The onboarding process for supply chain security exists to stop this. Every new developer, contractor, and service that touches your codebase must meet strict requirements before they can ship anything. This is where most teams fail. They add users to repos, give keys, and hope their existing rules are enough. They are not.
A strong onboarding process for supply chain security begins with identity verification. No account should be provisioned without confirming both the person and the purpose. Enforce single sign-on and hardware security keys from day one. Integrate access control with your version control and CI/CD systems so permissions are set at onboarding, not ad hoc later.
Next, examine toolchains. Every workstation, build agent, and deployment pipeline should be hardened before access is granted. Apply least privilege for credentials. Rotate tokens automatically. Require code signing for all commits. This locks down the supply chain entry points that attackers exploit in their first move.