Case Study
Fintech payment platform security testing
Appsecco ran scoped, non-disruptive security testing across the web app, APIs, and payment flows to validate launch readiness in a regulated market.
The Challenge
The team was preparing to launch in a regulated market and needed clear, defensible answers beyond a standard compliance pass. They wanted to remove ambiguity about product-level risk before sign-off.
- • Whether payment flows could be manipulated under real usage conditions
- • How well tenant boundaries held up across APIs and data access
- • Which issues would require fixes prior to launch approval
Predictable, scoped engagement
We agreed on a fixed scope and fixed price upfront, then ran a short, documented workflow so the team knew what would happen at each step. Findings were shared as they were confirmed -- no surprises at the end.
Define scope and testing window
A written scope covered the web app, APIs, and payment flows, with clear environments and a fixed timeline.
Hands-on testing with weekly checkpoints
Testing focused on payment logic, tenant boundaries, and authorization paths, with progress updates and clarifications as needed.
Evidence-first reporting
Each finding included reproduction steps, impact, and prioritized fixes to make approval reviews straightforward.
Findings tied to real abuse paths
We modeled how a motivated attacker would chain normal product actions, then verified those paths against the live system. Each issue was documented with clear evidence and a fix that engineers could implement quickly.
Payment workflow replay and double-processing
By replaying timing-sensitive steps in the payment flow, we could trigger duplicate processing under specific concurrency conditions.
Resolution: Idempotency keys and transaction-level locking were added to enforce single-processing semantics.
Cross-tenant access via crafted API references
A chained API sequence allowed access to another tenant's financial records by mixing identifiers from different contexts.
Resolution: Authorization checks were enforced at the data layer to ensure tenant scoping on every request.
Token validation edge case
A token parsing edge case allowed a privileged action path that normal requests could not reach.
Resolution: Token validation was tightened to a single accepted algorithm and verified claims at each boundary.
Outcome
The team entered launch review with a clear record of payment integrity, tenant isolation, and token handling. Fixes were prioritized and re-tested, giving leadership a defensible go/no-go decision and shared confidence in the scope tested.
"The findings were specific and easy to act on. It made our launch approval review calm and straightforward."
— CISO, Payment Processing Platform
Safe next step
Talk through your fintech scope.
No commitment required.
We can review the payment flows, APIs, and environments you want covered, explain how we scope testing, and share a fixed quote if it is useful.
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