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 evil.md 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: 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.

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.

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: 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.