Zero Trust for Software Supply Chains

Zero Trust as a security principle , never trust, always verify , emerged from the recognition that network perimeter security was insufficient because threats inside the perimeter could move laterally with as much ease as legitimate traffic. Applied to software supply chains, the same principle demands that software artifacts not be trusted on the basis of their source location (the registry URL, the vendor's distribution server) but instead verified on the basis of cryptographic evidence of their authenticity, integrity, and provenance. A software supply chain operating on implicit trust , accepting dependencies because they came from the expected registry, accepting build outputs because they came from the expected CI system , is as vulnerable to supply chain attacks as a perimeter-trusted network is to insider threats and lateral movement.

The publisher identity verification gap is the specific Zero Trust failure in the software supply chain context. When a build system pulls a dependency from a public package registry, it verifies the package is available at the registry URL and, if the package is signed, that the signature is valid. It does not verify the identity of the entity that published the package , specifically, it does not verify that the entity who published the package to the registry is the same entity that created the original project and has maintained it. A supply chain attack that involves a compromised publisher account or a typosquatted package name will pass URL-based and signature-based checks while delivering code from an unexpected publisher.

The build agent credential minimisation gap is the Zero Trust principle of least privilege applied to the build pipeline. Build agents that pull source code, resolve dependencies, build artifacts, and push to registries frequently operate with credentials scoped to the broadest access required by any single build step , because configuring separate credentials for each step is more complex. The least-privilege Zero Trust principle requires that each build step authenticate with the minimum credential scope required for that specific operation: source code pull requires read access to the repository, not write access to the registry; artifact push requires write access to the registry namespace, not read access to all source repositories.

Why this matters

Zero Trust for software supply chains matters for TPRM because the security improvements that Zero Trust provides in network contexts apply equally to the software supply chain , and enterprises that have invested in Zero Trust network architecture while leaving their software supply chains on implicit trust have created an asymmetry where the network is harder to attack through but the software supply chain remains as vulnerable as it was before.

• Zero Trust applied to network but not software supply chain
• Publisher identity verification absent , registry presence accepted as authenticity
• Build agent minimum privilege not implemented , broad credentials for all build steps
• Software update server identity verified only by HTTPS certificate
• Implicit trust in dependency sources not addressed by Zero Trust network initiative

What good looks like

A Zero Trust software supply chain verifies publisher identity beyond registry presence using provenance attestations and transparency logs, applies minimum-privilege credentials to each build pipeline step, verifies software update authenticity through provenance rather than just HTTPS, and implements signature verification as a cryptographic identity assertion rather than a distribution channel trust proxy.

• Publisher identity verification via provenance attestations and transparency logs
• Build agent step-level minimum privilege , separate credentials per build step
• Software update provenance verification beyond HTTPS certificate
• Dependency publisher identity verification beyond registry URL presence
• Cryptographic identity assertions as the Zero Trust verification mechanism

Tooling

Supply Chain Zero Trust , Sigstore for cryptographic identity and provenance; Workload Identity for build agent identity

Sigstore provides the identity infrastructure for Zero Trust software supply chains , OIDC-based signing that binds software artifacts to the CI/CD system identity rather than a persistent key, providing cryptographic verification of publisher identity that goes beyond registry URL presence. Workload Identity in cloud platforms provides step-level identity for build pipeline operations , allowing each build step to authenticate with minimum required permissions rather than shared build agent credentials.

Governance challenges

The governance challenge with Zero Trust for software supply chains is the maturity gap. Zero Trust network architectures have been implemented in enterprises for years with mature tooling and established patterns. Zero Trust supply chain architectures , provenance verification, publisher identity verification, step-level build credentials , are newer concepts with less mature tooling adoption. The governance resolution is progressive implementation, starting with the most critical controls: SLSA provenance verification for the highest-criticality software and step-level credential minimisation in the CI/CD pipeline.

• Apply Zero Trust principles explicitly to software supply chain
• Implement provenance verification as supply chain identity assertion
• Migrate to step-level build credentials , least privilege per pipeline step
• Verify publisher identity beyond registry presence for critical dependencies
• Include supply chain Zero Trust in vendor security programme assessment

If you are a small team

Map the implicit trust assumptions in your software supply chain: what dependencies are you trusting because they came from the expected registry URL? What build artifacts are you trusting because they came from the CI system? What software updates are you applying because they were served over HTTPS? Each implicit trust is a Zero Trust gap. Start with the highest-criticality implicit trust , typically the software updates for your most critical infrastructure , and implement provenance verification as the first explicit identity assertion in your supply chain Zero Trust programme.

• Map implicit trust assumptions in software supply chain
• Identify highest-criticality implicit trust to address first
• Implement SLSA provenance verification for critical software updates
• Migrate CI/CD to step-level minimum privilege credentials

What to require

Ask directly:

"Have you applied Zero Trust principles to your software supply chain , specifically, do you verify publisher identity beyond registry URL presence for your critical dependencies, and do your build agents use step-level minimum privilege credentials rather than shared service account credentials?"

Expect as evidence

• Publisher identity verification beyond registry presence
• Build agent step-level credential minimisation
• Software update provenance verification
• Zero Trust supply chain architecture description

A vendor who confirms Zero Trust network security should be asked about Zero Trust supply chain security. Network Zero Trust and supply chain Zero Trust are complementary. An enterprise that has one without the other has hardened the perimeter while leaving the software delivery mechanism on implicit trust.

How to evidence it

• Zero Trust supply chain assessment
• Build agent credential minimisation records
• Provenance verification implementation
• Publisher identity verification process

Key Takeaway

Network: Zero Trust. Software supply chain: implicit trust for dependencies from expected registries, build artifacts from expected CI systems, software updates from vendor HTTPS URLs. Zero Trust eliminates implicit trust and requires cryptographic verification of identity and integrity. Provenance attestations are the supply chain identity assertion , cryptographic evidence that binds the artifact to the build system that produced it. Step-level build credentials are the supply chain least privilege , minimum access per operation rather than shared credentials for all operations. The Zero Trust principle is the same. The implementation in the supply chain context is different from the network context.

More on Third-party oversight

• Build Pipeline Integrity
• Vendor Patch Cadence Reality
• Artifact Registry Security