Core specification version 1.0 is available now! See the newest documentation here.

SLSA Specification

SLSA is a specification for describing and incrementally improving supply chain security, established by industry consensus. It is organized into a series of levels that describe increasing security guarantees.

This is version 1.0-rc1 of the SLSA specification, which defines the SLSA levels. For other versions, use the chooser at the bottom of this section. For the recommended attestation formats, including provenance, see “Specifications” in the menu at the top.

About this release candidate

This release candidate is a preview of version 1.0. It contains all anticipated concepts and major changes for v1.0, but there are still outstanding TODOs and cleanups. We expect to cover all TODOs and address feedback before the 1.0 final release.

Known issues:

  • TODO: Use consistent terminology throughout the site: “publish” vs “release”, “publisher” vs “maintainer” vs “developer”, “consumer” vs “ecosystem” vs “downstream system”, “build” vs “produce.

  • Verifying artifacts and setting expectations are still in flux. We would like feedback on whether to move these parts out of the build track.

Table of contents

Section Description
What’s new in v1.0 What’s new in SLSA Version 1.0.
Security levels Overview of SLSA, intended for all audiences. If you read one section, read this.
Guiding principles Background on the guiding principles behind SLSA.
Terminology Terminology and model used by SLSA.
Producing artifacts Detailed technical requirements for producing software artifacts, intended for system implementers.
Verifying build systems Guidelines for securing SLSA Build L3+ builders, intended for system implementers.
Verifying artifacts Guidance for verifying software artifacts and their SLSA provenance, intended for system implementers and software consumers.
Threats & mitigations Specific supply chain attacks and how SLSA helps.
FAQ Questions and more information.
Future directions Additions and changes being considered for future SLSA versions.

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 section 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”.

Finally, SLSA is in the eye of the beholder: software consumers ultimately make their own SLSA determinations, though in practice they may delegate to some authority. For example, a build system may claim to conform to SLSA Build L3, but it is up to a consumer whether to trust that claim.

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 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 package 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

Note: The previous version of the specification used a single unnamed track, SLSA 1–4. For version 1.0 the Source aspects were removed to focus on the Build track. A Source track may be added in future versions.

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


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.





Build L1: Provenance


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.

  • 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.

  • 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.

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

Build L2: Build service


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 signs1 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.


All of Build L1, plus:

  • The build runs on a hosted build service that generates and signs1 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 validating the authenticity of the provenance attestation.


All of Build L1, plus:

  • Prevents tampering after the build through digital signatures1.

  • 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.

Build L3: Hardened builds


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.


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.

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.

Guiding principles

This section provides a background on the guiding principles behind SLSA. It is intended to help the reader better understand SLSA’s design decisions.

Trust systems, verify artifacts

One of the main ideas behind SLSA is to establish trust in a small number of systems and then automatically verify the many artifacts produced by those systems. In this way, SLSA is able to scale to support hundreds of thousands of software projects with a near-constant amount of central work.

We have no option but to manually analyze and reason about the behavior of some number of trusted systems, such as change management, build, and packaging systems. There will always be some trusted computing base on which we rely for security. We need to analyze and reason about this computing base to convince ourselves that it meets our security needs. Such analysis is difficult and expensive. We cannot possibly harden and audit thousands of bespoke build systems. We can, however, do this for a small number of general purpose systems.

Once we have a set of trusted systems, we can verify the supply chain of arbitrarily many artifacts by ensuring that artifacts were produced by trusted systems. This allows SLSA to scale to entire ecosystems and organizations regardless of size.

For example, a security engineer may analyze the architecture and implementation of a build system to ensure that it meets the SLSA build requirements. Once the analysis is complete, the public keys used by the build system to sign provenance can then be “trusted” up to a given SLSA level. Downstream systems can then automatically determine that an artifact meets a given level by verifying that it has provenance signed by that public key.

Corollary: Minimize the number of trusted systems

A corollary to the previous principle is to minimize the size of the trusted computing base. Every system we trust adds attack surface and increases the need for manual security analysis. Where possible:

  • Concentrate trust in shared infrastructure. For example, instead of each team maintaining their own build system, use a shared build system. This way hardening work can be shared across all teams.
  • Remove the need to trust components whenever possible. For example, use end-to-end signing to avoid the need to trust intermediate distribution systems.

Anchor trust in code, not individuals

Another idea behind SLSA is to anchor our trust in source code rather than individual maintainers who have write access to package registries. Code is static and analyzable. People, on the other hand, are prone to mistakes, credential compromise, and sometimes malicious action. Therefore, SLSA aims to securely trace all software back to source code, without the possibility for an individual maintainer to tamper with it after it has been committed.

Prefer attestations over inferences

SLSA places a strong emphasis on explicit attestations about security properties, particularly an artifact’s provenance, rather than inferring security properties from how a system is configured.

Theoretically one could configure access control such that the only path from source to release is through the official channels: the CI/CD system only pulls from the proper source, package registry only allows access to the CI/CD system, and so on. In practice, that is almost impossible to get right and keep right. There are often over-provisioning, confused deputy problems, or mistakes. Even if it is configured properly at one moment, there is no guarantee that it will continue to stay so. And humans almost always end up getting in the access control lists.

Instead, SLSA prefers explicit attestations about an artifact’s provenance. Access control is still important, but SLSA provides a defense in depth: it is not sufficient to be granted access to publish a package; one must also have proof in the form of attestations that a package was built correctly. This way, the intermediate systems are taken out of the trust base and any individuals that are accidentally granted access do not have sufficient permission to tamper with the package.


Before diving into the SLSA Levels, we need to establish a core set of terminology and models to describe what we’re protecting.

TODO: Terms we still need to define

TODO: Define these terms before the v1.0 release.

  • Ecosystem
  • Project

Software supply chain

SLSA’s framework addresses every step of the software supply chain - the sequence of steps resulting in the creation of an artifact. We represent a supply chain as a directed acyclic graph of sources, builds, dependencies, and packages. One artifact’s supply chain is a combination of its dependencies’ supply chains plus its own sources and builds.

Software Supply Chain Model

Term Description Example
Artifact An immutable blob of data; primarily refers to software, but SLSA can be used for any artifact. A file, a git commit, a directory of files (serialized in some way), a container image, a firmware image.
Attestation An authenticated statement (metadata) about a software artifact or collection of software artifacts. A signed SLSA Provenance file.
Source Artifact that was directly authored or reviewed by persons, without modification. It is the beginning of the supply chain; we do not trace the provenance back any further. Git commit (source) hosted on GitHub (platform).
Build Process that transforms a set of input artifacts into a set of output artifacts. The inputs may be sources, dependencies, or ephemeral build outputs. .travis.yml (process) run by Travis CI (platform).
Package Artifact that is “published” for use by others. In the model, it is always the output of a build process, though that build process can be a no-op. Docker image (package) distributed on DockerHub (platform). A ZIP file containing source code is a package, not a source, because it is built from some other source, such as a git commit.
Dependency Artifact that is an input to a build process but that is not a source. In the model, it is always a package. Alpine package (package) distributed on Alpine Linux (platform).

Build model

We model a build as running on a multi-tenant platform, where each execution is independent. A tenant defines the build by specifying parameters through some sort of external interface, often including a reference to a configuration file in source control. The platform then runs the build by interpreting the parameters, fetching some initial set of dependencies, initializing the build environment, and then starting execution. The build then performs arbitrary steps, possibly fetching additional dependencies, and outputs one or more artifacts. Finally, for SLSA Build L2+, the platform outputs provenance describing this whole process.

Model Build

Primary Term Description
Platform System that allows tenants to run builds. Technically, it is the transitive closure of software and services that must be trusted to faithfully execute the build.
Build Process that converts input sources and dependencies into output artifacts, defined by the tenant and executed within a single environment on a platform.
Steps The set of actions that comprise a build, defined by the tenant.
Environment Machine, container, VM, or similar in which the build runs, initialized by the platform. In the case of a distributed build, this is the collection of all such machines/containers/VMs that run steps.
External parameters The set of top-level, independent inputs to the build, specified by a tenant and used by the platform to initialize the build.
Dependencies Artifacts fetched during initialization or execution of the build process, such as configuration files, source artifacts, or build tools.
Outputs Collection of artifacts produced by the build.
Provenance Attestation (metadata) describing how the outputs were produced, including identification of the platform and external parameters.

Notably, there is no formal notion of “source” in the build model, just parameters and dependencies. Most build platforms have an explicit “source” artifact to be built, which is often a git repository; in the build model, the reference to this artifact is a parameter while the artifact itself is a dependency.

For examples on how this model applies to real-world build systems, see index of build types.

Verification model

Verification in SLSA is performed in two ways. Firstly, the build system is certified to ensure conformance with the requirements at the level claimed by the build system. This certification should happen on a recurring cadence with the outcomes published by the platform operator for their users to review and make informed decisions about which builders to trust.

Secondly, artifacts are verified to ensure they meet the producer defined expectations of where the package source code was retrieved from and on what build system the package was built.

Verification Model

Term Description
Expectations A set of constraints on the package’s provenance metadata. The package producer sets expectations for a package, whether explicitly or implicitly.
Provenance verification Artifacts are verified by the package ecosystem to ensure that the package’s expectations are met before the package is used.
Build system certification Build systems are certified for their conformance to the SLSA requirements at the stated level.

The examples below suggest some ways that expectations and verification may be implemented for different, broadly defined, package ecosystems.

Example: Small software team
Term Example
Expectations Defined by the producer’s security personnel and stored in a database.
Provenance verification Performed automatically on cluster nodes before execution by querying the expectations database.
Build system certification The build system implementer follows secure design and development best practices, does annual penetration testing exercises, and self-certifies their conformance to SLSA requirements.
Example: Open source language distribution
Term Example
Expectations Defined separately for each package and stored in the package registry.
Provenance verification The language distribution registry verifies newly uploaded packages meet expectations before publishing them. Further, the package manager client also verifies expectations prior to installing packages.
Build system certification Performed by the language ecosystem packaging authority.

Producing artifacts

This section covers the detailed technical requirements for producing artifacts at each SLSA level. The intended audience is system implementers and security engineers.

For an informative description of the levels intended for all audiences, see Levels. For background, see Terminology. To better understand the reasoning behind the requirements, see Threats and mitigations.

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119.


Build levels

Responsibility to implement SLSA is spread across the following parties.

Implementer Requirement Degree L1L2L3
Producer Choose an appropriate build system
Follow a consistent build process
Distribute provenance
Build system Provenance generation Exists
Isolation strength Build service
Ephemeral and isolated

Security Best Practices

While the exact definition of what constitutes a secure system is beyond the scope of this specification, to be conformant all implementations MUST use industry security best practices. This includes, but is not limited to, using proper access controls, securing communications, implementing proper management of cryptographic secrets, doing frequent updates, and promptly fixing known vulnerabilities.

Various relevant standards and guides can be consulted for that matter such as the CIS Critical Security Controls.


A package’s producer is the organization that owns and releases the software. It might be an open-source project, a company, a team within a company, or even an individual.

Choose an appropriate build system

The producer MUST select a build system that is capable of reaching their desired SLSA Build Level.

For example, if a producer wishes to produce a Build Level 3 artifact, they MUST choose a builder capable of producing Build Level 3 provenance.

Follow a consistent build process

The producer MUST build their artifact in a consistent manner such that verifiers can form expectations about the build process. In some implemenatations, the producer MAY provide explicit metadata to a verifier about their build process. In others, the verifier will form their expectations implicitly (e.g. trust on first use).

For example, if a producer wishes to distribute their artifact through a package ecosystem that requires explicit metadata about the build process in the form of a config file. That metadata includes the artifact’s source repository and build parameters that stay constant between builds. The producer MUST complete that config file and keep it up to date.

Distribute provenance

The producer MUST distribute provenance to artifact consumers. The producer MAY delegate this responsibility to the package ecosystem, provided that the package ecosystem is capable of distributing provenance.

Build system

A package’s build system is the infrastructure used to transform the software from source to package. This includes the transitive closure of all hardware, software, persons, and organizations that can influence the build. A build system is often a hosted, multi-tenant build service, but it could be a system of multiple independent rebuilders, a special-purpose build system used by a single software project, or even a developer’s workstation. Ideally, one build system is used by many different software packages so that consumers can minimize the number of trusted systems. For more background, see Build Model.

The build system is responsible for providing two things: provenance generation and isolation between builds. The Build level describes the degree to which each of these properties is met.

Provenance generation

The build system is responsible for generating provenance describing how the package was produced.

The SLSA Build level describes the minimum bound for:

  • Completeness: What information is contained in the provenance?
  • Authenticity: How strongly can the provenance be tied back to the builder?
  • Accuracy: How resistant is the provenance generation to tampering within the build process?
Provenance Exists

The build process MUST generate provenance that unambiguously identifies the output package and describes how that package was produced.

The format MUST be acceptable to the package ecosystem and/or consumer. It is RECOMMENDED to use the SLSA Provenance format and associated suite because it is designed to be interoperable, universal, and unambiguous when used for SLSA. See that format’s documentation for requirements and implementation guidelines. If using an alternate format, it MUST contain the equivalent information as SLSA Provenance at each level and SHOULD be bi-directionally translatable to SLSA Provenance.

  • Completeness: Best effort. The provenance at L1 SHOULD contain sufficient information to catch mistakes and simulate the user experience at higher levels in the absence of tampering. In other words, the contents of the provenance SHOULD be the same at all Build levels, but a few fields MAY be absent at L1 if they are prohibitively expensive to implement.
  • Authenticity: No requirements.
  • Accuracy: No requirements.
Provenance is Authentic

Authenticity: Consumers MUST be able to validate the authenticity of the provenance attestation in order to:

  • Ensure integrity: Verify that the digital signature of the provenance attestation is valid and the provenance was not tampered with after the build.
  • Define trust: Identify the build system and other entities that are necessary to trust in order to trust the artifact they produced.

This SHOULD be through a digital signature from a private key accessible only to the service that generated the provenance attestation.

This allows the consumer to trust the contents of the provenance attestation, such as the identity of the build system.

Accuracy: The provenance MUST be generated by the build system (i.e. within the trust boundary identified in the provenance) and not by a tenant of the build system (i.e. outside the trust boundary).

  • The data in the provenance MUST be obtained from the build service, either because the generator is the build service or because the provenance generator reads the data directly from the build service.
  • The build system MUST have some security control to prevent tenants from tampering with the provenance. However, there is no minimum bound on the strength. The purpose is to deter adversaries who might face legal or financial risk from evading controls.

Completeness: SHOULD be complete, but there MAY be external parameters that are not sufficiently captured in the provenance.

Provenance is Non-forgeable

Accuracy: Provenance MUST be strongly resistant to influence by tenants.

  • Any secret material used to demonstrate the non-forgeable nature of the provenance, for example the signing key used to generate a digital signature, MUST be stored in a secure management system appropriate for such material and accessible only to the build service account.
  • Such secret material MUST NOT be accessible to the environment running the user-defined build steps.
  • Every field in the provenance MUST be generated or verified by the build service in a trusted control plane. The user-controlled build steps MUST NOT be able to inject or alter the contents.

Completeness: MUST be complete. In particular, the external parameters MUST be fully enumerated in the provenance.

Note: This requirement was called “non-falsifiable” in the initial draft version (v0.1).

Isolation strength

The build system is responsible for isolating between builds, even within the same tenant project. In other words, how strong of a guarantee do we have that the build really executed correctly, without external influence?

The SLSA Build level describes the minimum bar for isolation strength. For more information on assessing a build system’s isolation strength, see Verifying build systems.

Build service

All build steps ran using some build service, not on a maintainer’s workstation.

Examples: GitHub Actions, Google Cloud Build, Travis CI.

Ephemeral and isolated

The build service ensured that the build steps ran in an ephemeral and isolated environment provisioned solely for this build, free of influence from other build instances, whether prior or concurrent.

  • It MUST NOT be possible for a build to access any secrets of the build service, such as the provenance signing key.
  • It MUST NOT be possible for two builds that overlap in time to influence one another.
  • It MUST NOT be possible for one build to persist or influence the build environment of a subsequent build.
  • Build caches, if used, MUST be purely content-addressable to prevent tampering.
  • The build SHOULD NOT call out to remote execution unless it’s considered part of the “builder” within the trust boundary.
  • The build SHOULD NOT open services that allow for remote influence.

Note: This requirement was split into “Isolated” and “Ephemeral Environment” the initial draft version (v0.1).

Verifying Build Systems

The provenance consumer is responsible for deciding whether they trust a builder to produce SLSA Build L3 provenance. However, assessing Build L3 capabilities requires information about a builder’s construction and operating procedures that the consumer cannot glean from the provenance itself. To aid with such assessments, we provide a common threat model and builder model for reasoning about builders’ security. We also provide a questionnaire that organizations can use to describe their builders to consumers along with sample answers that do and do not satisfy the SLSA Build L3 requirements.

Threat Model

Attacker Goal

The attacker’s primary goal is to tamper with a build to create unexpected, vulnerable, or malicious behavior in the output artifact while avoiding detection. Their means of doing so is generating build provenance that does not faithfully represent the built artifact’s origins or build process.

More formally, if a build with external parameters P would produce an artifact with binary hash X and a build with external parameters P’ would produce an artifact with binary hash Y, they wish to produce provenance indicating a build with external parameters P produced an artifact with binary hash Y.

This diagram represents a successful attack:


Note: Platform abuse (e.g. running non-build workloads) and attacks against builder availability are out of scope of this document.

TODO: Align/cross-reference with SLSA Provenance Model.

TODO: Redraw diagrams in the style used by the rest of the site.

Types of attackers

We consider three attacker profiles differentiated by the attacker’s capabilities and privileges as related to the build they wish to subvert (the “target build”).

TODO: Tie attack profiles into the rest of this section.

Project contributors


  • Create builds on the build service. These are the attacker’s controlled builds.
  • Modify one or more controlled builds’ external parameters.
  • Modify one or more controlled builds’ environments and run arbitrary code inside those environments.
  • Read the target build’s source repo.
  • Fork the target build’s source repo.
  • Modify a fork of the target build’s source repo and build from it.
Project maintainer


  • All listed under “low privilege”.
  • Create new builds under the target build’s project or identity.
  • Modify the target build’s source repo and build from it.
  • Modify the target build’s configuration.
Build service admin


  • All listed under “low privilege” and “medium privilege”.
  • Run arbitrary code on the build service.
  • Read and modify network traffic.
  • Access the control plane’s cryptographic secrets.
  • Remotely access build executors (e.g. via SSH).

TODO: List other high-privilege capabilities. TODO: Maybe differentiate between unilateral and non-unilateral privileges.

Build Model

The build model consists of five components: parameters, the control plane, one or more build executors, a build cache, and output storage. The data flow between these components is shown in the diagram below.


TODO: Align with provenance and build models.

The following subsections detail each element of the build model and prompts for assessing its ability to produce SLSA Build L3 provenance.

External Parameters

External parameters are the external interface to the builder and include all inputs to the build process. Examples include the source to be built, the build definition/script to be executed, user-provided instructions to the control plane for how to create the build executor (e.g. which operating system to use), and any additional user-provided strings.

Prompts for Assessing External Parameters
  • How does the control plane process user-provided external parameters? Examples: sanitizing, parsing, not at all
  • Which external parameters are processed by the control plane and which are processed by the executor?
  • What sort of external parameters does the control plane accept for executor configuration?
  • How do you ensure that all external parameters are represented in the provenance?
  • How will you ensure that future design changes will not add additional external parameters without representing them in the provenance?

Control Plane

The control plane is the build system component that orchestrates each independent build execution. It is responsible for setting up each build and cleaning up afterwards. At SLSA Build L2+ the control plane generates and signs provenance for each build performed on the build service. The control plane is operated by one or more administrators, who have privileges to modify the control plane.

Prompts for Assessing Control Planes
  • Administration

    • What are the ways an employee can use privileged access to influence a build or provenance generation? Examples: physical access, terminal access, access to cryptographic secrets
    • What controls are in place to detect or prevent the employee from abusing such access? Examples: two-person approvals, audit logging, workload identities
    • Roughly how many employees have such access?
    • How are privileged accounts protected? Examples: two-factor authentication, client device security policies
    • What plans do you have for recovering from security incidents and system outages? Are they tested? How frequently?
  • Provenance generation

    • How does the control plane observe the build to ensure the provenance’s accuracy?
    • Are there situations in which the control plane will not generate provenance for a completed build? What are they?
  • Development practices

    • How do you track the control plane’s software and configuration? Example: version control
    • How do you build confidence in the control plane’s software supply chain? Example: SLSA L3+ provenance, build from source
    • How do you secure communications between builder components? Example: TLS with certificate transparency.
    • Are you able to perform forensic analysis on compromised executors? How? Example: retain base images indefinitely
  • Creating executors

    • How does the control plane share data with executors? Example: mounting a shared file system partition
    • How does the control plane protect its integrity from executors? Example: not mount its own file system partitions on executors
    • How does the control plane prevent executors from accessing its cryptographic secrets? Examples: dedicated secret storage, not mounting its own file system partitions on executors, hardware security modules
  • Managing cryptographic secrets

    • How do you store the control plane’s cryptographic secrets?
    • Which parts of the organization have access to the control plane’s cryptographic secrets?
    • What controls are in place to detect or prevent employees abusing such access? Examples: two-person approvals, audit logging
    • How are secrets protected in memory? Examples: secrets are stored in hardware security modules and backed up in secure cold storage
    • How frequently are cryptographic secrets rotated? Describe the rotation process.
    • What is your plan for remediating cryptographic secret compromise? How frequently is this plan tested?


The build executor is the independent execution environment where the build takes place. Each executor must be isolated from the control plane and from all other executors, including executors running builds from the same build user or project. Build users are free to modify the environment inside the executor arbitrarily. Build executors must have a means to fetch input artifacts (source, dependencies, etc).

Prompts for Assessing Executors
  • Isolation technologies

    • How are executors isolated from the control plane and each other? Examples: VMs, containers, sandboxed processes
    • How have you hardened your executors against malicious tenants? Examples: configuration hardening, limiting attack surface
    • How frequently do you update your isolation software?
    • What is your process for responding to vulnerability disclosures? What about vulnerabilities in your dependencies?
    • What prevents a malicious build from gaining persistence and influencing subsequent builds?
  • Creation and destruction

    • What tools and environment are available in executors on creation? How were the elements of this environment chosen? Examples: A minimal Linux distribution with its package manager, OSX with HomeBrew
    • How long could a compromised executor remain active in the build system?
  • Network access

    • Are executors able to call out to remote execution? If so, how do you prevent them from tampering with the control plane or other executors over the network?
    • Are executors able to open services on the network? If so, how do you prevent remote interference through these services?


Builders may have zero or more caches to store frequently used dependencies. Build executors may have either read-only or read-write access to caches.

Prompts for Assessing Caches
  • What sorts of caches are available to build executors?
  • How are those caches populated?
  • How are cache contents validated before use?

Output Storage

Output Storage holds built artifacts and their provenance. Storage may either be shared between build projects or allocated separately per-project.

Prompts for Assessing Output Storage
  • How do you prevent builds from reading or overwriting files that belong to another build? Example: authorization on storage
  • What processing, if any, does the control plane do on output artifacts?

Builder Evaluation

Organizations can either self-attest to their answers or seek an audit/certification from a third party. Questionnaires for self-attestation should be published on the internet. Questionnaires for third-party certification need not be published. All provenance generated by L3+ builders must contain a non-forgeable attestation of the builder’s L3+ capabilities with a limited validity period. Any secret materials used to prove the non-forgeability of the attestation must belong to the attesting party.

TODO: Add build system attestation spec

Threats and mitigations

Attacks can occur at every link in a typical software supply chain, and these kinds of attacks are increasingly public, disruptive, and costly in today’s environment.

SLSA’s levels are designed to mitigate the risk of these attacks. This section enumerates possible attacks throughout the supply chain and shows how SLSA can help. For a background, see Terminology.


Supply Chain Threats

SLSA’s primary focus is supply chain integrity, with a secondary focus on availability. Integrity means protection against tampering or unauthorized modification at any stage of the software lifecycle. Within SLSA, we divide integrity into source integrity vs build integrity.

Source integrity: Ensure that all changes to the source code reflect the intent of the software producer. Intent of an organization is difficult to define, so SLSA approximates this as approval from two authorized representatives.

Build integrity: Ensure that the package is built from the correct, unmodified sources and dependencies according to the build recipe defined by the software producer, and that artifacts are not modified as they pass between development stages.

Availability: Ensure that the package can continue to be built and maintained in the future, and that all code and change history is available for investigations and incident response.

Real-world examples

TODO: Update this for v1.0.

Many recent high-profile attacks were consequences of supply-chain integrity vulnerabilities, and could have been prevented by SLSA’s framework. For example:

Integrity threat Known example How SLSA can help
A Submit unauthorized change (to source repo) Linux hypocrite commits: Researcher attempted to intentionally introduce vulnerabilities into the Linux kernel via patches on the mailing list. Two-person review caught most, but not all, of the vulnerabilities.
B Compromise source repo PHP: Attacker compromised PHP's self-hosted git server and injected two malicious commits. A better-protected source code platform would have been a much harder target for the attackers.
C Build from modified source (not matching source repo) Webmin: Attacker modified the build infrastructure to use source files not matching source control. A SLSA-compliant build server would have produced provenance identifying the actual sources used, allowing consumers to detect such tampering.
D Compromise build process SolarWinds: Attacker compromised the build platform and installed an implant that injected malicious behavior during each build. Higher SLSA levels require stronger security controls for the build platform, making it more difficult to compromise and gain persistence.
E Use compromised dependency (i.e. A-H, recursively) event-stream: Attacker added an innocuous dependency and then later updated the dependency to add malicious behavior. The update did not match the code submitted to GitHub (i.e. attack F). Applying SLSA recursively to all dependencies would have prevented this particular vector, because the provenance would have indicated that it either wasn't built from a proper builder or that the source did not come from GitHub.
F Upload modified package (not matching build process) CodeCov: Attacker used leaked credentials to upload a malicious artifact to a GCS bucket, from which users download directly. Provenance of the artifact in the GCS bucket would have shown that the artifact was not built in the expected manner from the expected source repo.
G Compromise package repo Attacks on Package Mirrors: Researcher ran mirrors for several popular package repositories, which could have been used to serve malicious packages. Similar to above (F), provenance of the malicious artifacts would have shown that they were not built as expected or from the expected source repo.
H Use compromised package Browserify typosquatting: Attacker uploaded a malicious package with a similar name as the original. SLSA does not directly address this threat, but provenance linking back to source control can enable and enhance other solutions.
Availability threat Known example How SLSA can help
E Dependency becomes unavailable Mimemagic: Maintainer intentionally removes package or version of package from repository with no warning. Network errors or service outages may also make packages unavailable temporarily. SLSA does not directly address this threat.

A SLSA level helps give consumers confidence that software has not been tampered with and can be securely traced back to source—something that is difficult, if not impossible, to do with most software today.

Threats in detail

IMPORTANT: This is a work in progress.

What follows is a comprehensive technical analysis of supply chain threats and their corresponding mitigations in SLSA. The goals are to:

  • Explain the reasons for each of the SLSA requirements.
  • Increase confidence that the SLSA requirements are sufficient to achieve the desired level of integrity protection.
  • Help implementers better understand what they are protecting against so that they can better design and implement controls.

Source integrity threats

A source integrity threat is a potential for an adversary to introduce a change to the source code that does not reflect the intent of the software producer. This includes the threat of an authorized developer introducing an unauthorized change—in other words, an insider threat.

SLSA v1.0 does not address source integrity, though we anticipate a Source track might do so in a future version. In the meantime, the threats and potential mitigations listed here show how SLSA v1.0 can fit into a broader supply chain security program.

(A) Submit unauthorized change

An adversary introduces a change through the official source control management interface without any special administrator privileges.

(A1) Submit change without review
Directly submit without review

Threat: Submit bad code to the source repository without another person reviewing.

Mitigation: Source repository requires two-person approval for all changes.

Example: Adversary directly pushes a change to a GitHub repo’s main branch. Solution: Configure GitHub’s “branch protection” feature to require pull request reviews on the main branch.

Review own change through a sock puppet account

Threat: Propose a change using one account and then approve it using another account.

Mitigation: Source repository requires approval from two different, trusted persons. If the proposer is trusted, only one approval is needed; otherwise two approvals are needed. The software producer maps accounts to trusted persons.

Example: Adversary creates a pull request using a secondary account and then approves and merges the pull request using their primary account. Solution: Configure branch protection to require two approvals and ensure that all repository contributors and owners map to unique persons.

Use a robot account to submit change

Threat: Exploit a robot account that has the ability to submit changes without two-person review.

Mitigation: All changes require two-person review, even changes authored by robots.

Example: A file within the source repository is automatically generated by a robot, which is allowed to submit without review. Adversary compromises the robot and submits a malicious change without review. Solution: Require human review for these changes.

RFC (#196): This solution may not be practical. Should there be an exception for locked down robot accounts?

Abuse review exceptions

Threat: Exploit a review exception to submit a bad change without review.

Mitigation: All changes require two-person review without exception.

Example: Source repository requires two-person review on all changes except for “documentation changes,” defined as only touching files ending with .md or .html. Adversary submits a malicious executable named without review using this exception, and then builds a malicious package containing this executable. This would pass expectations because the source repository is correct, and the source repository does require two-person review. Solution: Do not allow such exceptions.

RFC: This solution may not be practical in all circumstances. Are there any valid exceptions? If so, how do we ensure they cannot be exploited?

(A2) Evade code review requirements
Modify code after review

Threat: Modify the code after it has been reviewed but before submission.

Mitigation: Source control platform invalidates approvals whenever the proposed change is modified.

Example: Source repository requires two-person review on all changes. Adversary sends a “good” pull request to a peer, who approves it. Adversary then modifies it to contain “bad” code before submitting. Solution: Configure branch protection to dismiss stale approvals when new changes are pushed.

RFC: How do we handle the productivity hit? The cost of code review is already high for most projects, given current code review tooling, so making code review even costlier might not be considered warranted. Are there alternative solutions? Better tooling? Another SLSA level to represent this?

Submit a change that is unreviewable

Threat: Send a change that is meaningless for a human to review that looks benign but is actually malicious.

Mitigation: Code review system ensures that all reviews are informed and meaningful.

Example: A proposed change updates a file, but the reviewer is only presented with a diff of the cryptographic hash, not of the file contents. Thus, the reviewer does not have enough context to provide a meaningful review. Solution: the code review system should present the reviewer with a content diff or some other information to make an informed decision.

Copy a reviewed change to another context

Threat: Get a change reviewed in one context and then transfer it to a different context.

Mitigation: Approvals are context-specific.

Example: MyPackage’s source repository requires two-person review. Adversary forks the repo, submits a change in the fork with review from a colluding colleague (who is not trusted by MyPackage), then merges the change back into the upstream repo. Solution: The merge should still require review, even though the fork was reviewed.

Compromise another account

Threat: Compromise one or more trusted accounts and use those to submit and review own changes.

Mitigation: Source control platform verifies two-factor authentication, which increases the difficulty of compromising accounts.

Example: Trusted person uses a weak password on GitHub. Adversary guesses the weak password, logs in, and pushes changes to a GitHub repo. Solution: Configure GitHub organization to requires 2FA for all trusted persons. This would increase the difficulty of using the compromised password to log in to GitHub.

Hide bad change behind good one

Threat: Request review for a series of two commits, X and Y, where X is bad and Y is good. Reviewer thinks they are approving only the final Y state whereas they are also implicitly approving X.

Mitigation: Only the version that is actually reviewed is the one that is approved. Any intermediate revisions don’t count as being reviewed.

Example: Adversary sends a pull request containing malicious commit X and benign commit Y that undoes X. In the pull request UI, reviewer only reviews and approves “changes from all commits”, which is a delta from HEAD to Y; they don’t see X. Adversary then builds from the malicious revision X. Solution: Expectations do not accept this because the version X is not considered reviewed.

(A3) Code review bypasses that are out of scope of SLSA
Software producer intentionally submits bad code

Threat: Software producer intentionally submits “bad” code, following all proper processes.

Mitigation: Outside the scope of SLSA. Trust of the software producer is an important but separate property from integrity.

Example: A popular extension author sells the rights to a new owner, who then modifies the code to secretly mine bitcoin at the users’ expense. SLSA does not protect against this, though if the extension were open source, regular auditing may discourage this from happening.

Collude with another trusted person

Threat: Two trusted persons collude to author and approve a bad change.

Mitigation: Outside the scope of SLSA. We use “two trusted persons” as a proxy for “intent of the software producer”.

Trick reviewer into approving bad code

Threat: Construct a change that looks benign but is actually malicious, a.k.a. “bugdoor.”

Mitigation: Outside the scope of SLSA.

Reviewer blindly approves changes

Threat: Reviewer approves changes without actually reviewing, a.k.a. “rubber stamping.”

Mitigation: Outside the scope of SLSA.

(B) Compromise source repo

An adversary introduces a change to the source control repository through an administrative interface, or through a compromise of the underlying infrastructure.

Project owner bypasses or disables controls

Threat: Trusted person with “admin” privileges in a repository submits “bad” code bypassing existing controls.

Mitigation: All persons are subject to same controls, whether or not they have administrator privileges. Disabling the controls requires two-person review (and maybe notifies other trusted persons?)

Example 1: GitHub project owner pushes a change without review, even though GitHub branch protection is enabled. Solution: Enable the “Include Administrators” option for the branch protection.

Example 2: GitHub project owner disables “Include Administrators”, pushes a change without review, then re-enables “Include Administrators”. This currently has no solution on GitHub.

Platform admin abuses privileges

Threat: Platform administrator abuses their privileges to bypass controls or to push a malicious version of the software.

Mitigation: TBD

Example 1: GitHostingService employee uses an internal tool to push changes to the MyPackage source repo.

Example 2: GitHostingService employee uses an internal tool to push a malicious version of the server to serve malicious versions of MyPackage sources to a specific CI/CD client but the regular version to everyone else, in order to hide tracks.

Example 3: GitHostingService employee uses an internal tool to push a malicious version of the server that includes a backdoor allowing specific users to bypass branch protections. Adversary then uses this backdoor to submit a change to MyPackage without review.

Exploit vulnerability in SCM

Threat: Exploit a vulnerability in the implementation of the source code management system to bypass controls.

Mitigation: Outside the scope of SLSA.

Build integrity threats

A build integrity threat is a potential for an adversary to introduce behavior to a package that is not reflected in the source code, or to build from a source, dependency, and/or process that is not intended by the software producer.

The SLSA Build track covers these threats when combined with verifying artifacts against expectations.

(C) Build from modified source

An adversary builds from a version of the source code that does not match the official source control repository.

The mitigation here is to compare the provenance against expectations for the package, which depends on SLSA Build L1 for provenance. (Threats against the provenance itself are covered by (D) and (F).)

Build from unofficial fork of code (expectations)

Threat: Build using the expected CI/CD process but from an unofficial fork of the code that may contain unauthorized changes.

Mitigation: Verifier requires the provenance’s source location to match an expected value.

Example: MyPackage is supposed to be built from GitHub repo good/my-package. Instead, it is built from evilfork/my-package. Solution: Verifier rejects because the source location does not match.

Build from unofficial branch or tag (expectations)

Threat: Build using the expected CI/CD process and source location, but checking out an “experimental” branch or similar that may contain code not intended for release.

Mitigation: Verifier requires that the provenance’s source branch/tag matches an expected value, or that the source revision is reachable from an expected branch.

Example: MyPackage’s releases are tagged from the main branch, which has branch protections. Adversary builds from the unprotected experimental branch containing unofficial changes. Solution: Verifier rejects because the source revision is not reachable from main.

Build from unofficial build steps (expectations)

Threat: Build the package using the proper CI/CD platform but with unofficial build steps.

Mitigation: Verifier requires that the provenance’s build configuration source matches an expected value.

Example: MyPackage is expected to be built by Google Cloud Build using the build steps defined in the source’s cloudbuild.yaml file. Adversary builds with Google Cloud Build, but using custom build steps provided over RPC. Solution: Verifier rejects because the build steps did not come from the expected source.

Build from unofficial parameters (expectations)

Threat: Build using the expected CI/CD process, source location, and branch/tag, but using a parameter that injects unofficial behavior.

Mitigation: Verifier requires that the provenance’s external parameters all match expected values.

Example 1: MyPackage is supposed to be built from the release.yml workflow. Adversary builds from the debug.yml workflow. Solution: Verifier rejects because the workflow parameter does not match the expected value.

Example 2: MyPackage’s GitHub Actions Workflow uses github.event.inputs to allow users to specify custom compiler flags per invocation. Adversary sets a compiler flag that overrides a macro to inject malicious behavior into the output binary. Solution: Verifier rejects because the inputs parameter was not expected.

Build from modified version of code modified after checkout (expectations)

Threat: Build from a version of the code that includes modifications after checkout.

Mitigation: Build service pulls directly from the source repository and accurately records the source location in provenance.

Example: Adversary fetches from MyPackage’s source repo, makes a local commit, then requests a build from that local commit. Builder records the fact that it did not pull from the official source repo. Solution: Verifier rejects because the source repo does not match the expected value.

(D) Compromise build process

An adversary introduces an unauthorized change to a build output through tampering of the build process; or introduces false information into the provenance.

These threats are directly addressed by the SLSA Build track.

Compromise build environment of subsequent build (Build L3)

Threat: Perform a “bad” build that persists a change in the build environment, then run a subsequent “good” build using that environment.

Mitigation: Builder ensures that each build environment is ephemeral, with no way to persist changes between subsequent builds.

Example: Build service uses the same machine for subsequent builds. Adversary first runs a build that replaces the make binary with a malicious version, then runs a subsequent build that otherwise would pass expectations. Solution: Builder changes architecture to start each build with a clean machine image.

Compromise parallel build (Build L3)

Threat: Perform a “bad” build that alters the behavior of another “good” build running in parallel.

Mitigation: Builds are isolated from one another, with no way for one to affect the other.

Example: Build service runs all builds for project MyPackage on the same machine as the same Linux user. Adversary starts a “bad” build that listens for the “good” build and swaps out source files, then starts a “good” build that would otherwise pass expectations. Solution: Builder changes architecture to isolate each build in a separate VM or similar.

Steal cryptographic secrets (Build L3)

Threat: Use or exfiltrate the provenance signing key or some other cryptographic secret that should only be available to the build service.

Mitigation: Builds are isolated from the trusted build service control plane, and only the control plane has access to cryptographic secrets.

Example: Provence is signed on the build worker, which the adversary has control over. Adversary uses a malicious process that generates false provenance and signs it using the provenance signing key. Solution: Builder generates and signs provenance in the trusted control plane; the worker has no access to the key.

Set values of the provenance (Build L2-L3)

Threat: Generate false provenance and get the trusted control plane to sign it.

Mitigation: At Build L2+, trusted control plane generates all information that goes in the provenance, except (optionally) the output artifact hash. At Build L3+, this is hardened to prevent compromise even by determined adversaries.

Example 1 (Build L2): Provenance is generated on the build worker, which the adversary has control over. Adversary uses a malicious process to get the build service to claim that it was built from source repo good/my-package when it was really built from evil/my-package. Solution: Builder generates and signs the provenance in the trusted control plane; the worker reports the output artifacts but otherwise has no influence over the provenance.

Example 2 (Build L3): Provenance is generated in the trusted control plane, but workers can break out of the container to access the signing material. Solution: Builder is hardened to provide strong isolation against tenant projects.

Poison the build cache (Build L3)

Threat: Add a “bad” artifact to a build cache that is later picked up by a “good” build process.

Mitigation: Build caches must be isolate between builds to prevent such cache poisoning attacks.

Example: Build system uses a build cache across builds, keyed by the hash of the source file. Adversary runs a malicious build that creates a “poisoned” cache entry with a falsified key, meaning that the value wasn’t really produced from that source. A subsequent build then picks up that poisoned cache entry.

Project owner (TBD)

TODO: similar to Source (do the same threats apply here?)

Platform admin (TBD)

TODO: similar to Source

(E) Use compromised dependency

TODO: What exactly is this about? Is it about compromising the build process through a bad build tool, and/or is it about compromising the output package through a bad library? Does it involve all upstream threats to the dependency, or is it just about this particular use of the package (e.g. tampering on input, or choosing a bad dependency)?

TODO: Fill this out to give more examples of threats from compromised dependencies.

(F) Upload modified package

An adversary uploads a package not built from the proper build process.

Build with untrusted CI/CD (expectations)

Threat: Build using an unofficial CI/CD pipeline that does not build in the correct way.

Mitigation: Verifier requires provenance showing that the builder matched an expected value.

Example: MyPackage is expected to be built on Google Cloud Build, which is trusted up to Build L3. Adversary builds on SomeOtherBuildService, which is only trusted up to Build L2, and then exploits SomeOtherBuildService to inject bad behavior. Solution: Verifier rejects because builder is not as expected.

Upload package without provenance (Build L1)

Threat: Upload a package without provenance.

Mitigation: Verifier requires provenance before accepting the package.

Example: Adversary uploads a malicious version of MyPackage to the package repository without provenance. Solution: Verifier rejects because provenance is missing.

Tamper with artifact after CI/CD (Build L1)

Threat: Take a good version of the package, modify it in some way, then re-upload it using the original provenance.

Mitigation: Verifier checks that the provenance’s subject matches the hash of the package.

Example: Adversary performs a proper build, modifies the artifact, then uploads the modified version of the package to the repository along with the provenance. Solution: Verifier rejects because the hash of the artifact does not match the subject found within the provenance.

Tamper with provenance (Build L2)

Threat: Perform a build that would not pass expectations, then modify the provenance to make the expectations checks pass.

Mitigation: Verifier only accepts provenance with a valid cryptographic signature or equivalent proving that the provenance came from an acceptable builder.

Example: MyPackage is expected to be built by GitHub Actions from the good/my-package repo. Adversary builds with GitHub Actions from the evil/my-package repo and then modifies the provenance so that the source looks like it came from good/my-package. Solution: Verifier rejects because the cryptographic signature is no longer valid.

(G) Compromise package repo

An adversary modifies the package on the package repository using an administrative interface or through a compromise of the infrastructure.

TODO: fill this out

(H) Use compromised package

An adversary modifies the package after it has left the package repository, or tricks the user into using an unintended package.


Threat: Register a package name that is similar looking to a popular package and get users to use your malicious package instead of the benign one.

Mitigation: Mostly outside the scope of SLSA. That said, the requirement to make the source available can be a mild deterrent, can aid investigation or ad-hoc analysis, and can complement source-based typosquatting solutions.

Availability threats

An availability threat is a potential for an adversary to deny someone from reading a source and its associated change history, or from building a package.

SLSA v1.0 does not address availability threats, though future versions might.

(A)(B) Delete the code

Threat: Perform a build from a particular source revision and then delete that revision or cause it to get garbage collected, preventing anyone from inspecting the code.

Mitigation: Some system retains the revision and its version control history, making it available for inspection indefinitely. Users cannot delete the revision except as part of a transparent legal or privacy process.

Example: Adversary submits bad code to the MyPackage GitHub repo, builds from that revision, then does a force push to erase that revision from history (or requests GitHub to delete the repo.) This would make the revision unavailable for inspection. Solution: Verifier rejects the package because it lacks a positive attestation showing that some system, such as GitHub, ensured retention and availability of the source code.

(E) A dependency becomes temporarily or permanently unavailable to the build process

Threat: Unable to perform a build with the intended dependencies.

Mitigation: Outside the scope of SLSA. That said, some solutions to support hermetic and reproducible builds may also reduce the impact of this threat.

Other threats

Threats that can compromise the ability to prevent or detect the supply chain security threats above but that do not fall cleanly into any one category.

Tamper with expectations

Threat: Modify the expectations to accept something that would not otherwise be accepted.

Mitigation: Changes to expectations require some form of authorization, such as two-party review.

Example: Expectations for MyPackage only allows source repo good/my-package. Adversary modifies the expectations to also accept evil/my-package, then builds from that repo and uploads a bad version of the package. Solution: Expectation changes require two-party review.

Forge change metadata

Threat: Forge the change metadata to alter attribution, timestamp, or discoverability of a change.

Mitigation: Source control platform strongly authenticates actor identity, timestamp, and parent revisions.

Example: Adversary submits a git commit with a falsified author and timestamp, and then rewrites history with a non-fast-forward update to make it appear to have been made long ago. Solution: Consumer detects this by seeing that such changes are not strongly authenticated and thus not trustworthy.

Exploit cryptographic hash collisions

Threat: Exploit a cryptographic hash collision weakness to bypass one of the other controls.

Mitigation: Require cryptographically secure hash functions for code review and provenance, such as SHA-256.

Examples: Construct a “good” file and a “bad” file with the same SHA-1 hash. Get the “good” file reviewed and then submit the “bad” file, or get the “good” file reviewed and submitted and then build from the “bad” file. Solution: Only accept cryptographic hashes with strong collision resistance.

Frequently Asked Questions

Q: Why is SLSA not transitive?

SLSA Build levels only cover the trustworthiness of a single build, with no requirements about the build levels of transitive dependencies. The reason for this is to make the problem tractable. If a SLSA Build level required dependencies to be the same level, then reaching a level would require starting at the very beginning of the supply chain and working forward. This is backwards, forcing us to work on the least risky component first and blocking any progress further downstream. By making each artifact’s SLSA rating independent from one another, it allows parallel progress and prioritization based on risk. (This is a lesson we learned when deploying other security controls at scale throughout Google.) We expect SLSA ratings to be composed to describe a supply chain’s overall security stance, as described in the case study vision.

Q: What about reproducible builds?

When talking about reproducible builds, there are two related but distinct concepts: “reproducible” and “verified reproducible.”

“Reproducible” means that repeating the build with the same inputs results in bit-for-bit identical output. This property provides many benefits, including easier debugging, more confident cherry-pick releases, better build caching and storage efficiency, and accurate dependency tracking.

“Verified reproducible” means using two or more independent build systems to corroborate the provenance of a build. In this way, one can create an overall system that is more trustworthy than any of the individual components. This is often suggested as a solution to supply chain integrity. Indeed, this is one option to secure build steps of a supply chain. When designed correctly, such a system can satisfy all of the SLSA Build level requirements.

That said, verified reproducible builds are not a complete solution to supply chain integrity, nor are they practical in all cases:

  • Reproducible builds do not address source, dependency, or distribution threats.
  • Reproducers must truly be independent, lest they all be susceptible to the same attack. For example, if all rebuilders run the same pipeline software, and that software has a vulnerability that can be triggered by sending a build request, then an attacker can compromise all rebuilders, violating the assumption above.
  • Some builds cannot easily be made reproducible, as noted above.
  • Closed-source reproducible builds require the code owner to either grant source access to multiple independent rebuilders, which is unacceptable in many cases, or develop multiple, independent in-house rebuilders, which is likely prohibitively expensive.

Therefore, SLSA does not require verified reproducible builds directly. Instead, verified reproducible builds are one option for implementing the requirements.

For more on reproducibility, see Hermetic, Reproducible, or Verifiable?

Q: How does SLSA relate to in-toto?

in-toto is a framework to secure software supply chains hosted at the Cloud Native Computing Foundation. The in-toto specification provides a generalized workflow to secure different steps in a software supply chain. The SLSA specification recommends in-toto attestations as the vehicle to express Provenance and other attributes of software supply chains. Thus, in-toto can be thought of as the unopinionated layer to express information pertaining to a software supply chain, and SLSA as the opinionated layer specifying exactly what information must be captured in in-toto metadata to achieve the guarantees of a particular level.

in-toto’s official implementations written in Go, Java, and Rust include support for generating SLSA Provenance metadata. These APIs are used in other tools generating SLSA Provenance such as Sigstore’s cosign, the SLSA GitHub Generator, and the in-toto Jenkins plugin.

Future directions

The initial draft version (v0.1) of SLSA had a larger scope including protections against tampering with source code and a higher level of build integrity (Build L4). This section collects some early thoughts on how SLSA might evolve in future version to re-introduce those notions and add other additional aspects of automatable supply chain security.

Build track

Build L4

A build L4 could include further hardening of the build service and enabling corraboration of the provenance, for example by providing complete knowledge of the build inputs.

The initial draft version (v0.1) 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

A Source track could provide protection against tampering of the source code prior to the build.

The initial draft version (v0.1) of SLSA included the following source requirements, which may or may not form the basis for 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.
  1. Alternate means of verifying the authenticity of the provenance are also acceptable.