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, including AI/MCP integrations when they're in scope, explain how we scope testing, and share a fixed quote if it is useful.
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