Google Cloud Build fetch Identity token

Question

in my scenario, I would like to trigger an Google Cloud Function based on HTTP endpoint during a Google Cloud Build. The HTTP request is done using a step with a python:3.7-slim container.

Based on this and this examples from the documentation, I wanted to use the following code:

REGION = 'us-central1'
PROJECT_ID = 'name-of-project'
RECEIVING_FUNCTION = 'my-cloud-function'

function_url = f'https://{REGION}-{PROJECT_ID}.cloudfunctions.net/{RECEIVING_FUNCTION}'

metadata_server_url = 'http://metadata/computeMetadata/v1/instance/service-accounts/default/identity?audience='
token_full_url = metadata_server_url + function_url
token_headers = {'Metadata-Flavor': 'Google'}

token_response = requests.get(token_full_url, headers=token_headers)
jwt = token_response.text
print(jwt)

r = requests.post(url=function_url, headers=function_headers, json=payload)

Surprisingly, the code fails because jwt is Not Found (according to the print statement). I already tested the code and IAM settings by hard coding a valid identity token and also tested the exact same fetching mechanism on a test VM inside the same project. The problem seems to be that the meta data fetching some is not working inside cloud build.

Am I missing something? Thank you for any help!

Answer 1

The solution is to use a new IAM api to generate an ID_TOKEN, on a service account with an access token, if the requester (this one who generate the access token) has the role Service Account Token Creator on the service account (or widely on the project).

This first example use direct API calls

 - name: gcr.io/cloud-builders/gcloud
   entrypoint: "bash"
   args:
    - "-c"
    - |
        curl -X POST -H "content-type: application/json" \
        -H "Authorization: Bearer $(gcloud auth print-access-token)" \
        -d '{"audience": "YOUR AUDIENCE"}' \
         "https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/YOUR SERVICE ACCOUNT:generateIdToken"
        # Use Cloud Build Service Account
        # service_account_email=$(gcloud config get-value account) 

And here the Python code version

- name: python:3.7
          entrypoint: "bash"
          args:
            - "-c"
            - |
                    pip3 install google-auth requests
                    python3 extract-token.py

And extract-token.py content the following code

REGION = 'us-central1'
PROJECT_ID = 'name-of-project'
RECEIVING_FUNCTION = 'my-cloud-function'
function_url = f'https://{REGION}-{PROJECT_ID}.cloudfunctions.net/{RECEIVING_FUNCTION}'

import google.auth
credentials, project_id = google.auth.default(scopes='https://www.googleapis.com/auth/cloud-platform')

# To use the Cloud Build service account email
service_account_email = credentials.service_account_email
#service_account_email = "YOUR OWN SERVICE ACCOUNT"

metadata_server_url = f'https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/{service_account_email}:generateIdToken'
token_headers = {'content-type': 'application/json'}

from google.auth.transport.requests import AuthorizedSession
authed_session = AuthorizedSession(credentials)
import json
body = json.dumps({'audience': function_url})

token_response = authed_session.request('POST',metadata_server_url, data=body, headers=token_headers)
jwt = token_response.json()
print(jwt['token'])

Don't hesitate if you need more details.

I think I will write an article on this on Medium, if you want I name you, let me know