Case study · 2025 · Payment Platforms · Fintech
Security Hardening for a Payments Platform
A growing payments platform asked us to attack it before someone else did. Under a scoped, written rules-of-engagement, we ran an authorized penetration test focused on API abuse and authorization logic, then worked the findings with the platform's engineers through fix sprints and retesting — finishing with a hardened surface and an evidence pack their assessor could work from.
Screens are illustrative recreations — client interfaces stay confidential.
The challenge
The platform had grown API-first and fast — new endpoints shipping weekly, partner integrations multiplying, and no adversarial review since the early architecture. Authorization logic was the honest worry: who can move whose money.
Larger merchants and an upcoming assessment were raising the bar: the platform needed findings fixed and demonstrated fixed, with evidence tidy enough to hand to a qualified assessor.
What we built
Scoped, authorized testing
A written rules-of-engagement defined targets, test windows, and escalation contacts; testing ran against a production-parity environment with methodology aligned to OWASP ASVS and API Security Top 10.
Authorization logic first
The deepest effort went to object-level and function-level authorization across merchant, partner, and internal roles — where we found the four critical issues, including cross-tenant object references in two payout endpoints.
Fix sprints with engineers, not a PDF
Findings were worked in joint fix sprints with reproductions, severity rationale, and pattern-level fixes — a centralized authorization middleware replaced per-endpoint checks, closing the class, not just the instances.
Retest and evidence pack
Every finding was retested against the deployed fix; the closing package paired each with its remediation evidence, alongside hardening work on secrets handling, session lifetimes, and dependency policy.
The results
- 32
- Findings remediated & retested
- −60%
- Exposed API surface
- 0
- Critical or high on retest
including 4 critical, 7 high severity
routes and scopes retired after inventory
verified against deployed fixes
Remaining medium/low items are accepted risks with documented compensating controls.
View the data as a table
| Initial | Retest | |
|---|---|---|
| Critical | 4 | 0 |
| High | 7 | 0 |
| Medium | 12 | 2 |
| Low | 9 | 4 |
- All 32 findings — 4 critical, 7 high — remediated and verified in retest, with zero critical or high findings remaining.
- Publicly reachable API surface shrank about 60% after an endpoint inventory retired forgotten routes and over-broad scopes.
- The platform's assessor review proceeded on the engagement's evidence pack, and adversarial testing became a standing pre-release practice for the payment team.
Client identities stay confidential; figures are rounded from end-of-engagement delivery reporting.
Stack & expertise
- OWASP ASVS methodology
- Burp Suite
- custom API fuzzing harness
- AWS
- Node.js fixes
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