Secure by design principles
In short: Secure by design means building security into a system from the first requirements rather than adding it at the end. A small set of principles, including least privilege, defence in depth, secure defaults and failing securely, guides decisions at every stage and makes systems cheaper and safer to run.
What secure by design means
Many breaches come from systems where security was added late: a firewall in front of an application that trusts every request, or encryption bolted on after data had already spread. Secure by design means treating security as a design requirement, like performance or usability, from day one. Government agencies, including CISA in the United States and the NCSC in the UK, now push software makers to ship products that are secure out of the box rather than leaving hardening to customers.
Fixing a flaw during design costs a conversation; fixing it in production can mean an incident, an emergency patch and lost trust.
The core principles
| Principle | In practice |
|---|---|
| Least privilege | A reporting service gets read-only database access, not admin |
| Defence in depth | MFA, network segmentation and logging all protect the same data |
| Fail securely | If the authorisation service is unreachable, deny access rather than allow it |
| Secure defaults | New storage buckets are private; default passwords do not exist |
| Minimise attack surface | Disable unused ports, features, accounts and API endpoints |
| Separation of duties | The person who writes code cannot approve their own release to production |
| Complete mediation | Check permissions on every request, not just at login |
| Keep it simple | Fewer components and clear data flows are easier to secure and review |
| Open design | Security relies on keys and sound methods, not on hiding how the system works |
Two more ideas are increasingly important: zero trust, which never assumes a request is safe because of its network location (see zero trust basics), and privacy by design, which collects and keeps only the personal data you truly need.
Applying the principles
- Write security requirements alongside functional ones, such as "all admin actions require MFA and are logged".
- Run a threat model on the design and record the decisions.
- Use proven components: standard libraries for cryptography, a managed identity provider, hardened base images.
- Automate checks in the build pipeline: dependency scanning, static analysis and infrastructure-as-code scanning.
- Ship with secure defaults and document any setting that weakens security.
- Plan for operation: patching, logging, key rotation and incident response.
Common mistakes
- Trusting input because it comes from "inside" the network or from your own front end.
- Rolling your own encryption or authentication scheme.
- Granting broad permissions "for now" and never removing them.
- Error messages that leak stack traces, file paths or database details.
- Treating a penetration test at the end as the security plan.
Tip: in exam answers and design reviews, name the principle and show where it applies in your design. "Least privilege: the web tier uses a database account limited to these three stored procedures" is much stronger than "the system is secure".
Practice
A payment API allows transactions when the fraud-check service times out. Which principle does this break?
Give an example of a secure default.
Why is "open design" a principle?
Key takeaways
- Build security in from requirements, not at the end.
- Least privilege, defence in depth and secure defaults are core principles.
- Fail securely: errors should deny, not allow.
- Minimise attack surface and keep designs simple.
- Name the principle and show where it applies.
Original content written by Mohamed Fayas for FM TecHub. © 2026 FM TecHub. You're welcome to share a link to this page; please don't copy the text elsewhere without permission.