quotes matter in SQL injection attacks and how using Prepared Statements (also called Parameterized Queries) can effectively prevent these types of exploits.

LEARN MORE: How to prevent SQL Injection

If you ask someone how to check for an SQL injection vulnerability in a web application, their first suggestion might be to enter a single quote (‘) into an input field. If the application responds with an error, it could indicate that the input is interfering with the database query—a classic sign of SQL injection. In fact, some people even refer to SQL injection as “Single Quote Injection” because of how often this method is used to test for vulnerabilities.

However, attackers are not limited to simple single-quote injections. Our research explores Fragmented SQL Injection, a more advanced technique where hackers manipulate two separate input fields within the same query context to bypass authentication systems. Understanding how single quotes affect database queries is essential to recognizing and preventing these types of attacks. Let’s take a closer look.

The Role of Single Quotes in SQL Injection Attacks

In many systems—such as command interpreters, file systems, and databases—certain characters have special meanings. These characters, known as metacharacters, can change how a system processes commands. In SQL, single (‘) and double (“) quotes act as string delimiters, marking the beginning and end of a text-based input.

Because of this, injecting an unescaped single or double quote into a database query can break the query’s structure, often resulting in a syntax error. Consider the following SQL statement:

SELECT * FROM users WHERE username="USER_INPUT'

If an attacker enters a single quote (‘) as input, the database query may become malformed, leading to an error:

$username = "'";

$query = "SELECT * FROM users WHERE username="".$username."""

Resulting SQL Query

SELECT * FROM users WHERE username=""''

Here, the database is unable to process the query because the extra, unmatched single quote disrupts the expected syntax. This type of error is a key indicator that user input is not properly filtered or sanitized, making the system potentially vulnerable to SQL injection attacks.

When Single Quotes Aren’t Needed for SQL Injection

While string-based SQL queries are affected by quote injection, not all database queries rely on string inputs. Consider a scenario where the application queries a database using an integer-based identifier:

$query = "SELECT * FROM users WHERE id=" . $user_input;

In this case, single or double quotes are unnecessary. Instead, an attacker would need to inject a numeric value that modifies the SQL statement to execute unintended commands.

Blacklisting or Escaping Single Quotes

To defend against simple SQL injection attempts, some systems escape or blacklist single quotes, preventing them from breaking the query. However, this method is not foolproof.

For example, if a hacker attempts to inject the following payload:

$username = "' or 1=1 --";

$password = "qwerty123456";

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

The resulting SQL query would be:

SELECT * FROM users WHERE username="\" or 1=1 -- ' or password='qwerty123456';

Since the single quote (‘) is escaped with a backslash (\), the injection attempt fails, as the query no longer executes the intended malicious logic.

While escaping single quotes can reduce the risk of basic SQL injection attacks, it is not a complete solution. The most effective way to prevent SQL injection is by using Prepared Statements (Parameterized Queries), which separate user input from SQL commands entirely, ensuring that injected values cannot alter the intended logic of the query.

Understanding Fragmented SQL Injection

Fragmented SQL Injection is an attack technique where multiple input fields are manipulated together to bypass authentication or other security controls. While not originally named by its discoverer, this method allows attackers to split their malicious payloads across different input fields to evade detection mechanisms such as blacklists and character limits.

How Fragmented SQL Injection Works

In a typical SQL injection attack, a hacker might insert a single quote (‘) to break the query structure. However, some systems automatically escape special characters using a backslash (\), preventing direct injection. Fragmented SQL injection gets around this by splitting the payload between two input fields that are processed within the same SQL query context.

Consider the following authentication attempt:

Input Fields:

Username: \

Password: or 1 #

Resulting Query:

SELECT * FROM users WHERE username="\" and password=' or 1 # ';

Why This Works

• The backslash (\) entered in the username field escapes the next single quote (‘), neutralizing it.

• The password field then contains the payload: or 1 #, which modifies the logic of the SQL statement.

• Since or 1 is always true, the query successfully authenticates the attacker without needing a valid password.

• The # (hash) character acts as a comment marker, telling the database to ignore the rest of the query, effectively bypassing any remaining authentication checks.

The Impact

By leveraging this technique, an attacker can bypass login forms and authentication mechanisms, potentially gaining unauthorized access to user accounts or administrative controls. Traditional input validation and blacklists may fail to detect this attack since each input field alone appears harmless—but when processed together, they form a complete SQL injection payload.

Preventing Fragmented SQL Injection

To protect against this technique, applications should implement strong SQL injection defenses, including:

• Prepared Statements (Parameterized Queries) – These ensure user inputs are treated as data, not SQL commands.
• Strict Input Validation – Disallow escape characters and enforce input length constraints.
• Escaping and Encoding – Ensure user input cannot break query logic.
• Limiting Error Messages – Avoid revealing query structure through error responses.

The Limitations of Filtering Functions in SQL Injection Prevention

A referenced blog post suggests using PHP’s htmlentities() function to filter user inputs as a way to prevent SQL injection attacks. When configured with the ENT_QUOTES flag, this function converts special characters—such as single quotes (‘), double quotes (“), and HTML tags—into their corresponding HTML entities. For example, a double quote would be encoded as:

While this method may reduce the risk of injection in some cases, it is not a foolproof solution. SQL injection attacks can still be executed without using single or double quotes, making this approach insufficient as a primary defense mechanism. Additionally, legacy encoding tricks like GBK encoding can sometimes bypass security functions such as addslashes() in PHP, further weakening this type of input filtering.

Why Prepared Statements Are the Best Defense Against SQL Injection

The most effective and reliable way to prevent SQL injection attacks is by using Prepared Statements, also known as Parameterized Queries.

Why Prepared Statements Work:

• They separate SQL query structure from user input, ensuring that user data is treated strictly as a value, not as part of the SQL command.
• Unlike filtering-based approaches, they are resistant to evolving SQL injection techniques.
• They eliminate the need for manual escaping, making the code more secure and less prone to errors.

Many input filtering techniques, including htmlentities(), may offer partial protection, but attackers continue to find ways to bypass them. Relying on these methods alone leaves applications vulnerable to new attack techniques, making Prepared Statements the only consistently reliable approach for preventing SQL injection.

Implementing Parameterized Queries in PHP and .NET

Using Parameterized Queries is the most effective way to protect applications from SQL injection attacks. Below are examples of how to implement this approach in PHP and .NET to ensure secure database queries.

Parameterized Queries in PHP

In PHP, the prepare() method is used to define an SQL statement with placeholders, and bindParam() assigns values securely:

$stmt = $dbh->prepare("UPDATE users SET email=:new_email WHERE id=:user_id");

$stmt->bindParam(':new_email', $email);

$stmt->bindParam(':user_id', $id);

Parameterized Queries in .NET

For .NET applications, parameterized queries are implemented using the SqlCommand class. Instead of inserting raw user input into the SQL statement, parameters are explicitly defined and assigned values:

string sql = "SELECT * FROM Customers WHERE CustomerId = @CustomerId";

SqlCommand command = new SqlCommand(sql);

command.Parameters.Add(new SqlParameter("@CustomerId", System.Data.SqlDbType.Int));

command.Parameters["@CustomerId"].Value = 1;

Why Prepared Statements Are Essential

Many developers still rely on blacklist-based filtering to block SQL injection attempts, either manually or through functions like addslashes(). However, attackers continue to develop new techniques to bypass these defenses, making blacklist-based approaches unreliable.

The only consistently effective way to prevent SQL injection vulnerabilities is by using Prepared Statements (Parameterized Queries). This method ensures that user input is always treated as data rather than part of the SQL command, effectively neutralizing injection attempts at the database level.

THE AUTHOR

Acunetix

Acunetix developers and tech agents regularly contribute to the blog. All the Acunetix developers come with years of experience in the web security sphere.