Introduction – Why API Security Testing Is Critical in Interviews
APIs expose business data and core functionality to web, mobile, and third-party applications. A single insecure API can lead to data breaches, financial loss, and compliance violations.
Because of this risk, interviewers actively ask api security testing interview questions to check whether candidates can:
- Identify API security vulnerabilities
- Validate authentication and authorization
- Prevent data exposure and misuse
- Understand OWASP API risks
- Test APIs using tools like Postman, SoapUI, Rest Assured, Python
For QA engineers, API testers, and SDETs, security testing knowledge is now mandatory, not optional.
This article is a complete interview-focused guide with clear answers, real-time examples, response samples, status codes, and scenario-based questions, suitable for freshers to experienced professionals.
What Is API Testing? (Clear & Simple)
API testing verifies whether an API:
- Works correctly
- Returns valid responses
- Enforces business rules
- Handles errors properly
API security testing specifically checks whether the API:
- Allows access only to authorized users
- Protects sensitive data
- Prevents attacks and misuse
Simple Example
For a Get User Profile API:
- Valid token → 200 OK + user data
- Invalid token → 401 Unauthorized
- Accessing another user’s data → 403 Forbidden
REST vs SOAP vs GraphQL (Security Perspective)
| Feature | REST | SOAP | GraphQL |
| Data Format | JSON/XML | XML | JSON |
| Security | OAuth, JWT | WS-Security | Token-based |
| Exposure Risk | High if misconfigured | Moderate | High |
| Common Attacks | Auth bypass, IDOR | XML attacks | Over-fetching |
👉 Most api security testing interview questions focus on REST APIs, but SOAP security concepts are important in enterprise systems.
API Security Testing Interview Questions & Answers (90+)
Section 1: API Security Basics (Q1–Q20)
1. What is API security testing?
Testing APIs to ensure data is protected and only authorized access is allowed.
2. Why is API security important?
APIs expose backend data directly, making them prime attack targets.
3. Difference between API testing and API security testing?
API testing checks functionality; security testing checks protection.
4. What are common API security risks?
Authentication flaws, authorization issues, data exposure.
5. What is authentication?
Verifying the identity of a user or system.
6. What is authorization?
Verifying what actions a user is allowed to perform.
7. Difference between authentication and authorization?
Authentication = who you are; Authorization = what you can do.
8. What authentication methods are used in APIs?
Basic Auth, API keys, OAuth, JWT.
9. What is JWT?
JSON Web Token used for secure, stateless authentication.
10. What is OAuth?
Authorization framework for delegated access.
11. What is API key?
A unique identifier used to authenticate API requests.
12. What is token expiration?
Time limit after which token becomes invalid.
13. What is refresh token?
Used to generate a new access token.
14. What is HTTPS?
Secure communication using SSL/TLS encryption.
15. Why is HTTPS mandatory for APIs?
Prevents data interception and man-in-the-middle attacks.
16. What is sensitive data exposure?
When confidential data is returned in API responses.
17. What is rate limiting?
Restricting number of API calls per user.
18. What is throttling?
Controlling traffic to protect backend systems.
19. What is API gateway?
Central point for security, routing, and monitoring.
20. What is OWASP?
Organization that publishes security risk guidelines.
OWASP Top API Security Risks (Interview Must-Know)
| Risk | Description |
| Broken Object Level Authorization (BOLA) | Accessing other users’ data |
| Broken Authentication | Weak or missing auth |
| Excessive Data Exposure | Returning unnecessary data |
| Rate Limiting Issues | Unlimited requests |
| Injection | SQL/NoSQL injection |
| Security Misconfiguration | Default settings |
| Mass Assignment | Updating restricted fields |
HTTP Status Codes – Security Context
| Code | Meaning | Security Usage |
| 200 | OK | Authorized success |
| 201 | Created | Secure creation |
| 400 | Bad Request | Invalid input |
| 401 | Unauthorized | Invalid or missing auth |
| 403 | Forbidden | Access denied |
| 404 | Not Found | Hide resource existence |
| 409 | Conflict | Duplicate request |
| 429 | Too Many Requests | Rate limit exceeded |
| 500 | Server Error | Backend issue |
Section 2: Authentication & Authorization Questions (Q21–Q45)
21. How do you test authentication in APIs?
By sending valid, invalid, expired, and missing tokens.
22. How do you test authorization?
By accessing resources belonging to other users.
23. What is Broken Object Level Authorization?
When user can access another user’s object using ID manipulation.
24. Example of BOLA?
Changing /users/101 to /users/102 and getting data.
25. What is role-based access control?
Access based on user roles.
26. What is attribute-based access control?
Access based on attributes like region or department.
27. How do you test token expiration?
Wait until token expires and retry API call.
28. What is token reuse issue?
Using same token indefinitely.
29. What is privilege escalation?
User gaining higher access than allowed.
30. How do you test privilege escalation?
Attempt admin actions using normal user token.
31. What is session fixation?
Reusing old session identifiers.
32. How do you test logout functionality?
Check if token is invalid after logout.
33. What is API scope?
Defines allowed actions in OAuth.
34. How do you test missing scope?
Remove scope and verify access denial.
35. What is brute force attack?
Repeated attempts to guess credentials.
36. How do you prevent brute force?
Rate limiting and account lock.
37. What is IP whitelisting?
Allowing access only from specific IPs.
38. How do you test IP restrictions?
Send request from non-allowed IP.
39. What is CORS?
Cross-Origin Resource Sharing.
40. How do you test CORS issues?
Check headers for allowed origins.
41. What is API version security?
Ensuring old versions aren’t insecure.
42. What is API replay attack?
Reusing intercepted requests.
43. How to prevent replay attacks?
Use timestamps and nonces.
44. What is certificate-based authentication?
Using SSL certificates for auth.
45. What is mutual TLS?
Both client and server authenticate each other.
Real-Time API Security Validation Example
API Request
GET /api/users/101
Authorization: Bearer <token>
Secure Behavior
- Same user → 200 OK
- Different user ID → 403 Forbidden
Insecure Behavior
- Returns data for other users → Critical defect
Section 3: Data & Input Security (Q46–Q65)
46. What is input validation?
Ensuring only valid data is accepted.
47. What is SQL injection?
Injecting malicious SQL queries.
48. How do you test SQL injection in APIs?
Send payloads like ‘ OR 1=1 –.
49. What is NoSQL injection?
Injection in NoSQL queries.
50. What is mass assignment?
Updating restricted fields via request payload.
51. Example of mass assignment?
Updating isAdmin=true via API.
52. How do you prevent mass assignment?
Whitelist allowed fields.
53. What is excessive data exposure?
Returning internal fields like passwords.
54. How do you test data exposure?
Inspect API response fields.
55. What is sensitive data?
Passwords, tokens, card numbers.
56. What is data masking?
Hiding sensitive data.
57. How do you test data masking?
Verify partial values like ****1234.
58. What is file upload vulnerability?
Uploading malicious files.
59. How do you test file upload APIs?
Upload unsupported file types.
60. What is XML External Entity (XXE)?
XML parser vulnerability.
61. How do you test XXE?
Inject external entity references.
62. What is deserialization vulnerability?
Executing malicious serialized objects.
63. How do you prevent deserialization issues?
Disable unsafe deserialization.
64. What is API logging risk?
Logging sensitive data.
65. How do you test logging issues?
Check logs for secrets.
Postman / Automation Security Snippets
Postman – Unauthorized Access Test
pm.test(“Unauthorized user blocked”, function () {
pm.response.to.have.status(403);
});
Rest Assured – Auth Test
given()
.auth().oauth2(invalidToken)
.when()
.get(“/users/101”)
.then()
.statusCode(401);
Python – Token Test
import requests
res = requests.get(url, headers={“Authorization”: “Bearer invalid”})
assert res.status_code == 401
Scenario-Based API Security Testing Interview Questions (15)
- User accesses another user’s data—what issue?
- API returns password hash—severity?
- Token never expires—risk?
- Rate limiting missing—impact?
- API works without authentication—what defect?
- Admin API accessible to normal user—issue?
- SQL injection payload returns data—next step?
- Old API version still accessible—risk?
- Logout doesn’t invalidate token—impact?
- API allows unlimited login attempts—fix?
- Sensitive data visible in logs—what to do?
- File upload accepts .exe—issue?
- GraphQL API returns extra fields—risk?
- Error messages reveal DB details—impact?
- API gateway bypassed—how detect?
How Interviewers Evaluate Your Answers
Interviewers look for:
- Understanding of real security risks
- Ability to explain vulnerabilities clearly
- Practical testing approach
- Awareness of OWASP API risks
- Logical thinking, not hacking
👉 Security mindset matters more than tools.
API Security Testing Interview Cheatsheet
- Always test auth & authorization
- Check object-level access
- Validate input & output data
- Watch for excessive data exposure
- Enforce rate limits
- Use HTTPS everywhere
FAQs – API Security Testing Interview Questions
Q1. Is API security testing mandatory for QA roles?
Yes, basic security awareness is expected.
Q2. Do interviewers expect hacking skills?
No, logical testing and validation skills.
Q3. REST or SOAP—more security issues?
REST APIs are more commonly targeted.
Q4. Biggest mistake candidates make?
Ignoring authorization testing.
Q5. How to prepare quickly?
Practice OWASP API risks on sample APIs.