Input validation failures are at the root of many serious vulnerabilities, including injection attacks. APIs that accept unvalidated or poorly sanitised input can be tricked into executing unintended commands or queries. Testing input handling systematically is essential for preventing issues like SQL injection, command injection, and deserialization flaws.
Input Validation and Injection Vectors
APIs accept input through query parameters, path variables, headers, and request bodies. Attackers can embed payloads that target databases, operating systems, or other back-end components. Good validation enforces type, format, length, and allowed characters, while the application uses parameterised queries and safe APIs for back-end operations.
# Example of a suspicious input payload
curl -X GET "https://api.example.com/v1/customers?search=' OR '1'='1"Validation tests should cover missing fields, wrong types, overly long values, illegal characters, and unexpected structures. They should also verify that error responses are clear but do not reveal implementation details that help attackers refine payloads.
Designing Injection-Focused Tests
Create targeted tests for endpoints that touch databases, file systems, or external services. Try payloads that would break out of expected structures if concatenated unsafely, and confirm that the API rejects them gracefully. Where possible, combine manual exploration with automated scanning tools under controlled conditions.
Common Mistakes
Mistake 1 — Assuming frameworks make injection impossible
Safe defaults can be bypassed by unsafe coding patterns.
❌ Wrong: Skipping validation tests because “the ORM will handle it.”
✅ Correct: Test critical paths for injection behaviour regardless of framework claims.
Mistake 2 — Focusing only on SQL and ignoring other injection types
APIs may interact with many back-end systems.
❌ Wrong: Testing only SQL-like payloads, ignoring commands, LDAP, or template engines.
✅ Correct: Consider all back-end integrations when designing injection tests.