SQL injection is the most common web vulnerability and still in the OWASP Top 10. This guide explains how it works, how to find vulnerable code, and how to fix it permanently.
In this guide
A SQL injection attack inserts malicious SQL into an input field. Example of vulnerable code: const query = `SELECT * FROM users WHERE email = '${req.body.email}'`. If the attacker enters: ' OR '1'='1 — the query becomes: SELECT * FROM users WHERE email = '' OR '1'='1' — which returns all users. Or entering: '; DROP TABLE users; -- — deletes the entire table. Any string interpolation into a SQL query is vulnerable.
Never concatenate user input into SQL strings. Use parameterized queries (also called prepared statements): const result = await db.query('SELECT * FROM users WHERE email = $1', [req.body.email]). The database treats the parameter as a data value, never as SQL syntax — even if it contains quotes or SQL keywords. This single change eliminates SQL injection regardless of what the user inputs. In every ORM and database library, parameterized queries are the default.
ORMs like Prisma, Sequelize, and TypeORM use parameterized queries automatically: const user = await prisma.user.findFirst({ where: { email: req.body.email } }). This is safe regardless of what email contains. The only risk returns when you drop down to raw SQL within an ORM — always use the ORM's raw query method with parameters, never string concatenation: prisma.$queryRaw`SELECT * FROM users WHERE email = ${email}`.
Search your codebase for patterns that suggest string interpolation into SQL: grep -r 'query.*\$\{' src/ and grep -r 'execute.*+' src/. Any query built by concatenating or interpolating variables should be converted to parameterized form. Tools like Semgrep (free, open source) can scan automatically for SQL injection patterns in Node.js, Python, Java, and other languages.
Even with parameterized queries, the database user your app connects with should have only the permissions it needs. An app that only reads and writes user data does not need DROP TABLE or CREATE TABLE permissions. Create a dedicated database user with: GRANT SELECT, INSERT, UPDATE, DELETE ON ALL TABLES IN SCHEMA public TO appuser. If an injection somehow occurs despite parameterized queries, limited permissions reduce the blast radius.
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