API Penetration Testing for a High-Volume FinTech Platform

See how our certified pentesters secured a banking application with a high transaction volume. We identified serious logic flaws that automated scanners missed, preventing potential financial fraud.

The Client

Anonymized for security reasons: A rapidly growing European Neo-bank and payment processor. The client handles over 500,000 transactions daily and was preparing to launch a new mobile application backed by a complex REST API.

The Challenge

The client operated under strict regulations (PSD2/DORA) and faced high risks of financial fraud. Their internal team had already run automated scans, which showed “clean” results. However, the CISO needed a manual, in-depth API penetration test to verify if the business logic was truly secure against sophisticated attackers.

Key Objectives:

• Verify the security of the new REST API endpoints.
• Test for logic flaws that could lead to funds theft.
• Ensure zero leakage of Personally Identifiable Information (PII).

Scope of the Engagement

Two senior pentesters, 10 tester-days in total, one production-equivalent (staging) environment of the REST API. The scope was agreed in writing before any traffic was sent, and it covered:

• The REST API behind the new mobile application, including authentication, transfers, transaction history, and merchant endpoints.
• Three user roles: standard customer, merchant, and support operator. Every endpoint was tested against every role, not just the role it was designed for.
• Object level authorization, tested account against account rather than only checking that a token was present.
• Excluded by agreement: denial of service, physical access, and social engineering against staff.

Rules of engagement defined the testing window, the contacts on both sides, and the abort conditions, so the client’s fraud monitoring team knew which alerts were ours.

The Solution: Grey Box Pentest

Pentestica assigned two senior pentesters, 10 tester-days in total. We adopted a Grey Box approach, where we had access to API documentation and valid user accounts for all three roles (customers, merchants and a support operator) to simulate insider threats and privilege escalation.

Our methodology combined industry standards (OWASP API Security Top 10) with attack paths built specifically around the client’s payment flows. Documentation-first reconnaissance mapped every declared endpoint, then traffic capture from the mobile client exposed a set of endpoints that were live but absent from the specification.

Why the Automated Scans Came Back Clean

A scanner checks whether a request is authenticated. It does not know which account is supposed to own which record. The IDOR below required a tester to hold two accounts at once and compare what each one could reach, and the race condition required knowing what a transfer is supposed to do. That is the practical difference between a scan and a test, and it is covered in more depth in our comparison of penetration testing and vulnerability scanning.

Key Findings

While automated tools found nothing significant, our manual pentesting uncovered three significant vulnerabilities: one critical, one high and one medium severity.

• Critical: IDOR (Insecure Direct Object Reference) Our pentesters discovered that by manipulating the Transaction ID in the API call, User A could view the full transaction history and sensitive data of User B.
  • Business Impact: Massive data breach and GDPR violation.
• High: Race Condition (Double Spending) We identified a flaw in the transfer logic. By sending multiple simultaneous requests to the transfer endpoint, we successfully withdrew funds twice before the database updated the account balance.
  • Business Impact: Direct financial loss and potential insolvency.
• Medium: Improper Rate Limiting The login API lacked sufficient protection, allowing for brute-force attacks against user PINs.
  • Business Impact: Account takeover for users with guessable PINs if an attacker holds their phone numbers.

Each finding was delivered with the exact request and response that produced it, so the development team could reproduce the issue before touching the code.

The Result & Remediation

We delivered a draft report with critical findings flagged during testing, and the full report a few days after testing ended.

• Immediate Fixes: The client’s development team patched the IDOR and Race Condition vulnerabilities within 48 hours using our remediation guidelines.
• Re-testing: We conducted a verification pentest (re-test) to confirm the fixes were effective.
• Launch: The application launched on schedule with no security incidents reported to date.

The report came in two parts: an Executive Summary written for the board and the regulator, and a Technical Report for the engineering team with the vulnerability list, exploitation evidence, and step-by-step remediation. For a fictional example of what such a summary covers, see our executive summary example.

“Pentestica found logic bugs our automated scans had never flagged. Their manual approach saved us from a potential PR disaster.” – CISO of the Client

Need the same test for your API?

This engagement was a grey box API penetration test of the REST API in a production-equivalent (staging) environment, run by two senior pentesters, 10 tester-days in total. A standard web application or API test at Pentestica falls in the $2,500–$5,500 range, and larger scopes like this one are quoted in tester-days; see the full penetration testing cost breakdown or request a pentest quote with your endpoint count and user roles.

Testing a mobile app that calls the same API? See mobile application penetration testing.