How We Secured a FinTech API Processing $2M Daily

Challenge

• Critical API endpoints accepted over-scoped tokens and lacked consistent authorization boundaries.
• Audit pressure increased due to SOC 2 readiness and PCI-aligned controls needed by enterprise buyers.
• Daily transaction volume was rising quickly, creating performance risk across fraud checks and settlement workflows.

Our Process

1. 1. Threat Modeling

   We mapped trust boundaries, identified misuse paths for payout APIs, and ranked remediation by exploitability and blast radius.

   

2. 2. Architecture Redesign

   We moved from a fragile point-to-point API model to a segmented service architecture with policy enforcement and event-driven settlement orchestration.

   

3. 3. Implementation

   Security controls were embedded in endpoint policies, token scope checks, and risk-scored command execution patterns.

   app.MapPost("/v1/payouts", async (
       PayoutRequest request,
       ClaimsPrincipal user,
       IFraudPolicyEngine fraud,
       IPayoutService payouts,
       CancellationToken ct) =>
   {
       if (!user.HasClaim("scope", "payouts:write"))
       {
           return Results.Forbid();
       }
   
       var riskResult = await fraud.EvaluateAsync(request, ct);
       if (!riskResult.Allowed)
       {
           return Results.BadRequest(new { error = "Blocked by risk policy" });
       }
   
       var result = await payouts.CreateAsync(request, ct);
       return Results.Ok(result);
   })
   .RequireAuthorization("payout_writer")
   .RequireRateLimiting("payments-api")
   .WithName("CreatePayout");

4. 4. Testing

   We introduced an automated security pipeline spanning SAST, dependency scanning, and baseline dynamic tests before release.

   name: security-quality-gate
   on:
     pull_request:
     push:
       branches: [dev, main]
   
   jobs:
     security:
       runs-on: ubuntu-latest
       steps:
         - uses: actions/checkout@v4
         - uses: returntocorp/semgrep-action@v1
         - name: Dependency scan
           run: dotnet list src/Platform.Api package --vulnerable
         - name: OWASP ZAP baseline
           run: docker run --rm -t owasp/zap2docker-stable zap-baseline.py -t https://staging.api.example.com

Results

Team

Led by Feroze Basha, .NET Security Specialist.