gh-trusted-builds-app

An example of using GitHub Actions reusable workflows to create and deploy a trusted software artifact.

Background

These days, companies of all sizes and industries create software, from small scripts to complicated back-of-house inventory forecasting systems. For both security and compliance reasons, companies want to be able to demonstrate that the software they're using is secure. One way to achieve this goal is to have a trusted build system that can produce signed statements about the software artifacts it produces. An authenticated statement like this is called an attestation. Throughout the lifecycle of the artifact, the build system can produce attestations of different types in order to demonstrate that certain events occurred. For example:

multiple reviewers approved a particular code change
this container image was built from this code commit
a container image was scanned for vulnerabilities

Later on, when the software is deployed, the deployment system can check for these attestations and verify that they meet the company's policy for software. Any attempted deployments that fail to meet the policy can be rejected, thereby preventing low quality or malicious software from entering the production environment.

This repository is a demonstration of using GitHub Actions as that trusted build system. It's part of a number of repositories that you can find using the automated-governance topic:

liatrio/github-trusted-builds-app: a very simple Go application
liatrio/github-trusted-builds-workflows: reusable workflows designed to be used by application teams
liatrio/github-trusted-builds-attestations: a tool for creating attestations of different types
liatrio/github-trusted-builds-policy: Rego policy used to evaluate attestations

Scenario

In this demonstration, multiple teams must approve a software artifact for deployment to production. As a shorthand, we'll call these teams the "central" teams for their particular domain; as opposed to "application" teams, who are responsible for the development and maintenance of one or more applications.

First up is the platform team, which is responsible for providing a Kubernetes cluster and deployment tooling. To facilitate automatic approvals, the platform team maintains a reusable GitHub Actions workflow that builds container images from source and pushes them to GitHub Container Registry. The images are annotated and signed by the workflow; in addition, a record of the signature is also uploaded to Rekor, which is a transparency log for supply chain security. In order to use the workflow, application teams only have to specify the reference to it in a job:

jobs:
  build-and-push:
    uses: liatrio/gh-trusted-builds-workflows/.github/workflows/build-and-push.yaml@main

The security team also needs to approve which artifacts are deployed to production. Like the platform team, they provide a reusable workflow that scans container images for vulnerabilities and another workflow that evaluates if an artifact meets enterprise policy. This latter workflow produces a verification summary attestation (VSA) which indicates if the artifact passed or failed policy. Later on, the platform team can check for the existence of this VSA and approve or deny deployment accordingly.

Technologies

Before diving into the workflows, it may be helpful to briefly review the core technologies used by the demo.

in-toto Attestations

Much of this demo is involved with producing attestations, or signed statements about a software artifact. A popular format for attestations comes from the in-toto project, which is focused on software supply-chain security. While this demonstration does not use in-toto directly, many open source projects outside in-toto use the in-toto attestation format; this includes all the attestations created in this demo.

The in-toto format consists of several pieces:

predicate: structured information about a software artifact. The predicate type determines the structure and type of data available.
statement: contains the predicate and provides a list of subjects (software artifacts) described by the predicate.
envelope: the wrapper for the statement and any signatures.

Here's an example of an in-toto attestation:

{
  "payloadType": "application/vnd.in-toto+json",
  "payload": "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",
  "signatures": [
    {
      "keyid": "",
      "sig": "MEUCICYn68n2eOij6SLpgnzz1lyrW5dSixGRambvA/625DwiAiEAktVa8wx6mqSYpzzsVWUzcaAZcLsQYs/paYFRJGpSx2o="
    }
  ]
}

This is the envelope, which contains the in-toto statement in the payload field. Decoding the payload field shows the statement:

example in-toto statement (click to expand)

{
  "_type": "https://in-toto.io/Statement/v1",
  "subject": [
    {
      "name": "ghcr.io/liatrio/gh-trusted-builds-app",
      "digest": {
        "sha256": "4d9e77fedca29393dff9a39b065d158aaa9644c18b2c51f75fcaf84b8c941abf"
      }
    }
  ],
  "predicateType": "https://slsa.dev/verification_summary/v0.2",
  "predicate": {
    "input_attestations": [
      {
        "digest": {
          "sha256": "f140f88609749ad99e0583c52ecd5cf16c2e5e2c50cd6140364a991cc1af22da"
        },
        "uri": "https://rekor.sigstore.dev/api/v1/log/entries?logIndex=21449831"
      },
      {
        "digest": {
          "sha256": "e7cc98968e902f0063b9ad492b22ec2d0a647f343ed1689c0a37cb3ef76f9c31"
        },
        "uri": "https://rekor.sigstore.dev/api/v1/log/entries?logIndex=21449834"
      },
      {
        "digest": {
          "sha256": "995ef5f608938f03bf7415b6523a5ec1ca20d1f1cd37d650c233aeb3658fb1fa"
        },
        "uri": "https://rekor.sigstore.dev/api/v1/log/entries?logIndex=21449861"
      }
    ],
    "policy": {
      "uri": "https://github.com/liatrio/gh-trusted-builds-policy/releases/download/v1.1.1/bundle.tar.gz"
    },
    "policy_level": "SLSA_LEVEL_3",
    "resource_uri": "ghcr.io/liatrio/gh-trusted-builds-app",
    "time_verified": "2023-05-23T17:52:14.186161165Z",
    "verification_result": "PASSED",
    "verifier": {
      "id": "https://github.com/liatrio/gh-trusted-builds-workflows/.github/workflows/policy-verification.yaml@refs/heads/main"
    }
  }
}

This statement links a particular container image (ghcr.io/liatrio/gh-trusted-builds-app@sha256:4d9e77fedca29393dff9a39b065d158aaa9644c18b2c51f75fcaf84b8c941abf) to a predicate (https://slsa.dev/verification_summary/v0.2). We'll see more on this predicate type later. For more information about in-toto attestations, the official specification is a good place to start.

SLSA

Software supply-chain security has been a growing concern in the past few years. SLSA (supply-chain levels for software artifacts) is an attempt to codify best practices around producing software artifacts by describing a series of standards and levels that a software artifact can obtain. A higher level indicates a more secure and robust software delivery process. The SLSA website gives a more comprehensive overview of the levels.

The initial focus of SLSA is around how software is built, and the framework describes a provenance attestation. Provenance links a software artifact to the source code from which it was created, as well as the method and environment in which the build ran. The project also provides some tools for creating provenance, like the slsa-github-generator, which provides ways to create provenance attestations for different artifact types built in GitHub Actions. This demonstration uses the SLSA container generator to create a provenance attestation.

Open Policy Agent

Open Policy Agent (OPA) is a set of tools for writing and executing policies in a language called Rego. The policies are used to authorize some request or event, given input data. For example, this is part of a larger policy used in this demo to check that a pull request had at least one reviewer:

package security.pullrequest

default allow = false

allow {
	count(violation) == 0
}

violation[msg] {
	count(input.predicate.reviewers) < 1
	msg := "pull request reviewers is less than 1"
}

This demonstration uses Rego in multiple places: first liatrio/github-trusted-builds-attestations uses policy on the attestations generated by the workflow in order to produce a verification summary attestation (VSA). Later on, when the workflow attempts to deploy to a local Kubernetes cluster, the Sigstore policy controller is configured with a Rego policy that checks the attributes of the VSA produced earlier.

GitHub Actions: Reusable Workflows

GitHub Actions has multiple ways to minimize duplication in a workflow -- one method is to create a custom action. With composite actions, it's even possible to use wrap multiple external actions in a single reusable unit.

However, for this demonstration, we're relying on a larger unit -- the reusable workflow. A reusable workflow functions similarly to a composite action, in that it allows you to bundle multiple actions together and pass inputs/outputs between them. Instead of wrapping individual actions, the workflow can include one or more complete jobs, so we can perform separate tasks in parallel. In addition, a reusable workflow has different security features that make it desirable over other ways to reduce duplication in GitHub Actions.

First and foremost is the fact that a reusable workflow runs in a separate virtual machine than other jobs in the caller's workflow. This is in contrast to an individual action, which always runs in the larger context of a job. Running on the same virtual machine as the workflow caller makes it difficult to access secrets in a way that doesn't also allow the caller to use them. It also means that a caller can manipulate the environment in order to do things like capture environment variables or snoop on network traffic. With reusable workflows, we can eliminate these concerns which would otherwise make it difficult for us to trust the attestations produced by the workflows.

This demonstration also shows the use of least-privilege access with reusable workflows. Each reusable workflow is specifically granted the token permissions the caller will allow. This enables the caller repository to have a safer, read-only default to GitHub token permissions.

GitHub Actions: OpenID Connect

A GitHub Actions feature that complements reusable workflows is the OpenID Connect (OIDC) integration, which allows a job to acquire an OIDC id token, which is a JSON web token (JWT) signed by GitHub Actions.

A JWT is a popular token format that contains three sections: a header, claims, and signature. The header contains metadata about the token, while the claims typically contain information about the entity described by the token, as well as details needed to validate the token, such as the token issuer and expiry. Finally, the signature encompasses the header and claims, so that by verifying the signature, we know that we can trust the information in the other two sections.

In the case of GitHub Actions, the claims in the token describe the running workflow:

{
   "jti": "79ba45c1-a46f-4605-b68d-8207b5a5287f",
   "sub": "repo:liatrio/gh-trusted-builds-app:ref:refs/heads/main",
   "aud": "https://github.com/liatrio",
   "ref": "refs/heads/main",
   "sha": "0c960da8e1b1844d14ac4048b60f2ec892ed21a3",
   "repository": "liatrio/gh-trusted-builds-app",
   "repository_owner": "liatrio",
   "repository_owner_id": "5726618",
   "run_id": "4724225373",
   "run_number": "22",
   "run_attempt": "1",
   "repository_visibility": "public",
   "repository_id": "627556067",
   "actor_id": "9082799",
   "actor": "alexashley",
   "workflow": "app",
   "head_ref": "",
   "base_ref": "",
   "event_name": "workflow_dispatch",
   "ref_type": "branch",
   "workflow_ref": "liatrio/gh-trusted-builds-app/.github/workflows/app.yaml@refs/heads/main",
   "workflow_sha": "0c960da8e1b1844d14ac4048b60f2ec892ed21a3",
   "job_workflow_ref": "liatrio/gh-trusted-builds-workflows/.github/workflows/build-and-push.yaml@refs/heads/main",
   "job_workflow_sha": "f1e5fbbfa4f750e221038adca773bca75ec38c3d",
   "runner_environment": "github-hosted",
   "iss": "https://token.actions.githubusercontent.com",
   "nbf": 1681753278,
   "exp": 1681754178,
   "iat": 1681753878
}

One claim in particular, job_workflow_ref, is instrumental to the security properties of this demonstration; it indicates the workflow path and Git reference that were used in this job. You may also notice the similarly-named workflow_ref claim, which is the path to the running workflow definition. When using a normal workflow, the values of these two claims is the same, but when a reusable workflow is called by another workflow, the value of job_workflow_ref will be the path and Git ref of the reusable workflow. Because the token is signed by GitHub Actions, as long as we validate the token signature and claims, we can trust that only a running instance of the workflow is in possession of the token. This property allows us to create federated trust in order to access cloud provider resources or even request code-signing certificates.

In order to validate tokens, we need to know what keys were used to sign the tokens. GitHub Actions hosts an OpenID Connect provider at https://token.actions.githubusercontent.com, which includes an OIDC discovery document at the /.well-known/openid-configuration endpoint:

{
   "issuer": "https://token.actions.githubusercontent.com",
   "jwks_uri": "https://token.actions.githubusercontent.com/.well-known/jwks",
   "subject_types_supported": [
      "public",
      "pairwise"
   ],
   "response_types_supported": [
      "id_token"
   ],
   "claims_supported": [
      "sub",
      "aud",
      "exp",
      "iat",
      "iss",
      "jti",
      "nbf",
      "ref",
      "repository",
      "repository_id",
      "repository_owner",
      "repository_owner_id",
      "run_id",
      "run_number",
      "run_attempt",
      "actor",
      "actor_id",
      "workflow",
      "workflow_ref",
      "workflow_sha",
      "head_ref",
      "base_ref",
      "event_name",
      "ref_type",
      "environment",
      "environment_node_id",
      "job_workflow_ref",
      "job_workflow_sha",
      "repository_visibility",
      "runner_environment"
   ],
   "id_token_signing_alg_values_supported": [
      "RS256"
   ],
   "scopes_supported": [
      "openid"
   ]
}

The jwks_uri field (JSON web key set) contains a URL that can be used to grab the public keys which are needed to verify the signature on the JWTs issued by GitHub Actions.

Finally, it's worth mentioning how a job receives an id token, as it's not one of the standard OAuth2 grants. In order to acquire an id token, a job must first be configured with the id-token permission:

jobs:
   demo:
      permissions:
         id-token: write

Later on, once the job is running, it will have access to the following environment variables:

ACTIONS_ID_TOKEN_REQUEST_URL
ACTIONS_RUNTIME_TOKEN

The job can then make an HTTP POST request to $ACTIONS_ID_TOKEN_REQUEST_URL with $ACTIONS_RUNTIME_TOKEN in the authorization header (using the bearer token scheme), and the job will receive an id token in the value field of the JSON response.

For a more in-depth overview of using GitHub Actions' OIDC feature with reusable workflows, the SLSA GitHub Generator specifications go into additional detail.

Sigstore: Cosign

Cosign is a tool from the Sigstore project for signing and verifying container images, blobs, and attestations. It also supports running OPA or CUE policies against those attestations. In this demonstration, we'll use cosign to sign both a container image and a number of attestations about the image.

Sigstore: Policy Controller

Another Sigstore project, the policy controller, is used to gate Kubernetes deployments with OPA or CUE policy. It can verify that images are signed and check the attestations against a supplied policy. We'll use it in this demo to check that our container image has an attestation signed by the right identity.

Sigstore: Rekor Transparency Log

Rekor is an open-source software supply chain tool that’s part of the larger Sigstore project. It consists of a transparency log server and a command line tool to interact with the server. While there is a public instance, enterprises can also deploy and run Rekor on their own infrastructure.

Rekor has a number of properties that make it useful as an attestation store; first, its underlying storage is a robust, append-only log – new entries can be added, but old entries cannot be updated or deleted. Rekor’s implementation also provides the ability to demonstrate that an entry is actually included in the log, which users can verify through what’s called an inclusion proof. Similarly, by recording certain metadata about the log, users can also verify over time that the log hasn’t been tampered with and that only new entries have been created (through a consistency proof).

Rekor is used to store container image signatures, along with the public key or certificate that's needed to verify the signature. It can also store in-toto attestations, along with a few other built-in record types.

Sigstore: Fulcio Certificate Authority

Fulcio is a certificate authority for code-signing -- it issues short-lived (10 minute) certificates that anyone can use to sign images or attestations. The official documentation goes into more detail, but at a high level this is the flow for acquiring a code-signing certificate:

Acquire an id token from an issuer that Fulcio is configured to trust. This demo uses the GitHub Actions id token
Generate a temporary public/private key pair - cosign will do this automatically
Prove that you control the key pair by doing one of the following:
- Create a certificate signing request (CSR)
- Grab the sub claim from the id token and sign it
Make a request to Fulcio with the id token and the proof from the previous step
Fulcio will validate the id token and check the provided proof
If the request is valid, Fulcio will issue a code-signing certificate and populate the certificate fields with information from the id token
- When using a GitHub Actions OIDC token, the issuer will be https://token.actions.githubusercontent.com and the subject will be the value of the job_workflow_ref claim
Depending on the deployment type, Fulcio will also submit a request to a certificate transparency log to include the certificate in the logs. The response from the certificate transparency log will include a signed timestamp that will be added to the certificate.

After receiving the certificate, cosign or other tools can sign images or attestations with the private key generated earlier, and then upload the signature and certificate to Rekor. Once the signing is finished, the key pair can be discarded, as the certificate will be in Rekor for validation later.

For a more concrete example, this is a sequence diagram that shows how a GitHub Actions workflow might build & sign a container image:

Later, when someone tries to validate the signature, they can check that the certificate chains up to the Fulcio root, that the signature happened during the window when the certificate was valid, and, most importantly, verify that the certificate identity matches the expected signer. Anyone can use the Sigstore public good Fulcio instance to get a certificate, so it's important that you only trust signatures from identities that you trust.

Software Bill of Materials (SBOM)

A software bill of materials describes the components that make up a software artifact. The SBOM includes the dependencies of an artifact and may include licensing information about the artifact and its dependencies. While an SBOM can be useful to enforce internal licensing standards, it's also valuable for vulnerability analysis; this is especially true when there's a need to retroactively look at software running in production and determine if it's vulnerable to a new attack. If an organization already has SBOMs for the software it's running, it becomes much easier to tell if there's an impact from a new vulnerability.

There are several standard formats, like SPDX from the Linux Foundation and CycloneDX from OWASP. Fortunately, there are a number of tools for generating and working with SBOMs and most tools understand multiple standards. In this demonstration, we're using SPDX as the format and Syft as the tool to generate SBOMs.

Workflows

The demo makes use of several reusable workflows defined in liatrio/gh-trusted-builds-workflows. Each workflow is owned by either the platform or security teams.

Platform: Build & Push

The platform team's build-and-push workflow is split into several jobs:

detect-workflow
build
push
sign
source-attestations
provenance
sbom

The build job uses Docker to build a container image, but doesn't push it to a container registry. Instead, the job outputs a tar file that will be pushed by the next job. The reason to split build and push is that the build step is executing untrusted code from the application team; consequently, we don't want that job to be able to request the id token that's used for signing or to try to push a malicious image. It's also often the case that organizations will use a central registry for all teams, and the authenticated machine identity is likely to have access to more repositories than the one used by the application image. In that case, the build job could even push images to other repositories; while the authentication in this demo is scoped to the liatrio/gh-trusted-builds-app repository, it's still important to separate these steps for those reasons.

So the build job only has the permissions it needs in order to checkout the repo:

jobs:
  build:
    permissions:
      contents: read

Next up is the push job, which loads the tar file from the build job and pushes it to GitHub Container Registry (GHCR). Like the build job, it only has the permissions it needs to write to the registry:

jobs:
  push:
    permissions:
      packages: write

With the image built and stored in GHCR, we can now sign the image in the sign job. This job has access to the workflow id token so that it can request a signing certificate from Fulcio; the job also needs permission to push the signature to the registry, so it has that access as well:

jobs:
  sign:
    permissions:
      id-token: write
      contents: read
      packages: write

The sign job uses cosign to sign the image and annotate it with the workflow run:

- name: Sign
  run: |
    cosign sign \
        --annotations liatr.io/github-actions-run-link='${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}' \
        --annotations liatr.io/signed-off-by=platform-team \
        --rekor-url ${{ inputs.rekorUrl }} \
        --fulcio-url ${{ inputs.fulcioUrl }} \
        --yes ghcr.io/${{ github.repository }}@${{ needs.push.outputs.digest }}

Next, the provenance and source-attestations job both produce attestations that are signed by the workflow. The provenance job uses the container generator from slsa-framework/slsa-github-generator to produce a provenance attestation that links the container image and source code, along with some metadata about how the artifact was produced. Because the workflow is only using the generator, it doesn't have the full context necessary to populate the entire provenance, so some fields aren't present.

Here's an example of the provenance generated by this job (Rekor entry):

SLSA Provenance attestation (click to expand)

_type: https://in-toto.io/Statement/v0.1
predicateType: https://slsa.dev/provenance/v0.2
subject:
  - name: ghcr.io/liatrio/gh-trusted-builds-app
    digest:
      sha256: 6c3bf887638f7c0d86731e6208befa1b439e465cb435465d982c50609553b514
predicate:
  builder:
    id: >-
      https://github.com/liatrio/gh-trusted-builds-workflows/.github/workflows/build-and-push.yaml@refs/heads/main
  buildType: https://github.com/slsa-framework/slsa-github-generator/container@v1
  invocation:
    configSource:
      uri: git+https://github.com/liatrio/gh-trusted-builds-app@refs/heads/main
      digest:
        sha1: e1f1d4396181766e12fca22f2ba856e8154b4304
      entryPoint: .github/workflows/app.yaml
    parameters: {}
    environment:
      github_actor: rcoy-v
      github_actor_id: '9846738'
      github_base_ref: ''
      github_event_name: push
      github_event_payload:
        after: e1f1d4396181766e12fca22f2ba856e8154b4304
        base_ref: null
        before: c4df379485ab80b62ae0cc57c611348a3015f944
        commits:
          - author:
              email: [email protected]
              name: Ryan Vance
              username: rcoy-v
            committer:
              email: [email protected]
              name: GitHub
              username: web-flow
            distinct: true
            id: e1f1d4396181766e12fca22f2ba856e8154b4304
            message: 'docs: remove extra newline (#1)'
            timestamp: '2023-05-22T10:27:27-05:00'
            tree_id: e5dc0b7046c70012c4b84bc7ede6f969efe79edf
            url: >-
              https://github.com/liatrio/gh-trusted-builds-app/commit/e1f1d4396181766e12fca22f2ba856e8154b4304
        compare: >-
          https://github.com/liatrio/gh-trusted-builds-app/compare/c4df379485ab...e1f1d4396181
        created: false
        deleted: false
        forced: false
        head_commit:
          author:
            email: [email protected]
            name: Ryan Vance
            username: rcoy-v
          committer:
            email: [email protected]
            name: GitHub
            username: web-flow
          distinct: true
          id: e1f1d4396181766e12fca22f2ba856e8154b4304
          message: 'docs: remove extra newline (#1)'
          timestamp: '2023-05-22T10:27:27-05:00'
          tree_id: e5dc0b7046c70012c4b84bc7ede6f969efe79edf
          url: >-
            https://github.com/liatrio/gh-trusted-builds-app/commit/e1f1d4396181766e12fca22f2ba856e8154b4304
        organization:
          avatar_url: https://avatars.githubusercontent.com/u/5726618?v=4
          description: Enterprise Delivery Transformation, DevOps, Cloud Native Automation
          events_url: https://api.github.com/orgs/liatrio/events
          hooks_url: https://api.github.com/orgs/liatrio/hooks
          id: 5726618
          issues_url: https://api.github.com/orgs/liatrio/issues
          login: liatrio
          members_url: https://api.github.com/orgs/liatrio/members{/member}
          node_id: MDEyOk9yZ2FuaXphdGlvbjU3MjY2MTg=
          public_members_url: https://api.github.com/orgs/liatrio/public_members{/member}
          repos_url: https://api.github.com/orgs/liatrio/repos
          url: https://api.github.com/orgs/liatrio
        pusher:
          email: [email protected]
          name: rcoy-v
        ref: refs/heads/main
        repository:
          allow_forking: true
          archive_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/{archive_format}{/ref}
          archived: false
          assignees_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/assignees{/user}
          blobs_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/git/blobs{/sha}
          branches_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/branches{/branch}
          clone_url: https://github.com/liatrio/gh-trusted-builds-app.git
          collaborators_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/collaborators{/collaborator}
          comments_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/comments{/number}
          commits_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/commits{/sha}
          compare_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/compare/{base}...{head}
          contents_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/contents/{+path}
          contributors_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/contributors
          created_at: 1684768749
          default_branch: main
          deployments_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/deployments
          description: null
          disabled: false
          downloads_url: https://api.github.com/repos/liatrio/gh-trusted-builds-app/downloads
          events_url: https://api.github.com/repos/liatrio/gh-trusted-builds-app/events
          fork: false
          forks: 0
          forks_count: 0
          forks_url: https://api.github.com/repos/liatrio/gh-trusted-builds-app/forks
          full_name: liatrio/gh-trusted-builds-app
          git_commits_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/git/commits{/sha}
          git_refs_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/git/refs{/sha}
          git_tags_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/git/tags{/sha}
          git_url: git://github.com/liatrio/gh-trusted-builds-app.git
          has_discussions: false
          has_downloads: true
          has_issues: true
          has_pages: false
          has_projects: true
          has_wiki: true
          homepage: null
          hooks_url: https://api.github.com/repos/liatrio/gh-trusted-builds-app/hooks
          html_url: https://github.com/liatrio/gh-trusted-builds-app
          id: 643991426
          is_template: false
          issue_comment_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/issues/comments{/number}
          issue_events_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/issues/events{/number}
          issues_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/issues{/number}
          keys_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/keys{/key_id}
          labels_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/labels{/name}
          language: Go
          languages_url: https://api.github.com/repos/liatrio/gh-trusted-builds-app/languages
          license: null
          master_branch: main
          merges_url: https://api.github.com/repos/liatrio/gh-trusted-builds-app/merges
          milestones_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/milestones{/number}
          mirror_url: null
          name: gh-trusted-builds-app
          node_id: R_kgDOJmKHgg
          notifications_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/notifications{?since,all,participating}
          open_issues: 0
          open_issues_count: 0
          organization: liatrio
          owner:
            avatar_url: https://avatars.githubusercontent.com/u/5726618?v=4
            email: [email protected]
            events_url: https://api.github.com/users/liatrio/events{/privacy}
            followers_url: https://api.github.com/users/liatrio/followers
            following_url: https://api.github.com/users/liatrio/following{/other_user}
            gists_url: https://api.github.com/users/liatrio/gists{/gist_id}
            gravatar_id: ''
            html_url: https://github.com/liatrio
            id: 5726618
            login: liatrio
            name: liatrio
            node_id: MDEyOk9yZ2FuaXphdGlvbjU3MjY2MTg=
            organizations_url: https://api.github.com/users/liatrio/orgs
            received_events_url: https://api.github.com/users/liatrio/received_events
            repos_url: https://api.github.com/users/liatrio/repos
            site_admin: false
            starred_url: https://api.github.com/users/liatrio/starred{/owner}{/repo}
            subscriptions_url: https://api.github.com/users/liatrio/subscriptions
            type: Organization
            url: https://api.github.com/users/liatrio
          private: false
          pulls_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/pulls{/number}
          pushed_at: 1684769247
          releases_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/releases{/id}
          size: 0
          ssh_url: [email protected]:liatrio/gh-trusted-builds-app.git
          stargazers: 0
          stargazers_count: 0
          stargazers_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/stargazers
          statuses_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/statuses/{sha}
          subscribers_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/subscribers
          subscription_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/subscription
          svn_url: https://github.com/liatrio/gh-trusted-builds-app
          tags_url: https://api.github.com/repos/liatrio/gh-trusted-builds-app/tags
          teams_url: https://api.github.com/repos/liatrio/gh-trusted-builds-app/teams
          topics: []
          trees_url: >-
            https://api.github.com/repos/liatrio/gh-trusted-builds-app/git/trees{/sha}
          updated_at: '2023-05-22T15:19:19Z'
          url: https://github.com/liatrio/gh-trusted-builds-app
          visibility: public
          watchers: 0
          watchers_count: 0
          web_commit_signoff_required: false
        sender:
          avatar_url: https://avatars.githubusercontent.com/u/9846738?v=4
          events_url: https://api.github.com/users/rcoy-v/events{/privacy}
          followers_url: https://api.github.com/users/rcoy-v/followers
          following_url: https://api.github.com/users/rcoy-v/following{/other_user}
          gists_url: https://api.github.com/users/rcoy-v/gists{/gist_id}
          gravatar_id: ''
          html_url: https://github.com/rcoy-v
          id: 9846738
          login: rcoy-v
          node_id: MDQ6VXNlcjk4NDY3Mzg=
          organizations_url: https://api.github.com/users/rcoy-v/orgs
          received_events_url: https://api.github.com/users/rcoy-v/received_events
          repos_url: https://api.github.com/users/rcoy-v/repos
          site_admin: false
          starred_url: https://api.github.com/users/rcoy-v/starred{/owner}{/repo}
          subscriptions_url: https://api.github.com/users/rcoy-v/subscriptions
          type: User
          url: https://api.github.com/users/rcoy-v
      github_head_ref: ''
      github_ref: refs/heads/main
      github_ref_type: branch
      github_repository_id: '643991426'
      github_repository_owner: liatrio
      github_repository_owner_id: '5726618'
      github_run_attempt: '1'
      github_run_id: '5047631192'
      github_run_number: '4'
      github_sha1: e1f1d4396181766e12fca22f2ba856e8154b4304
  metadata:
    buildInvocationID: 5047631192-1
    completeness:
      parameters: true
      environment: false
      materials: false
    reproducible: false
  materials:
    - uri: git+https://github.com/liatrio/gh-trusted-builds-app@refs/heads/main
      digest:
        sha1: e1f1d4396181766e12fca22f2ba856e8154b4304

In addition to the provenance, the build-and-push workflow's source-attestation job produces a custom pull request attestation using liatrio/gh-trusted-builds-attestations. This attestation ties the commit to the pull request and includes information about whether the pull request was approved and which individuals reviewed it. Many enterprises have policies that require a minimum number of reviewers, so this attestation can be used to require that artifacts were built only from approved source code changes.

Here's an example of a pull request attestation produced by the workflow (Rekor entry):

_type: https://in-toto.io/Statement/v0.1
predicateType: https://liatr.io/attestations/github-pull-request/v1
subject:
  - name: git+https://github.com/liatrio/gh-trusted-builds-app.git
    digest:
      sha1: e1f1d4396181766e12fca22f2ba856e8154b4304
  - name: ghcr.io/liatrio/gh-trusted-builds-app
    digest:
      sha256: 6c3bf887638f7c0d86731e6208befa1b439e465cb435465d982c50609553b514
predicate:
  link: https://github.com/liatrio/gh-trusted-builds-app/pull/1
  title: 'docs: remove extra newline'
  author: rcoy-v
  mergedBy: rcoy-v
  createdAt: '2023-05-22T15:27:05Z'
  mergedAt: '2023-05-22T15:27:27Z'
  base: main
  head: rcoy-v-patch-1
  approved: true
  reviewers:
    - name: alexashley
      approved: true
      reviewLink: >-
        https://github.com/liatrio/gh-trusted-builds-app/pull/1#pullrequestreview-1436887240
      timestamp: '2023-05-22T15:27:18Z'
  contributors:
    - name: rcoy-v
  predicateCreatedAt: '2023-05-22T15:28:48.369418041Z'

Finally, the last job in this workflow, called sbom, produces a software bill of materials using Syft. The workflow runs Syft against the pushed image and produces an SBOM in the SPDX format. Then the job runs cosign attest in order to sign and upload the SBOM as an attestation. While there's support in Syft for attesting the SBOM through sfyt attest, we're using cosign directly for more fine-grained configuration:

$ syft -o spdx-json --file sbom.spdx.json ghcr.io/${{ github.repository }}@${{ needs.push.outputs.digest }}

$ cosign attest --predicate="sbom.spdx.json" \
   --rekor-url ${{ inputs.rekorUrl }} \
   --type spdxjson \
   --fulcio-url ${{ inputs.fulcioUrl }} \
   --yes \
   ghcr.io/${{ github.repository }}@${{ needs.push.outputs.digest }}

For an example of an SBOM, here's an attestation from one of the workflow runs (Rekor log entry):

SPDX bill of materials (click to expand)

_type: https://in-toto.io/Statement/v0.1
predicateType: https://spdx.dev/Document
subject:
  - name: ghcr.io/liatrio/gh-trusted-builds-app
    digest:
      sha256: 294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc
predicate:
  SPDXID: SPDXRef-DOCUMENT
  creationInfo:
    created: '2023-05-26T22:02:06Z'
    creators:
      - 'Organization: Anchore, Inc'
      - 'Tool: syft-0.82.0'
    licenseListVersion: '3.20'
  dataLicense: CC0-1.0
  documentNamespace: >-
    https://anchore.com/syft/image/ghcr.io/liatrio/gh-trusted-builds-app@sha256-294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc-5b3e747f-d8dc-4a0b-a01f-60d9ed082d68
  files:
    - SPDXID: SPDXRef-File-app-d20c3eddd3b3b879
      checksums:
        - algorithm: SHA1
          checksumValue: '0000000000000000000000000000000000000000'
      comment: >-
        layerID:
        sha256:53ea96ed00f53fed01d48a16b049a99938902ed5ee4517e57f464d7fabacb33f
      copyrightText: ''
      fileName: /app
      fileTypes:
        - OTHER
      licenseConcluded: NOASSERTION
    - SPDXID: SPDXRef-File-app-server-50b4875c6ae53f25
      checksums:
        - algorithm: SHA256
          checksumValue: a947dcd63d19e76e860f32f8bdd33ca47297d10d8e6f1a2eb913224d93fe9fe4
      comment: >-
        layerID:
        sha256:53ea96ed00f53fed01d48a16b049a99938902ed5ee4517e57f464d7fabacb33f
      copyrightText: ''
      fileName: /app/server
      fileTypes:
        - APPLICATION
        - BINARY
      licenseConcluded: NOASSERTION
  name: >-
    ghcr.io/liatrio/gh-trusted-builds-app@sha256:294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc
  packages:
    - SPDXID: >-
        SPDXRef-Package-go-module-github.com-liatrio-gh-trusted-builds-app-1808e23a377c2b90
      copyrightText: NOASSERTION
      downloadLocation: NOASSERTION
      externalRefs:
        - referenceCategory: SECURITY
          referenceLocator: cpe:2.3:a:liatrio:gh-trusted-builds-app:\(devel\):*:*:*:*:*:*:*
          referenceType: cpe23Type
        - referenceCategory: SECURITY
          referenceLocator: cpe:2.3:a:liatrio:gh_trusted_builds_app:\(devel\):*:*:*:*:*:*:*
          referenceType: cpe23Type
        - referenceCategory: PACKAGE-MANAGER
          referenceLocator: pkg:golang/github.com/liatrio/gh-trusted-builds-app@(devel)
          referenceType: purl
      licenseConcluded: NOASSERTION
      licenseDeclared: NOASSERTION
      name: github.com/liatrio/gh-trusted-builds-app
      sourceInfo: 'acquired package info from go module information: /app/server'
      versionInfo: (devel)
  relationships:
    - comment: >-
        evident-by: indicates the package's existence is evident by the given
        file
      relatedSpdxElement: SPDXRef-File-app-server-50b4875c6ae53f25
      relationshipType: OTHER
      spdxElementId: >-
        SPDXRef-Package-go-module-github.com-liatrio-gh-trusted-builds-app-1808e23a377c2b90
    - relatedSpdxElement: SPDXRef-DOCUMENT
      relationshipType: DESCRIBES
      spdxElementId: SPDXRef-DOCUMENT
  spdxVersion: SPDX-2.3

Security: Image Scan

Now that the image is built, it needs to be scanned for vulnerabilities by an approved image scanner. The security team uses Trivy for this, because it supports outputting the scan results in a format that cosign can use to create an attestation:

steps:
 - name: Trivy Scan
   uses: aquasecurity/[email protected]
   with:
      image-ref: ghcr.io/${{ github.repository }}@${{ inputs.digest }}
      format: 'cosign-vuln'
      output: trivy.report.json
   env:
      TRIVY_USERNAME: ${{ github.actor }}
      TRIVY_PASSWORD: ${{ secrets.GITHUB_TOKEN }}

The scan-image workflow only contains a single job called scan, which takes as an input the digest of the image created in the build-and-push workflow. It invokes the Trivy GitHub Action and then uses cosign attest to upload the vulnerability results as an attestation to Rekor.

This is an example of what the vulnerability attestation looks like (Rekor entry):

Vulnerability attestation (click to expand)

_type: https://in-toto.io/Statement/v0.1
predicateType: https://cosign.sigstore.dev/attestation/vuln/v1
subject:
  - name: ghcr.io/liatrio/gh-trusted-builds-app
    digest:
      sha256: 6c3bf887638f7c0d86731e6208befa1b439e465cb435465d982c50609553b514
predicate:
  invocation:
    parameters: null
    uri: ''
    event_id: ''
    builder.id: ''
  scanner:
    uri: pkg:github/aquasecurity/[email protected]
    version: 0.38.1
    db:
      uri: ''
      version: ''
    result:
      ArtifactName: >-
        ghcr.io/liatrio/gh-trusted-builds-app@sha256:6c3bf887638f7c0d86731e6208befa1b439e465cb435465d982c50609553b514
      ArtifactType: container_image
      Metadata:
        DiffIDs:
          - >-
            sha256:0c2717ceeb1e6dec4b2a748974b77fd8f13ac5c3d9d434f80f2c1b58c83f31ae
          - >-
            sha256:8a90007133baa3918410eca530757f8e7c65aff8421ac010a185ff51c2f88e80
        ImageConfig:
          architecture: amd64
          config:
            Entrypoint:
              - /app/server
            Env:
              - >-
                PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
            Labels:
              org.opencontainers.image.created: '2023-05-22T15:27:46.155Z'
              org.opencontainers.image.description: ''
              org.opencontainers.image.licenses: ''
              org.opencontainers.image.revision: e1f1d4396181766e12fca22f2ba856e8154b4304
              org.opencontainers.image.source: https://github.com/liatrio/gh-trusted-builds-app
              org.opencontainers.image.title: gh-trusted-builds-app
              org.opencontainers.image.url: https://github.com/liatrio/gh-trusted-builds-app
              org.opencontainers.image.version: main
            WorkingDir: /app
          created: '2023-05-22T15:28:09.391753905Z'
          history:
            - comment: buildkit.dockerfile.v0
              created: '2023-05-22T15:27:48.389191081Z'
              created_by: WORKDIR /app
            - comment: buildkit.dockerfile.v0
              created: '2023-05-22T15:28:09.391753905Z'
              created_by: 'COPY /app/server . # buildkit'
            - comment: buildkit.dockerfile.v0
              created: '2023-05-22T15:28:09.391753905Z'
              created_by: ENTRYPOINT ["/app/server"]
              empty_layer: true
          os: linux
          rootfs:
            diff_ids:
              - >-
                sha256:0c2717ceeb1e6dec4b2a748974b77fd8f13ac5c3d9d434f80f2c1b58c83f31ae
              - >-
                sha256:8a90007133baa3918410eca530757f8e7c65aff8421ac010a185ff51c2f88e80
            type: layers
        ImageID: >-
          sha256:dfde59c7ffd4a99f202c45669cc6311dadc906d82f7e40f445ebc7a226bd8781
        RepoDigests:
          - >-
            ghcr.io/liatrio/gh-trusted-builds-app@sha256:6c3bf887638f7c0d86731e6208befa1b439e465cb435465d982c50609553b514
      SchemaVersion: 2
  metadata:
    scanStartedOn: '2023-05-22T15:29:20.193667225Z'
    scanFinishedOn: '2023-05-22T15:29:20.193667225Z'

The image produced by the demo is very simple, and as of this writing doesn't contain any vulnerabilities. However, if there were vulnerabilities in the image, they would appear in the attestation and policy could be used to reject artifacts that either had too many vulnerabilities or had certain high-risk CVEs.

Security: Policy Verification

Now that we've attested to the number of code reviewers, built and signed an image, attested to image provenance and vulnerabilities, it's time to evaluate those attestations. While it would be possible to use a workflow or Kubernetes validating admission webhook to evaluate all the attestations, it would be cumbersome to have that controller know everything that needs to be validated about the image. Especially considering that enterprises will refine their internal policies over time and need to make adjustments to every deployment gate.

Instead, we can evaluate all the attestations up to this point and produce a verification summary attestation, which will attest that the container image either passed or failed a particular policy.

In order to do this, we use liatrio/gh-trusted-builds-attestations to pull the attestations for an image, verify that the attestations were signed by the expected workflows, and then run policy against those attestations in order to check if the image meets the standards enforced by the policy.

Either way, the attestations tool will produce a VSA that can be used later to prevent low quality or malicious images from being deployed. Here's an example of what the VSA produced by the workflow looks like (Rekor entry):

_type: https://in-toto.io/Statement/v1
subject:
  - name: ghcr.io/liatrio/gh-trusted-builds-app
    digest:
      sha256: 6c3bf887638f7c0d86731e6208befa1b439e465cb435465d982c50609553b514
predicateType: https://slsa.dev/verification_summary/v0.2
predicate:
  input_attestations:
    - digest:
        sha256: 1c735dab58c44079863b1a5e209617f8357409d63203e2309894e0b0d1e0ffaa
      uri: https://rekor.sigstore.dev/api/v1/log/entries?logIndex=21341724
    - digest:
        sha256: cdcf6759b454b26b6f151b2913c2949d4b1bef8c97d78085553f55ededc79f15
      uri: https://rekor.sigstore.dev/api/v1/log/entries?logIndex=21341756
  policy:
    uri: >-
      https://github.com/liatrio/gh-trusted-builds-policy/releases/download/v1.1.1/bundle.tar.gz
  policy_level: SLSA_LEVEL_3
  resource_uri: ghcr.io/liatrio/gh-trusted-builds-app
  time_verified: '2023-05-22T15:29:58.061215474Z'
  verification_result: PASSED
  verifier:
    id: >-
      https://github.com/liatrio/gh-trusted-builds-workflows/.github/workflows/policy-verification.yaml@refs/heads/main

Internally, the policy-verification workflow has two jobs:

detect-workflow
verify

The detect-workflow job determines the reusable workflow ref by requesting an id token and grabbing the job_workflow_ref claim, which is used later by the verify job to populate the verifier.id field in the SLSA VSA. This job may be removed in the future if GitHub enhances the github context by making the job_workflow_ref information available directly.

Next, the verify job invokes liatrio/gh-trusted-builds-attestations with the vsa subcommand:

- name: Create Verification Summary Attestation
  env:
    GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
  run: |
    attestation vsa \
      --artifact-uri ghcr.io/${{ github.repository }} \
      --artifact-digest ${{ inputs.digest }} \
      --policy-url "https://github.com/liatrio/gh-trusted-builds-policy/releases/download/v1.1.1/bundle.tar.gz" \
      --verifier-id ${{ github.server_url }}/${{ needs.metadata.outputs.jobWorkflowRef }} \
      --fulcio-url ${{ inputs.fulcioUrl }} \
      --rekor-url ${{ inputs.rekorUrl }}

The GITHUB_TOKEN environment variable is used to query GHCR for attestations and also to push the VSA to the registry after verification is finished. Like the other jobs that produce attestations, it also needs the ability to request an id token:

jobs:
   verify:
      permissions:
         id-token: write
         contents: read
         packages: write

Platform: Deployment

Finally, we'd like to actually deploy the artifact that the workflow built. In an enterprise setting, this may not be part of the same workflow that built the image, but the deployment is included here to show an end-to-end picture.

In this workflow, we first download k3d, which is a lightweight Kubernetes distribution that runs in Docker. It'll serve as our deployment target for the demo, as we can use it to spin up a temporary Kubernetes cluster in the GitHub Actions runner.

Once it's installed, we can create a simple cluster called demo:

$  k3d cluster create --agents 1 --no-lb --wait demo

Now we can install and configure the Sigstore policy-controller, which is what we'll use to verify the VSA produced by the policy-verification workflow. The policy-controller has a custom ClusterImagePolicy Kubernetes resource that we need to create in order to tell the controller what attestations to verify. For the ease of development, this policy is hard-coded in the workflow:

apiVersion: policy.sigstore.dev/v1alpha1
kind: ClusterImagePolicy
metadata:
  name: demo
spec:
  images:
    - glob: "ghcr.io/liatrio/gh-trusted-builds-app**"
  authorities:
    - name: attestation
      keyless:
        url: https://fulcio.sigstore.dev
        trustRootRef: 
        identities:
          - issuer: https://token.actions.githubusercontent.com
            subject: https://github.com/liatrio/gh-trusted-builds-workflows/.github/workflows/policy-verification.yaml@refs/heads/main
      ctlog:
        url: https://rekor.sigstore.dev
        trustRootRef: 
      attestations:
          - name: has-passing-vsa
            predicateType: "https://slsa.dev/verification_summary/v0.2"
            policy:
              type: rego
              data: |
                package sigstore
                default isCompliant = false
                isCompliant {
                  input.predicate.verifier.id == "https://github.com/liatrio/gh-trusted-builds-workflows/.github/workflows/policy-verification.yaml@refs/heads/main"
                  input.predicate.verification_result == "PASSED"
                }

This ClusterImagePolicy says we're expecting an attestation of type https://slsa.dev/verification_summary/v0.2 that was signed with a certificate issued from the GitHub Actions issuer via Fulcio, and that the certificate was issued to the policy-verification workflow. In the attestation policy, we also check that the VSA result was PASSED and that the verifier was the one we expect.

If the policy controller allows the deployment, then the end result won't look any different from a normal Kubernetes deployment. However, if image fails to meet policy, the deployment will be blocked and this message will be returned from the Kubernetes API server:

error: failed to create deployment: admission webhook "policy.sigstore.dev" denied the request: validation failed: failed policy: demo: spec.template.spec.containers[0].image
ghcr.io/liatrio/gh-trusted-builds-app@sha256:4c2a8a95f7ecc73abc62abd699dfb33579dd29ecde4f44f009dbbca6305609e4 failed evaluating rego policy for type has-passing-vsa: policy is not compliant for query 'isCompliant = data.sigstore.isCompliant'

Unfortunately, it doesn't appear that there's much feedback on what aspect of the policy failed. In this case, it's a very simple policy, which would make it easier to understand what failed. However, there's more that could be done to make the error understandable for end-users.

Verification

While the image attestations are validated by liatrio/gh-trusted-builds-attestations and the policy-controller, it's also possible to validate the steps that were done in this demo by using cosign directly.

To demonstrate, we'll use this workflow run which produced the image ghcr.io/liatrio/gh-trusted-builds-app@sha256:294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc.

To get an overview of the images' attestations and signatures, we can use cosign tree:

$ 📦 Supply Chain Security Related artifacts for an image: ghcr.io/liatrio/gh-trusted-builds-app@sha256:294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc
└── 💾 Attestations for an image tag: ghcr.io/liatrio/gh-trusted-builds-app:sha256-294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc.att
   ├── 🍒 sha256:86f0c07ab8720d5676dd38aa3c46c2179475f760d7fdbdeb8374292a00f86226
   ├── 🍒 sha256:937fea3f79cff9469147cadb127e7721fb66eaee20e228756cad7c20b77296d8
   ├── 🍒 sha256:e4a4239487d5a33cd3836d2654c7b76148b1810d1005c34796be3499021ed297
   ├── 🍒 sha256:28dc5117fe59531af747159a7227f3f0a21175581b13873cce15912e3aaef204
   └── 🍒 sha256:c4b6fb8b2b0e352135d21635d6724c93587370a179e61c39a32089d4d85274e8
└── 🔐 Signatures for an image tag: ghcr.io/liatrio/gh-trusted-builds-app:sha256-294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc.sig
   └── 🍒 sha256:010e941fc66633c54e29f2576dcb1e1c642a8c9313781b668dec26903a7998af

First, we'll verify that the image was signed by the platform team's build-and-push workflow:

$ cosign verify \
    --annotations liatr.io/signed-off-by=platform-team \
    --certificate-oidc-issuer https://token.actions.githubusercontent.com \
    --certificate-identity https://github.com/liatrio/gh-trusted-builds-workflows/.github/workflows/build-and-push.yaml@refs/heads/main \
    --certificate-github-workflow-repository liatrio/gh-trusted-builds-app \
    --rekor-url https://rekor.sigstore.dev \
    ghcr.io/liatrio/gh-trusted-builds-app@sha256:294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc

Verification for ghcr.io/liatrio/gh-trusted-builds-app@sha256:294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc --
The following checks were performed on each of these signatures:
  - The specified annotations were verified.
  - The cosign claims were validated
  - Existence of the claims in the transparency log was verified offline
  - The code-signing certificate was verified using trusted certificate authority certificates

The verify subcommand will also output the signature, so we can manually inspect the other fields for more information:

Image signature (click to expand)

[
  {
    "critical": {
      "identity": {
        "docker-reference": "ghcr.io/liatrio/gh-trusted-builds-app"
      },
      "image": {
        "docker-manifest-digest": "sha256:294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc"
      },
      "type": "cosign container image signature"
    },
    "optional": {
      "1.3.6.1.4.1.57264.1.1": "https://token.actions.githubusercontent.com",
      "1.3.6.1.4.1.57264.1.2": "workflow_dispatch",
      "1.3.6.1.4.1.57264.1.3": "54a0e5823b30c4fb8d0ff93b532e64d9478e012d",
      "1.3.6.1.4.1.57264.1.4": "app",
      "1.3.6.1.4.1.57264.1.5": "liatrio/gh-trusted-builds-app",
      "1.3.6.1.4.1.57264.1.6": "refs/heads/main",
      "Bundle": {
        "SignedEntryTimestamp": "MEUCIQDLeeJTlGROlwuXen9V8c0vA0gNjh1kCgFI21I7hDlBDQIgQ9mpdPXzkNt3Sg/KupxNTUH6JvcOEa0JvAxdimybue8=",
        "Payload": {
          "body": "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",
          "integratedTime": 1685138477,
          "logIndex": 21791129,
          "logID": "c0d23d6ad406973f9559f3ba2d1ca01f84147d8ffc5b8445c224f98b9591801d"
        }
      },
      "Issuer": "https://token.actions.githubusercontent.com",
      "Subject": "https://github.com/liatrio/gh-trusted-builds-workflows/.github/workflows/build-and-push.yaml@refs/heads/main",
      "githubWorkflowName": "app",
      "githubWorkflowRef": "refs/heads/main",
      "githubWorkflowRepository": "liatrio/gh-trusted-builds-app",
      "githubWorkflowSha": "54a0e5823b30c4fb8d0ff93b532e64d9478e012d",
      "githubWorkflowTrigger": "workflow_dispatch",
      "liatr.io/github-actions-run-link": "https://github.com/liatrio/gh-trusted-builds-app/actions/runs/5095256269",
      "liatr.io/signed-off-by": "platform-team"
    }
  }
]

Now we'll verify each attestation that the pipeline produced using the cosign verify-attestation command

https://liatr.io/attestations/github-pull-request/v1
https://slsa.dev/provenance/v0.2
https://cosign.sigstore.dev/attestation/vuln/v1
https://spdx.dev/Document
https://slsa.dev/verification_summary/v0.2

First up is the custom pull request attestation. It was produced in the build-and-push workflow, so attestation verification looks similar to verifying the image:

$ cosign verify-attestation \
  --type https://liatr.io/attestations/github-pull-request/v1 \
  --certificate-oidc-issuer https://token.actions.githubusercontent.com \
  --certificate-identity https://github.com/liatrio/gh-trusted-builds-workflows/.github/workflows/build-and-push.yaml@refs/heads/main \
  --certificate-github-workflow-repository liatrio/gh-trusted-builds-app \
  --rekor-url https://rekor.sigstore.dev \
  ghcr.io/liatrio/gh-trusted-builds-app@sha256:294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc

Verification for ghcr.io/liatrio/gh-trusted-builds-app@sha256:294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc --
The following checks were performed on each of these signatures:
  - The cosign claims were validated
  - Existence of the claims in the transparency log was verified offline
  - The code-signing certificate was verified using trusted certificate authority certificates
Certificate subject:  https://github.com/liatrio/gh-trusted-builds-workflows/.github/workflows/build-and-push.yaml@refs/heads/main
Certificate issuer URL:  https://token.actions.githubusercontent.com
GitHub Workflow Trigger: workflow_dispatch
GitHub Workflow SHA: 54a0e5823b30c4fb8d0ff93b532e64d9478e012d
GitHub Workflow Name: app
GitHub Workflow Trigger liatrio/gh-trusted-builds-app
GitHub Workflow Ref: refs/heads/main

Now we can do the same for the SLSA provenance attestation. This is an attestation type that's natively understood by cosign, so we can use --type slsaprovenance:

$ cosign verify-attestation \
  --type slsaprovenance \
  --certificate-oidc-issuer https://token.actions.githubusercontent.com \
  --certificate-identity https://github.com/liatrio/gh-trusted-builds-workflows/.github/workflows/build-and-push.yaml@refs/heads/main \
  --certificate-github-workflow-repository liatrio/gh-trusted-builds-app \
  --rekor-url https://rekor.sigstore.dev \
  ghcr.io/liatrio/gh-trusted-builds-app@sha256:294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc
 
Verification for ghcr.io/liatrio/gh-trusted-builds-app@sha256:294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc --
The following checks were performed on each of these signatures:
  - The cosign claims were validated
  - Existence of the claims in the transparency log was verified offline
  - The code-signing certificate was verified using trusted certificate authority certificates
Certificate subject:  https://github.com/liatrio/gh-trusted-builds-workflows/.github/workflows/build-and-push.yaml@refs/heads/main
Certificate issuer URL:  https://token.actions.githubusercontent.com
GitHub Workflow Trigger: workflow_dispatch
GitHub Workflow SHA: 54a0e5823b30c4fb8d0ff93b532e64d9478e012d
GitHub Workflow Name: app
GitHub Workflow Trigger liatrio/gh-trusted-builds-app
GitHub Workflow Ref: refs/heads/main

Next, we can check for the vulnerability attestation produced in the scan-image workflow. This is another attestation type that cosign is familiar with, so we can use --type vuln:

$ cosign verify-attestation \
  --type vuln \
  --certificate-oidc-issuer https://token.actions.githubusercontent.com \
  --certificate-identity https://github.com/liatrio/gh-trusted-builds-workflows/.github/workflows/scan-image.yaml@refs/heads/main \
  --certificate-github-workflow-repository liatrio/gh-trusted-builds-app \
  --rekor-url https://rekor.sigstore.dev \
  ghcr.io/liatrio/gh-trusted-builds-app@sha256:294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc
  
Verification for ghcr.io/liatrio/gh-trusted-builds-app@sha256:294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc --
The following checks were performed on each of these signatures:
  - The cosign claims were validated
  - Existence of the claims in the transparency log was verified offline
  - The code-signing certificate was verified using trusted certificate authority certificates
Certificate subject:  https://github.com/liatrio/gh-trusted-builds-workflows/.github/workflows/scan-image.yaml@refs/heads/main
Certificate issuer URL:  https://token.actions.githubusercontent.com
GitHub Workflow Trigger: workflow_dispatch
GitHub Workflow SHA: 54a0e5823b30c4fb8d0ff93b532e64d9478e012d
GitHub Workflow Name: app
GitHub Workflow Trigger liatrio/gh-trusted-builds-app
GitHub Workflow Ref: refs/heads/main

Similarly, we can verify the SBOM attestation. Like the SLSA provenance and vulnerability attestation types, this is another format the cosign understands natively:

$ cosign verify-attestation \
  --type spdxjson \
  --certificate-oidc-issuer https://token.actions.githubusercontent.com \
  --certificate-identity https://github.com/liatrio/gh-trusted-builds-workflows/.github/workflows/build-and-push.yaml@refs/heads/main \
  --certificate-github-workflow-repository liatrio/gh-trusted-builds-app \
  --rekor-url https://rekor.sigstore.dev \
  ghcr.io/liatrio/gh-trusted-builds-app@sha256:294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc
  
Verification for ghcr.io/liatrio/gh-trusted-builds-app@sha256:294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc --
The following checks were performed on each of these signatures:
  - The cosign claims were validated
  - Existence of the claims in the transparency log was verified offline
  - The code-signing certificate was verified using trusted certificate authority certificates
Certificate subject:  https://github.com/liatrio/gh-trusted-builds-workflows/.github/workflows/build-and-push.yaml@refs/heads/main
Certificate issuer URL:  https://token.actions.githubusercontent.com
GitHub Workflow Trigger: workflow_dispatch
GitHub Workflow SHA: 54a0e5823b30c4fb8d0ff93b532e64d9478e012d
GitHub Workflow Name: app
GitHub Workflow Trigger liatrio/gh-trusted-builds-app
GitHub Workflow Ref: refs/heads/main

Lastly, we can check the verification summary attestation produced by liatrio/gh-trusted-builds-attestations:

$ cosign verify-attestation \
  --type https://slsa.dev/verification_summary/v0.2 \
  --certificate-oidc-issuer https://token.actions.githubusercontent.com \
  --certificate-identity https://github.com/liatrio/gh-trusted-builds-workflows/.github/workflows/policy-verification.yaml@refs/heads/main \
  --certificate-github-workflow-repository liatrio/gh-trusted-builds-app \
  ghcr.io/liatrio/gh-trusted-builds-app@sha256:294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc

Verification for ghcr.io/liatrio/gh-trusted-builds-app@sha256:294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc --
The following checks were performed on each of these signatures:
  - The cosign claims were validated
  - Existence of the claims in the transparency log was verified offline
  - The code-signing certificate was verified using trusted certificate authority certificates
Certificate subject:  https://github.com/liatrio/gh-trusted-builds-workflows/.github/workflows/policy-verification.yaml@refs/heads/main
Certificate issuer URL:  https://token.actions.githubusercontent.com
GitHub Workflow Trigger: workflow_dispatch
GitHub Workflow SHA: 54a0e5823b30c4fb8d0ff93b532e64d9478e012d
GitHub Workflow Name: app
GitHub Workflow Trigger liatrio/gh-trusted-builds-app
GitHub Workflow Ref: refs/heads/main

Even after verifying the image signature and attestations, there may still be checks we wish to do on the individual attestations, which is where policy comes in. For instance, we could check the pull request attestation indicates that multiple reviewers approved a change or that our container isn't vulnerable to a particular CVE.

Of course, it's also helpful to know what failed verification looks like. What happens if an image is missing an attestation? We can simulate that by asking cosign to verify an attestation we know doesn't exist:

$ cosign verify-attestation \
  --type foo \
  --certificate-oidc-issuer https://token.actions.githubusercontent.com \
  --certificate-identity https://github.com/liatrio/gh-trusted-builds-workflows/.github/workflows/build-and-push.yaml@refs/heads/main \
  --certificate-github-workflow-repository liatrio/gh-trusted-builds-app \
  ghcr.io/liatrio/gh-trusted-builds-app@sha256:294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc
  
Error: none of the attestations matched the predicate type: foo, found: https://liatr.io/attestations/github-pull-request/v1,https://slsa.dev/provenance/v0.2,https://spdx.dev/Document
main.go:74: error during command execution: none of the attestations matched the predicate type: foo, found: https://liatr.io/attestations/github-pull-request/v1,https://slsa.dev/provenance/v0.2,https://spdx.dev/Document

We tried to ask cosign to verify the existence of a foo attestation, signed by the build-and-push workflow, and because there is no foo attestation, cosign will report that it wasn't able to find one.

The output will be similar if we ask cosign to verify an attestation that does exist, but was signed by a different identity. In this case, we'll try to verify that the vuln attestation was signed by the build-and-push workflow, when it was actually signed by the scan-image workflow.

$ cosign verify-attestation \
  --type vuln \
  --certificate-oidc-issuer https://token.actions.githubusercontent.com \
  --certificate-identity https://github.com/liatrio/gh-trusted-builds-workflows/.github/workflows/build-and-push.yaml@refs/heads/main \
  --certificate-github-workflow-repository liatrio/gh-trusted-builds-app \
  ghcr.io/liatrio/gh-trusted-builds-app@sha256:294bafb143807a4afe6b90e6b8b208b9616798effc48e4018b6b9eef9a6ef6bc
  
Error: none of the attestations matched the predicate type: vuln, found: https://liatr.io/attestations/github-pull-request/v1,https://slsa.dev/provenance/v0.2,https://spdx.dev/Document
main.go:74: error during command execution: none of the attestations matched the predicate type: vuln, found: https://liatr.io/attestations/github-pull-request/v1,https://slsa.dev/provenance/v0.2,https://spdx.dev/Document

In this case, the output is very similar, even though we know that the vuln attestation does exist. However, cosign first filters the signatures by the signer identities, so it's only looking at the identity that we specified (i.e., the build-and-push workflow).

Additional Resources

Sigstore Security Model - an overview of the security model for different Sigstore components.
in-toto specification
Fulcio certificate issuance overview
Fulcio certificate extensions
transparency.dev - an overview of the verifiable data structures behind Rekor and Fulcio's certificate transparency log.

Name		Name	Last commit message	Last commit date
Latest commit History 51 Commits
.github/workflows		.github/workflows
assets		assets
.gitignore		.gitignore
Dockerfile		Dockerfile
README.md		README.md
go.mod		go.mod
go.sum		go.sum
main.go		main.go
renovate.json		renovate.json

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