Secure Software Design
Secure defaults, fail closed, complete mediation, minimal trusted computing base, error handling that does not leak, and audit logs that answer questions.
Every lesson below identifies the asset, attacker capability and boundary before naming the vulnerability. Controls are split into prevention, detection and recovery; residual risk is explicit.
Overlay public entry points, identities, authorization, trust boundaries, encrypted links, sensitive stores, privileged services and audit sinks on the ordinary system diagram.
Secure defaults, deny by default, least privilege, complete mediation, separation of duties and explicit boundaries make the safe path the easy path.
Users need a stable safe error; operators need correlated internal diagnostics. Sending the latter to the former reveals implementation and sometimes secrets.
Logs are production data stores: they should contain decision evidence, not passwords, tokens, full payment details or unnecessary personal data.
Who did what to which resource, when, from where, with what result—and which policy allowed it.