421 The secure development lifecycle
Discover how secure software is planned, designed, and delivered using a structured lifecycle that incorporates risk, testing, and maintenance from the outset.
Objectives
Describe the phases of a secure development lifecycle
Understand how security is embedded across planning, design, and testing
Recognise how SDL reduces risk and supports sustainable software
Syllabus references
What is a Secure Development Lifecycle?
A Secure Development Lifecycle (SDL) is a structured approach to building software that places security at the heart of each development phase. It ensures that design decisions, coding practices, and testing strategies anticipate and address potential vulnerabilities before software is deployed.
Unlike traditional development models that treat security as a final step, SDL integrates secure thinking throughout the process.
Why SDL matters
Security added as an afterthought is often too late — or too expensive. Fixing vulnerabilities after a product is released is far more costly than addressing them during the planning and design stages.
Benefits of following SDL include:
Fewer vulnerabilities in production
Clear documentation and accountability
Increased trust in system behaviour
Compliance with industry security standards
Typical SDL phases
Each organisation may have its version of SDL, but most follow a pattern similar to this:
1. Requirements definition
Identify security goals, constraints, and compliance obligations
2. Design Specification
Document how the system should behave, including authentication and authorisation needs, and apply design patterns that reduce risk (e.g. least privilege, sandboxing)
3. Development
Use secure coding practices, linters, and coding standards
4. Testing
Run static and dynamic tests to detect vulnerabilities
5. Deployment
Configure secure defaults and access controls
6. Maintenance
Monitor for issues, apply updates, and respond to new threats
Using SDL as a thinking tool
In professional environments, SDL is more than a checklist — it's a mindset. Developers, project managers, and security analysts work together across phases to:
Evaluate threats
Document decisions
Ensure that each step reduces risk, not just adds features
Good SDL practice supports both security and quality assurance.
Summary
SDL integrates security throughout the software development process
Each phase has its own security focus — from requirements to maintenance
Following SDL improves trust, reduces cost, and prevents vulnerabilities in production
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