What is the root cause of SQL injection?

At its core, SQL injection is only possible when unsanitized user input is directly embedded into dynamically constructed SQL queries. This security flaw allows attackers to alter the structure of a query and potentially gain unauthorized access to data or execute other database commands. Instead of treating user input as data, vulnerable applications treat it as executable code—exposing one of the most common and damaging web security issues.

SQL injection example

Before we go into the causes of SQLi vulnerabilities, here’s a basic PHP example of a vulnerable query in the login process. Let’s say you’re taking the username and password from a submitted login form and directly inserting those values into an SQL query string as follows:

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

Typically, an application would send this query to the database, and if a non-empty result set is returned, it means the specified credentials are valid. To get around this, an attacker might submit an SQL injection payload as the username, such as the following SQL code fragment (ending with a comment symbol to bypass the rest of the query after the injection point):

' OR 1=1;--

The resulting SQL query sent to the database by the vulnerable application becomes:

SELECT * FROM users WHERE username = '' OR 1=1;-- AND password = ''

Because 1=1 is always true and the password check is commented out, this query bypasses authentication, potentially granting unauthorized access. Adding some basic validation would make the attacker’s job more difficult, but the proper fix is to use parameterized queries to ensure that user input is treated strictly as data and mixed with executable code.

See the Invicti SQL injection cheat sheet to learn more about SQLi payloads and the many ways to manipulate database queries.

Causes of SQL injection

While the root cause of SQL injection is the unsafe handling of user input in SQL queries, several underlying factors make this vulnerability possible (or more likely). Understanding these contributing causes is essential for identifying weak spots in application design and development practices.

Insufficient input validation and sanitization

When applications fail to rigorously validate or sanitize user inputs, attackers can embed malicious SQL statements in fields meant for benign data. Without input validation, the application can’t distinguish between legitimate data and harmful payloads, allowing dangerous commands to reach the database.

Improper use of dynamic SQL

Applications that build SQL queries using string concatenation directly with user input are highly vulnerable. Constructing a query in this way incorporates raw user input into the query logic, making it easy for attackers to manipulate. Using parameterized queries or prepared statements is a safer alternative that enforces separation between code and data.

Lack of context-aware encoding

Output encoding must match the context in which data is used. For example, HTML encoding is appropriate for web pages, but SQL query inputs should be suitably encoded as part of the parameterization process rather than manually. Misusing or skipping encoding during insecure query construction greatly increases the risk of injection, as the database or application may not properly neutralize malicious input.

Unsecured legacy code

Older applications often rely on hardcoded SQL logic and outdated development practices. These legacy systems may lack input validation, use unsafe database access patterns, include insecure coding constructs, or even use long-outdated database versions with known vulnerabilities. If not reviewed and updated, these legacy apps and databases can become persistent sources of injection risk.

Overly permissive database privileges

When applications connect to the database using accounts with administrative or broad access, attackers may be able to access system and user tables that the app shouldn’t need. In that situation, any successful injection could result in significant damage. Limiting database privileges to only what’s necessary for each operation is a critical control that helps contain the impact of potential exploits. This also includes running the database process itself from a restricted rather than root account. 

Poor error handling

Exposing detailed error messages to users can give attackers insight into the database structure, making it easier to craft successful injection payloads. Proper error handling ensures that internal issues are logged securely while returning only generic error messages to end users. In some cases, attackers can also detect error conditions indirectly based on changes in database reaction time, allowing for time-based blind SQL injection attacks.

Inadequate security testing

SQL injection vulnerabilities are easily missed without regular security testing. Static analysis is the first step, but code checking alone cannot predict how user input flows into live queries. Any gaps in dynamic security testing, both automated and manual, greatly increase the risk of undetected vulnerabilities making it into production environments.

Preventing SQL injection attacks with a DAST-first approach to application security

The most effective way to prevent SQL injection is to combine secure coding practices with thorough testing of the running application. This is where a dynamic application security testing (DAST) solution plays a critical role.

A DAST-first approach, as championed by Invicti, provides visibility into real, exploitable risks by scanning live applications under realistic conditions. This helps security teams prioritize vulnerabilities based on actual impact rather than theoretical flaws. With automated confirmation in the form of Invicti’s proof-based scanning, confirmed vulnerabilities are automatically validated, reducing false positives and streamlining remediation.

By integrating DAST into the development workflow, organizations can continuously identify and fix SQL injection issues before attackers have a chance to exploit them. This is a practical and scalable strategy for addressing one of the web’s oldest—and still most dangerous—application security threats.