Cannot access Google Cloud Storage bucket from GitHub Actions Workflow

I have the Django test case test_retrieve_bucket to test access to a GCP storage bucket.

from django.test import SimpleTestCase, TestCase
from google.cloud import storage

class DemoTest(TestCase):
def setUp(self):
    self.expense = Expense.objects.create(
        invoice_number = "ABC",
        account_number = "123",
        customer_name = "XYZ",
        invoice_amount = 12.50,
        invoice_date = datetime.now()
    )
def test1(self):
    return self.expense.invoice_number == "ABC"
def test2(self):
    return self.expense.account_number == "123"
def test3(self):
    return self.expense.customer_name == "XYZ"

def test_retrieve_bucket(self):
    bucket = "test_bucket_8866"
    client = storage.Client()
    bucket = client.bucket(bucket)
    return self.assertTrue(bucket.exists())

However, the test fails, and this is the error I am receiving:

google.api_core.exceptions.Forbidden: 403 GET https://storage.googleapis.com/storage/v1/b/test_bucket_8866?fields=name&prettyPrint=false: test-service-account@tbi-finance.iam.gserviceaccount.com does not have storage.buckets.get access to the Google Cloud Storage bucket. Permission 'storage.buckets.get' denied on resource (or it may not exist).

I successful authenticated the service account with Workload Identity Federation in the previous step:

The service account I used also has Storage Object Admin permission, which should give me access to the bucket:

Here is my workflow file:

name: Django CI

on:
  push:
    branches: [ "main" ]
  pull_request:
    branches: [ "main" ]

jobs:
  build:

    runs-on: ubuntu-latest
    strategy:
      max-parallel: 4
      matrix:
        python-version: [3.12.4]

    steps:
    - uses: actions/checkout@v4
    - name: auth
      uses: google-github-actions/auth@v2.0.0
      with:
        workload_identity_provider: 'projects/334572487877/locations/global/workloadIdentityPools/learn-github-actions-pool/providers/github-cdci'
        service_account: 'test-service-account@tbi-finance.iam.gserviceaccount.com'
    - name: Set up Python ${{ matrix.python-version }}
      uses: actions/setup-python@v3
      with:
        python-version: ${{ matrix.python-version }}
    - name: Install Dependencies
      run: |
        pip install pipenv && pipenv install --system
    - name: Run Tests
      run: |
        python manage.py test
permissions:
  contents: 'read'
  id-token: 'write'

When I ran the Django tests locally with the service account above, all of the tests passed. Is there anything else I'm missing?

Edit: This was the command I used to add the role WorkloadIdentityUser to Workload Identity Pool:

gcloud iam service-accounts add-iam-policy-binding "test-service-account@tbi-finance.iam.gserviceaccount.com" \
        --project="tbi-finance" \
        --role="roles/iam.workloadIdentityUser" \
        --member="principalSet://iam.googleapis.com/projects/334572487877/locations/global/workloadIdentityPools/learn-github-actions-pool/attribute.repository/duybtr/django_cdci"

I was able to get a repro of your solution working but....Oh boy!

In the interests of simplicity, I'm going to enumerate the various resources that I created and I hope that you're able to infer the difference that get you working.

Per this comment I encourage you to add:

the GitHub OIDC Debugger after checkout and before auth
token_format: access_token (if not already)

steps:
- name: checkout
  uses: actions/checkout@v4
- name: oidc-debugger
  uses: github/actions-oidc-debugger@main
  with:
    audience: https://iam.googleapis.com/{PROVIDER}
- name: auth
  uses: google-github-actions/auth@v2
  with:
    workload_identity_provider: {PROVIDER}
    service_account: {EMAIL}
    create_credentials_file: true
    token_format: access_token

Be wary of red-herrings. I received a different error than you but this is the result of misconfiguration and not because the Google Project's IAM was incorrect:

Error: google-github-actions/auth failed with: failed to generate Google Cloud OAuth 2.0 Access Token for ${EMAIL}:

{
  "error": {
    "code": 403,
    "message": "Permission 'iam.serviceAccounts.getAccessToken' denied on resource (or it may not exist).",
    "status": "PERMISSION_DENIED",
    "details": [
      {
        "@type": "type.googleapis.com/google.rpc.ErrorInfo",
        "reason": "IAM_PERMISSION_DENIED",
        "domain": "iam.googleapis.com",
        "metadata": {
          "permission": "iam.serviceAccounts.getAccessToken"
        }
      }
    ]
  }
}

Pool

gcloud iam workload-identity-pools describe ${POOL} \
  --project="${PROJECT}" \
  --location="global"

displayName: GitHub Actions Pool
name: projects/{NUMBER}/locations/global/workloadIdentityPools/{POOL}
state: ACTIVE

Provider

gcloud iam workload-identity-pools providers describe ${PROVIDER} \
--project=${PROJECT} \
--location="global" \
--workload-identity-pool=${POOL}

attributeCondition: assertion.repository_owner=="{OWNER}" && assertion.repository=="{OWNER}/{REPO}"
attributeMapping:
  attribute.actor: assertion.actor
  attribute.repository: assertion.repository
  attribute.repository_owner: assertion.repository_owner
  google.subject: assertion.sub
displayName: {PROVIDER}
name: projects/{NUMBER}/locations/global/workloadIdentityPools/{POOL}/providers/{PROVIDER}
oidc:
  issuerUri: https://token.actions.githubusercontent.com
state: ACTIVE

Project IAM

gcloud projects get-iam-policy ${PROJECT}

bindings:
- members:
  - serviceAccount:{EMAIL}
  role: roles/iam.serviceAccountTokenCreator
- members:
  - user:{ME}
  role: roles/owner
- members:
  - serviceAccount:{EMAIL}
  role: roles/storage.admin
etag: [REDACTED]
version: 1

NOTE Per my comment, there's no roles/iam.workloadIdentityUser on the Project's policy; this role is present on the Service Account's policy

Service Account IAM

gcloud iam service-accounts get-iam-policy ${EMAIL}

bindings:
- members:
  - principalSet://iam.googleapis.com/projects/{NUMBER}/locations/global/workloadIdentityPools/{POOL}/attribute.repository/{OWNER}/{REPO}
  role: roles/iam.workloadIdentityUser
etag: [REDACTED]
version: 1

Variables

Name	Value
`ACCOUNT`	Service Account
`EMAIL`	Service Account email address: `${ACCOUNT}@${PROJECT}.iam.gserviceaccount.com`
`ME`	My Google account
`NUMBER`	Google Cloud Project number
`OWNER`	GitHub User Account or `${ORG}`
`POOL`	Workload Identity Pool
`PROJECT`	Google Cloud Project ID
`PROVIDER`	Workload Identity Provider
`REPO`	GitHub repo name