Anonymized case study. Client name and identifying metrics are withheld. Patterns reflect representative senior .NET programs.
Executive summary
A payments ISV processed card-not-present flows through ASP.NET Core APIs. A QSA-led review flagged logging of sensitive fields, weak rate limits, and ad hoc key rotation.
The challenge
Engineers needed velocity while security required reproducible controls. Legacy endpoints mixed capture and settlement concerns.
Technical approach
Separate bounded contexts for capture vs settlement, structured logging with field redaction, ASP.NET rate limiting and WAF rules, Key Vault rotation runbooks, OWASP ASVS checklist in PR template, and contract tests on auth scopes.
Outcomes
Critical audit items remediated within the agreed window. Product resumed feature work with security gates in CI. Operations rotated keys without unplanned downtime.
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