What is a security shift-left strategy in CI/CD and how is it implemented?

Shift-left security moves security testing and enforcement as early in the development lifecycle as possible — into the developer's workflow and CI pipeline — rather than relying on a security team review gate at the end. The earlier a vulnerability is found, the cheaper it is to fix. Implementation layers: Pre-commit hooks (Husky, pre-commit framework) run fast local checks before a commit is even pushed — secret scanning (detect-secrets, gitleaks), IaC security scanning (tfsec, Checkov), and basic linting. PR pipeline: SAST (Semgrep, CodeQL), dependency scanning (Snyk, OWASP Dependency Check), container image scanning (Trivy), and IaC policy validation (OPA, Checkov). Merge gates: block merges if critical vulnerabilities are found — defined severity thresholds prevent high/critical CVEs from entering the main branch. Continuous dependency monitoring: tools like Dependabot and Renovate automatically open PRs for dependency updates with security fixes. Secret scanning: GitHub Advanced Security, GitGuardian, and truffleHog scan commits and history for accidentally committed secrets. The cultural shift is equally important: developers are given security training, immediate actionable feedback in their workflow, and are responsible for fixing issues rather than handing them to a security team.