A secure development lifecycle (SDL) is a software engineering process that adds security checks at every stage of development, from design and coding to testing, release, and maintenance. Instead of treating security as a final audit, SDL builds in code review, threat modeling, dependency checking, vulnerability scanning, and remediation planning as routine work.
SDL matters because many attacks exploit preventable mistakes such as weak input handling, exposed secrets, insecure defaults, or unpatched libraries. In a mature SDL, developers catch these issues earlier, when fixes are cheaper and less disruptive. In practice, defenders use SDL to reduce the chance that insecure code reaches production, while attackers benefit when organizations skip reviews or ship quickly without verification. For AI-assisted development, SDL is even more important: model-generated code can speed delivery, but it also needs human review, testing, and repeatable validation before deployment.