API Penetration Testing for a High-Volume FinTech Platform

See how our certified pentesters secured a banking application processing €50M+ monthly. We identified critical logic flaws that automated scanners missed, preventing potential financial fraud.

1. 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.

2. 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 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).

3. The Solution: Grey Box Pentest

Pentestica assigned two senior pentesters for a 10-day engagement. We adopted a Grey Box approach, where we had access to API documentation and valid user accounts (both regular users and merchants) to simulate insider threats and privilege escalation.

Our methodology combined industry standards (OWASP API Security Top 10) with proprietary attack vectors developed by our Red Team.

4. Critical Findings

While automated tools found nothing significant, our manual pentesting uncovered three critical vulnerabilities:

• 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.

5. The Result & Remediation

We delivered a comprehensive report within 24 hours of finishing the tests.

• 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.

“Pentestica found logic bugs that three previous vendors missed. Their manual approach saved us from a potential PR disaster.” — CISO of the Client