Security levels

SLSA is organized into a series of levels that provide increasing supply chain security guarantees. This gives you confidence that software hasn’t been tampered with and can be securely traced back to its source.

This page is an informative overview of the SLSA levels, describing their purpose and guarantees. For the normative requirements at each level, see Requirements. For a background, see Terminology.

What is SLSA?

SLSA is a set of incrementally adoptable security guidelines, established by industry consensus. The standards set by SLSA are guiding principles for both software producers and consumers: producers can follow the guidelines to make their software more secure, and consumers can make decisions based on a software package’s security posture. SLSA’s levels are designed to be incremental and actionable, and to protect against specific classes of supply chain attacks. The highest level in each track represents an ideal end state, and the lower levels represent intermediate milestones with commensurate security guarantees.

Importantly, SLSA is intended to be a primitive in a broader determination of a software’s risk. SLSA measures specific aspects of supply chain security, particularly those that can be fully automated; other aspects, such as developer trust and code quality, are out of scope. Furthermore, each link in the software supply chain has its own, independent SLSA level—in other words, it is not transitive (FAQ). The benefit of this approach is to break up the large supply chain security problem into tractable subproblems that can be prioritized based on risk and tackled in parallel. But this does mean that SLSA alone is not sufficient to determine if an artifact is “safe”.

TODO: SLSA is in the eye of the beholder: software consumers make their own SLSA determinations, though in practice they may delegate to some authority.

Who is SLSA for?

SLSA is intended to serve multiple populations:

  • Project maintainers, who are often small teams that know their build process and trust their teammates. Their primary goal is protection against compromise with as low overhead as possible. They may also benefit from easier maintenance and increased consumer confidence.

  • Consumers, who use a variety of software and do not know the maintainers or their build processes. Their primary goal is confidence that the software is authentic and has not been tampered with. They are concerned about rogue maintainers, compromised credentials, and compromised infrastructure.

  • Organizations, who are both producers and consumers of software. In addition to the goals above, organizations also want to broadly understand and reduce risk across the organization.

  • Infrastructure providers, such as build services and packaging ecosystems, who are critical to achieving SLSA. While not the primary beneficiary of SLSA, they may benefit from increased security and user trust.

Levels and tracks

SLSA levels are split into tracks. Each track has its own set of levels that measure a particular aspect of supply chain security. The purpose of tracks is to recognize progress made in one aspect of security without blocking on an unrelated aspect. Tracks also allow the SLSA spec to evolve: we can add more tracks without invalidating previous levels.

Track/Level Requirements Benefits
Build L0 (none) (n/a)
Build L1 Attestation showing that the package was built as expected Documentation, mistake prevention, inventorying
Build L2 Signed attestation, generated by a hosted build service Reduced attack surface, weak tamper protection
Build L3 Hardened build service Strong tamper protection
Build L4 (not yet defined)
Source L… (not yet defined)

Note: The previous version of the specification used a single unnamed track, SLSA 1–4, roughly corresponding to the equivalently numbered Build + Source levels.

Build track

The SLSA build track describes the level of protection against tampering during or after the build, and the trustworthiness of provenance metadata. Higher SLSA build levels provide increased confidence that a package truly came from the correct sources, without unauthorized modification or influence.

TODO: Add a diagram visualizing the following.

Summary of the build track:

  • Set project-specific expectations for how the package should be built.
  • Generate a provenance attestation automatically during each build.
  • Automatically verify that each package’s provenance meets expectations before allowing its publication and/or consumption.

What sets the levels apart is how much trust there is in the accuracy of the provenance and the degree to which adversaries are detected or prevented from tampering with the package. Higher levels require hardened builds and protection against more sophisticated adversaries.

Each ecosystem (for open source) or organization (for closed source) defines exactly how this is implemented, including: means of defining expectations, what provenance format is accepted, whether reproducible builds are used, how provenance is distributed, when verification happens, and what happens on failure. Guidelines for implementers can be found in the requirements.

Build L0: No guarantees

Summary

No requirements—L0 represents the lack of SLSA.

Intended for

Development or test builds of software that are built and run on the same machine, such as unit tests.

Requirements

n/a

Benefits

n/a

Build L1: Provenance

Summary

Package has a provenance attestation showing how it was built, and a downstream system automatically verifies that packages were built as expected. Prevents mistakes but is trivial to bypass or forge.

Intended for

Projects and organizations wanting to easily and quickly gain some benefits of SLSA—other than tamper protection—without changing their build workflows.

Requirements
  • Up front, the package maintainer defines how the package is expected to be built, including the canonical source repository and build command.

  • On each build, the release process automatically generates and distributes a provenance attestation describing how the artifact was actually built, including: who built the package (person or system), what process/command was used, and what the input artifacts were.

  • Downstream tooling automatically verifies that the artifact’s provenance exists and matches the expectation.

Benefits
  • Makes it easier for both maintainers and consumers to debug, patch, rebuild, and/or analyze the software by knowing its precise source version and build process.

  • Prevents mistakes during the release process, such as building from a commit that is not present in the upstream repo.

  • Aids organizations in creating an inventory of software and build systems used across a variety of teams.

Notes
  • Provenance may be incomplete and/or unsigned at L1. Higher levels require more complete and trustworthy provenance.

Build L2: Build service

Summary

Forging the provenance or evading verification requires an explicit “attack”, though this may be easy to perform. Deters unsophisticated adversaries or those who face legal or financial risk.

In practice, this means that builds run on a hosted service that generates and signs the provenance.

Intended for

Projects and organizations wanting to gain moderate security benefits of SLSA by switching to a hosted build service, while waiting for changes to the build service itself required by Build L3.

Requirements

All of Build L1, plus:

  • The build runs on a hosted build service that generates and signs the provenance itself. This may be the original build, an after-the-fact reproducible build, or some equivalent system that ensures the trustworthiness of the provenance.

  • Downstream verification of provenance includes authenticating the provenance.

Benefits

All of Build L1, plus:

  • Prevents tampering after the build through digital signatures.

  • Deters adversaries who face legal or financial risk by evading security controls, such as employees who face risk of getting fired.

  • Reduces attack surface by limiting builds to specific build services that can be audited and hardened.

  • Allows large-scale migration of teams to supported build services early while further hardening work (Build L3) is done in parallel.

TODO: If possible, avoid being overly specific about “signing”.

Build L3: Hardened builds

Summary

Forging the provenance or evading verification requires exploiting a vulnerability that is beyond the capabilities of most adversaries.

In practice, this means that builds run on a hardened build service that offers strong tamper protection.

Intended for

Most software releases. Build L3 usually requires significant changes to existing build services.

Requirements

All of Build L2, plus:

  • Build service implements strong controls to:

    • prevent runs from influencing one another, even within the same project.
    • prevent secret material used to sign the provenance from being accessible to the user-defined build steps.

TODO: Add requirement about survey and audit as per the v1.0 proposal.

Benefits

All of Build L2, plus:

  • Prevents tampering during the build—by maintainers, compromised credentials, or other tenants.

  • Greatly reduces the impact of compromised package upload credentials by requiring attacker to perform a difficult exploit of the build process.

  • Provides strong confidence that the package was built from the official source and build process.

Build L4: (not yet defined)

Build L4 is not yet defined. For reference, previous version of SLSA defined a “SLSA 4” that included the following requirements, which may or may not be part of a future Build L4:

  • Pinned dependencies, which guarantee that each build runs on exactly the same set of inputs.
  • Hermetic builds, which guarantee that no extraneous dependencies are used.
  • All dependencies listed in the provenance, which enables downstream systems to recursively apply SLSA to dependencies.
  • Reproducible builds, which enable other systems to corroborate the provenance.

Source track (not yet defined)

The Source track is not yet defined but is expected to measure protection against tampering of the source code. It will be defined in a future version of the specification.

For reference, the previous version of SLSA included the following requirements, which may or may not be part of a future Source track:

  • Strong authentication of author and reviewer identities, such as 2-factor authentication using a hardware security key, to resist account and credential compromise.
  • Retention of the source code to allow for after-the-fact inspection and future rebuilds.
  • Mandatory two-person review of all changes to the source to prevent a single compromised actor or account from introducing malicious changes.