Supply-chain Levels for Software Artifacts (SLSA)

The foundational requirement that source code is protected from unauthorized modification. This involves strong authentication for code commits, branch protection rules requiring peer review, and cryptographic signing of tags and commits. The source repository must maintain an immutable history, ensuring every change is attributed to a verified identity. This prevents malicious actors from injecting vulnerabilities directly into the codebase without detection.

The requirement that the build process itself is hardened against tampering. This mandates hermetic builds executed in isolated, ephemeral environments with no network access. The build pipeline must generate a cryptographically signed provenance attestation that records all inputs (source hash, dependencies, build commands) and outputs (artifact hashes). This creates a non-repudiable statement that can be independently verified.

The process of cryptographically validating the attestation that accompanies every artifact. A provenance document, typically generated in in-toto format, is checked against a trusted policy engine like Open Policy Agent (OPA). Verification confirms that the artifact was produced by a trusted builder, from a specific source repository, using an expected build recipe. This step closes the loop between source and artifact.

A set of cross-cutting controls that apply across all SLSA levels. These include:

• Scripted Builds: All build steps defined as code, not manual actions.
• Ephemeral Environments: Builds run in fresh, disposable containers.
• Isolation: No build can influence another concurrently. These requirements ensure consistency and prevent contamination between builds, forming the baseline hygiene for any SLSA-compliant pipeline.

The practice of explicitly declaring, pinning, and verifying all third-party dependencies. SLSA requires dependency pinning to exact cryptographic hashes, preventing dependency confusion attacks where malicious packages are substituted. All dependencies must be resolved from a trusted, private artifact registry that performs vulnerability scanning. This ensures the transitive dependency graph is fully known and verified.