SQL injection (SQLi) is a security flaw in which untrusted input changes the structure or meaning of a database query. It usually happens when an application builds SQL by joining query text with user-supplied values instead of keeping the command and its data separate. A database may then interpret part of the input as SQL code.
How SQL injection works
Applications often use SQL to retrieve or change data. In a safe design, the application defines the query and supplies user input separately as a value. In an unsafe design, it inserts input directly into SQL text. If that input is interpreted as SQL syntax, it can alter what the query does.
A simplified example
Imagine a sign-in or account lookup that constructs a query like this:
SELECT account_balance FROM user_data WHERE user_name = 'submitted name'If an application forms that statement by concatenating the submitted name into the text, SQL syntax in the input could escape the intended value and change the condition. OWASP illustrates how an injected condition can turn a lookup into a query that returns all account records. This is a toy illustration of the vulnerability, not a procedure for testing a real system.
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The underlying failure is a breakdown in the boundary between data and code: the database receives one statement in which user-controlled content can be parsed as part of the command. The same risk can occur in any feature that builds SQL dynamically, not just a login form. OWASP’s SQL Injection Prevention Cheat Sheet and its Top 10:2025 injection overview describe this class of flaw.
What an SQL injection flaw can allow
Depending on the query, database, and permissions of the application’s database account, an altered query may expose or change records, or perform other unintended database actions. SQL injection does not automatically mean an attacker can control the entire database server or operating system. More serious outcomes depend on the database system’s features and configuration, the vulnerable code path, and the account’s privileges.
Injection can also be categorized by how results are observed: in-band results return through the same channel, out-of-band results arrive through another channel, and inferential or blind cases involve drawing conclusions from application or database behavior rather than seeing returned data directly. Consequently, a page that displays no database results does not, by itself, establish that it is safe.
How to prevent SQL injection
Use parameterized queries
The primary defense is to define SQL structure first and bind user-supplied values separately with a prepared statement or parameterized query. In OWASP’s Java example, the query uses a ? placeholder for the value, then binds the supplied customer name with pstmt.setString(1, custname). The database treats the bound value as data rather than letting it rewrite the SQL command. OWASP summarizes the protection this way: “If database queries use this coding style, the database will always distinguish between code and data, regardless of what user input is supplied.” This applies when parameter binding is used as intended.
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Handle dynamic table, column, and sort choices separately
Ordinary bound parameters represent values; they generally cannot stand in for SQL structure such as a table name, column name, or sort direction. If users can choose among such options, map their selection in application code to a fixed, legal choice. Do not splice arbitrary identifiers or ordering text into a query.
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Use stored procedures only when safely implemented
A stored procedure can provide protection when it keeps SQL structure separate from input values. It is not automatically safe: a procedure that assembles and executes dynamic SQL from untrusted input can reintroduce the same vulnerability.
Use validation and least privilege as supporting controls
- Validate with allow-lists where appropriate. Server-side checks can restrict input to expected values, especially for constrained choices, but validation alone does not make unsafe string concatenation safe.
- Limit database-account permissions. Give the application’s account only the tables and operations it needs; do not run it with database-administrator privileges. This can limit damage if a flaw remains, but it does not fix unsafe query construction.
- Do not rely on escaping as the main defense. Escaping is database-specific and fragile compared with keeping code and data separate through parameter binding.
These defenses and their caveats are covered in the OWASP SQL Injection Prevention Cheat Sheet.
How to assess an application responsibly
Code review can identify query construction that mixes untrusted input with SQL text. Automated testing can complement review; OWASP Top 10:2025 points to static, dynamic, and interactive application security testing (SAST, DAST, and IAST) as useful tools in a CI/CD process. Test only systems you own or have explicit authorization to assess.
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