Overview

This threat model covers the software supply chain, meaning the process by which software is produced and consumed. We describe and cluster threats based on where in the software development pipeline those threats occur, labeled (A) through (I). This is useful because priorities and mitigations mostly cluster along those same lines. Keep in mind that dependencies are highly recursive, so each dependency has its own threats (A) through (I), and the same for their dependencies, and so on. For a more detailed explanation of the supply chain model, see Terminology.

Importantly, producers and consumers face aggregate risk across all of the software they produce and consume, respectively. Many organizations produce and/or consume thousands of software packages, both first- and third-party, and it is not practical to rely on every individual team in the organization to do the right thing. For this reason, SLSA prioritizes mitigations that can be broadly adopted in an automated fashion, minimizing the chance of mistakes.

Source 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 individual introducing an unauthorized change—in other words, an insider threat.

SLSA v1.0 does not address source threats, but we anticipate doing 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) Producer

The producer of the software intentionally produces code that harms the consumer, or the producer otherwise uses practices that are not deserving of the consumer’s trust.

Threats in this category likely cannot be mitigated through controls placed during the authoring/reviewing process, in contrast with (B).

Software producer intentionally submits bad code

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

Mitigation: TODO

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

(B) Authoring & reviewing

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

Threats in this category can be mitigated by code review or some other controls during the authoring/reviewing process, at least in theory. Contrast this with (A), where such controls are likely ineffective.

(B1) 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.

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 evil.md without review using this exception, and then builds a malicious package containing this executable. This would pass the policy because the source repository is correct, and the source repository does require two-person review. Solution: Do not allow such exceptions.

(B2) 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.

Note: This is not currently a SLSA requirement because the productivity hit is considered too great to outweigh the security benefit. The cost of code review is already too high for most projects, given current code review tooling, so making code review even costlier would not further our goals. However, this should be considered for future SLSA revisions once the state-of-the-art for code review has improved and the cost can be minimized.

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: Policy does not accept this because the version X is not considered reviewed.

(B3) Render code review ineffective

Collude with another trusted person

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

Mitigation: This threat is not currently addressed by 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: This threat is not currently addressed by SLSA.

Reviewer blindly approves changes

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

Mitigation: This threat is not currently addressed by SLSA.

(C) Source code management

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: The source platform must have controls in place to prevent and detect abusive behavior from administrators (e.g. two-person approvals for changes to the infrastructure, audit logging). A future Platform Operations Track may provide more specific guidance on how to secure the underlying platform.

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: This threat is not currently addressed by SLSA.

Build threats

A build integrity threat is a potential for an adversary to introduce behavior to an artifact without changing its source code, or to build from a source, dependency, and/or process that is not intended by the software producer.

The SLSA Build track mitigates these threats when the consumer verifies artifacts against expectations, confirming that the artifact they received was built in the expected manner.

(D) External build parameters

An adversary builds from a version of the source code that does not match the official source control repository, or changes the build parameters to inject behavior that was not intended by the official source.

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 (E) 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 platform 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.

(E) 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.

Forge values of the provenance (other than output digest) (Build L2+)

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

Mitigation: At Build L2+, the 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 platform 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.

Forge output digest of the provenance (n/a)

Threat: The tenant-controlled build process sets output artifact digest (subject in SLSA Provenance) without the trusted control plane verifying that such an artifact was actually produced.

Mitigation: None; this is not a problem. Any build claiming to produce a given artifact could have actually produced it by copying it verbatim from input to output.² (Reminder: Provenance is only a claim that a particular artifact was built, not that it was published to a particular registry.)

Example: A legitimate MyPackage artifact has digest abcdef and is built from source repo good/my-package. A malicious build from source repo evil/my-package claims that it built artifact abcdef when it did not. Solution: Verifier rejects because the source location does not match; the forged digest is irrelevant.

Compromise project owner (Build L2+)

Threat: An adversary gains owner permissions for the artifact’s build project.

Mitigation: The build project owner must not have the ability to influence the build process or provenance generation.

Example: MyPackage is built on Awesome Builder under the project “mypackage”. Adversary is an administrator of the “mypackage” project. Awesome Builder allows administrators to debug build machines via SSH. An adversary uses this feature to alter a build in progress.

Compromise other build (Build L3)

Threat: Perform a malicious build that alters the behavior of a benign build running in parallel or subsequent environments.

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

Example 1: A build platform runs all builds for project MyPackage on the same machine as the same Linux user. An adversary starts a malicious build that listens for another build and swaps out source files, then starts a benign build. The benign build uses the malicious build’s source files, but its provenance says it used benign source files. Solution: The build platform changes architecture to isolate each build in a separate VM or similar.

Example 2: A build platform uses the same machine for subsequent builds. An adversary first runs a build that replaces the make binary with a malicious version, then subsequently runs an otherwise benign build. Solution: The builder changes architecture to start each build with a clean machine image.

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 platform.

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

Example: Provenance 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.

Poison the build cache (Build L3)

Threat: Add a malicious artifact to a build cache that is later picked up by a benign build process.

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

Example: Build platform 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.

Compromise build platform admin (verification)

Threat: An adversary gains admin permissions for the artifact’s build platform.

Mitigation: The build platform must have controls in place to prevent and detect abusive behavior from administrators (e.g. two-person approvals, audit logging).

Example: MyPackage is built on Awesome Builder. Awesome Builder allows engineers on-call to SSH into build machines to debug production issues. An adversary uses this access to modify a build in progress. Solution: Consumers do not accept provenance from the build platform unless they trust sufficient controls are in place to prevent abusing admin privileges.

(F) Artifact publication

An adversary uploads a package artifact that does not reflect the intent of the package’s official source control repository.

This is the most direct threat because it is the easiest to pull off. If there are no mitigations for this threat, then (D) and (E) are often indistinguishable from this threat.

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 SomeOtherBuildPlatform, which is only trusted up to Build L2, and then exploits SomeOtherBuildPlatform to inject malicious 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 benign 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 meet 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) Distribution channel

An adversary modifies the package on the package registry using an administrative interface or through a compromise of the infrastructure including modification of the package in transit to the consumer.

The distribution channel threats and mitigations look very similar to the Artifact Publication (F) threats and mitigations with the main difference being that these threats are mitigated by having the consumer perform verification.

The consumer’s actions may be simplified if (F) produces a VSA. In this case the consumer may replace provenance verification with VSA verification.

Build with untrusted CI/CD (expectations)

Threat: Replace the package with one built 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 or a VSA for corresponding resourceUri.

Example: MyPackage is expected to be built on Google Cloud Build, which is trusted up to Build L3. Adversary builds on SomeOtherBuildPlatform, which is only trusted up to Build L2, and then exploits SomeOtherBuildPlatform to inject malicious behavior. Adversary then replaces the original package within the repository with the malicious package. Solution: Verifier rejects because builder is not as expected.

Issue VSA from untrusted intermediary (expectations)

Threat: Have an unofficial intermediary issue a VSA for a malicious package.

Mitigation: Verifier requires VSAs to be issued by a trusted intermediary.

Example: Verifier expects VSAs to be issued by TheRepository. Adversary builds a malicious package and then issues a VSA of their own for the malicious package. Solution: Verifier rejects because they only accept VSAs from TheRepository which the adversary cannot issue since they do not have the corresponding signing key.

Upload package without provenance or VSA (Build L1)

Threat: Replace the original package with a malicious one without provenance.

Mitigation: Verifier requires provenance or a VSA before accepting the package.

Example: Adversary replaces MyPackage with a malicious version of MyPackage on the package repository and deletes existing provenance. Solution: Verifier rejects because provenance is missing.

Replace package and VSA with another (expectations)

Threat: Replace a package and its VSA with a malicious package and its valid VSA.

Mitigation: Consumer ensures that the VSA matches the package they’ve requested (not just the package they received) by following the verification process.

Example: Adversary uploads a malicious package to repo/evil-package, getting a valid VSA for repo/evil-package. Adversary then replaces repo/my-package and its VSA with repo/evil-package and its VSA. Solution: Verifier rejects because the VSA resourceUri field lists repo/evil-package and not the expected repo/my-package.

Tamper with artifact after upload (Build L1)

Threat: Take a benign version of the package, modify it in some way, then replace it while retaining the original provenance or VSA.

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

Example: Adversary performs a proper build, modifies the artifact, then replaces the modified version of the package in the repository and retains the original provenance. Solution: Verifier rejects because the hash of the artifact does not match the subject found within the provenance.

Tamper with provenance or VSA (Build L2)

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

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

Example 1: 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.

Example 2: Verifier expects VSAs to be issued by TheRepository. Adversary builds a malicious package and then modifies the original VSA’s subject field to match the digest of the malicious package. Solution: Verifier rejects because the cryptographic signature is no longer valid.

Usage threats

A usage threat is a potential for an adversary to exploit behavior of the consumer.

(H) Package selection

The consumer requests a package that it did not intend.

Dependency confusion

Threat: Register a package name in a public registry that shadows a name used on the victim’s internal registry, and wait for a misconfigured victim to fetch from the public registry instead of the internal one.

Mitigation: The mitigation is for the software producer to build internal packages on a SLSA Level 2+ compliant build system and define expectations for build provenance. Expectations must be verified on installation of the internal packages. If a misconfigured victim attempts to install attacker’s package with an internal name but from the public registry, then verification against expectations will fail.

For more information see Verifying artifacts and Defender’s Perspective: Dependency Confusion and Typosquatting Attacks.

Typosquatting

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: This threat is not currently addressed by 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.

(I) Usage

The consumer uses a package in an unsafe manner.

Improper usage

Threat: The software can be used in an insecure manner, allowing an adversary to compromise the consumer.

Mitigation: This threat is not addressed by SLSA, but may be addressed by efforts like Secure by Design.

Dependency threats

A dependency threat is a potential for an adversary to introduce unintended behavior in one artifact by compromising some other artifact that the former depends on at build time. (Runtime dependencies are excluded from the model, as noted below.)

Unlike other threat categories, dependency threats develop recursively through the supply chain and can only be exploited indirectly. For example, if application A includes library B as part of its build process, then a build or source threat to B is also a dependency threat to A. Furthermore, if library B uses build tool C, then a source or build threat to C is also a dependency threat to both A and B.

This version of SLSA does not explicitly address dependency threats, but we expect that a future version will. In the meantime, you can apply SLSA recursively to your dependencies in order to reduce the risk of dependency threats.

Build dependency

An adversary compromises the target artifact through one of its build dependencies. Any artifact that is present in the build environment and has the ability to influence the output is considered a build dependency.

Include a vulnerable dependency (library, base image, bundled file, etc.)

Threat: Statically link, bundle, or otherwise include an artifact that is compromised or has some vulnerability, causing the output artifact to have the same vulnerability.

Example: The C++ program MyPackage statically links libDep at build time. A contributor accidentally introduces a security vulnerability into libDep. The next time MyPackage is built, it picks up and includes the vulnerable version of libDep, resulting in MyPackage also having the security vulnerability.

Mitigation: TODO

Use a compromised build tool (compiler, utility, interpreter, OS package, etc.)

Threat: Use a compromised tool or other software artifact during the build process, which alters the build process and injects unintended behavior into the output artifact.

Mitigation: This can be partially mitigated by treating build tooling, including OS images, as any other artifact to be verified prior to use. The threats described in this document apply recursively to build tooling as do the mitigations and examples. A future Build Environment track may provide more comprehensive guidance on how to address more specfiic aspects of this threat.

Example: MyPackage is a tarball containing an ELF executable, created by running /usr/bin/tar during its build process. An adversary compromises the tar OS package such that /usr/bin/tar injects a backdoor into every ELF executable it writes. The next time MyPackage is built, the build picks up the vulnerable tar package, which injects the backdoor into the resulting MyPackage artifact. Solution: apply SLSA recursively to all build tools prior to the build. The build platform verifies the disk image, or the individual components on the disk image, against the associated provenance or VSAs prior to running a build. Depending on where the initial compromise took place (i.e. before/during vs after the build of the build tool itself), the modified /usr/bin/tar will fail this verification.

Reminder: dependencies that look like runtime dependencies actually become build dependencies if they get loaded at build time.

Use a compromised runtime dependency during the build (for tests, dynamic linking, etc.)

Threat: During the build process, use a compromised runtime dependency (such as during testing or dynamic linking), which alters the build process and injects unwanted behavior into the output.

NOTE: This is technically the same case as Use a compromised build tool. We call it out to remind the reader that runtime dependencies can become build dependencies if they are loaded during the build.

Example: MyPackage has a runtime dependency on package Dep, meaning that Dep is not included in MyPackage but required to be installed on the user’s machine at the time MyPackage is run. However, Dep is also loaded during the build process of MyPackage as part of a test. An adversary compromises Dep such that, when run during a build, it injects a backdoor into the output artifact. The next time MyPackage is built, it picks up and loads Dep during the build process. The malicious code then injects the backdoor into the new MyPackage artifact.

Mitigation: In addition to all the mitigations for build tools, you can often avoid runtime dependencies becoming build dependencies by isolating tests to a separate environment that does not have write access to the output artifact.

Related threats

The following threats are related to “dependencies” but are not modeled as “dependency threats”.

Use a compromised dependency at runtime (modeled separately)

Threat: Load a compromised artifact at runtime, thereby compromising the user or environment where the software ran.

Example: MyPackage lists package Dep as a runtime dependency. Adversary publishes a compromised version of Dep that runs malicious code on the user’s machine when Dep is loaded at runtime. An end user installs MyPackage, which in turn installs the compromised version of Dep. When the user runs MyPackage, it loads and executes the malicious code from Dep.

Mitigation: N/A - This threat is not currently addressed by SLSA. SLSA’s threat model does not explicitly model runtime dependencies. Instead, each runtime dependency is considered a distinct artifact with its own threats.

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: An adversary submits malicious code to the MyPackage GitHub repo, builds from that revision, then does a force push to erase that revision from history (or requests that GitHub 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.

A dependency becomes temporarily or permanently unavailable to the build process

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

Mitigation: This threat is not currently addressed by SLSA. That said, some solutions to support hermetic and reproducible builds may also reduce the impact of this threat.

De-list artifact

Threat: The package registry stops serving the artifact.

Mitigation: N/A - This threat is not currently addressed by SLSA.

De-list provenance

Threat: The package registry stops serving the provenance.

Mitigation: N/A - This threat is not currently addressed by SLSA.

Verification threats

Threats that can compromise the ability to prevent or detect the supply chain security threats above.

Tamper with recorded expectations

Threat: Modify the verifier’s recorded expectations, causing the verifier to accept an unofficial package artifact.

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

Example: The package ecosystem records its expectations for a given package name in a configuration file that is modifiable by that package’s producer. The configuration for MyPackage expects the source repository to be good/my-package. The adversary modifies the configuration to also accept evil/my-package, and then builds from that repository and uploads a malicious version of the package. Solution: Changes to the recorded expectations 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 commit checksums and provenance subjects, such as SHA-256.

Examples: Construct a benign file and a malicious file with the same SHA-1 hash. Get the benign file reviewed and then submit the malicious file. Alternatively, get the benign file reviewed and submitted and then build from the malicious file. Solution: Only accept cryptographic hashes with strong collision resistance.