compute engine's service account has insufficient scopes for cloud vision api

I need to use Cloud Vision API in my python solution, I've been relying on an API key for a while now, but at the moment I'm trying to give my Compute Engine's default service account the scope needed to call Vision, with little luck so far.

I have enabled vision API in my project via cloud console, but I still get that 403 error:

Request had insufficient authentication scopes.

I would set access individually for each API from my gce's edit details tab, but couldn't find Vision listed along the other API's. The only way I managed to correctly receive a correct response from Vision API is by flagging the "Allow full access to all Cloud APIs" checkbox, again from my gce's edit details tab, but that doesn't sound too secure to me.

Hopefully there are better ways to do this, but I couldn't find any on Vision's documentation on authentication, nor in any question here on stack overflow (some had a close topic, but none of the proposed answers quite fitted my case, or provided a working solution).

Thank you in advance for your help.

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EDIT

I'm adding the list of every API I can individually enable in my gce's default service account from cloud console:

BigQuery; Bigtable Admin; Bigtable Data; Cloud Datastore; Cloud Debugger; Cloud Pub/Sub; Cloud Source Repositories; Cloud SQL; Compute Engine; Service Control; Service Management; Stackdriver Logging API; Stackdriver Monitoring API; Stackdriver Trace; Storage; Task queue; User info

None of them seems useful to my needs, although the fact that enabling full access to them all solves my problem is pretty confusing to me.

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EDIT #2

I'll try and state my question(s) more concisely: How do I add https://www.googleapis.com/auth/cloud-vision to my gce instance's default account?

I'm looking for a way to do that via any of the following: GCP console, gcloud command line, or even through Python (at the moment I'm using googleapiclient.discovery.build, I don't know if there is any way to ask for vision api scope through the library).

Or is it ok to enable all the scopes as long as limit the roles via IAM? And if that's the case how do I do that?

I really can't find my way around the documentation, thank you once again.

Google Cloud APIs (Vision, Natural Language, Translation, etc) do not need any special permissions, you should just enable them in your project (going to the API Library tab in the Console) and create an API key or a Service account to access them.

Your decision to move from API keys to Service Accounts is the correct one, given that Service Accounts are the recommended approach for authentication with Google Cloud Platform services, and for security reasons, Google recommends to use them instead of API keys.

That being said, I see that you are using the old Python API Client Libraries, which make use of the googleapiclient.discovery.build service that you mentioned. As of now, the newer idiomatic Client Libraries are the recommended approach, and they superseded the legacy API Client Libraries that you are using, so I would strongly encourage to move in that direction. They are easier to use, more understandable, better documented and are the recommended approach to access Cloud APIs programatically.

Getting that as the starting point, I will divide this answer in two parts:

Using Client Libraries

If you decided to follow my advice and migrate to the new Client Libraries, authentication will be really easy for you, given that Client Libraries use Application Default Credentials (ADC) for authentication. ADC make use of the default service account for Compute Engine in order to provide authentication, so you should not worry about it at all, as it will work by default.

Once that part is clear, you can move on to create a sample code (such as the one available in the documentation), and as soon as you test that everything is working as expected, you can move on to the complete Vision API Client Library reference page to get the information about how the library works.

Using (legacy) API Client Libraries If, despite my words, you want to stick to the old API Client Libraries, you might be interested in this other documentation page, where there is some complete information about Authentication using the API Client Libraries. More specifically, there is a whole chapter devoted to explaining OAuth 2.0 authentication using Service Accounts.

With a simple code like the one below, you can use the google.oauth2.service_account module in order to load the credentials from the JSON key file of your preferred SA, specify the required scopes, and use it when building the Vision client by specifying credentials=credentials:

from google.oauth2 import service_account
import googleapiclient.discovery

SCOPES = ['https://www.googleapis.com/auth/cloud-vision']
SERVICE_ACCOUNT_FILE = '/path/to/SA_key.json'

credentials = service_account.Credentials.from_service_account_file(
        SERVICE_ACCOUNT_FILE, scopes=SCOPES)

vision = googleapiclient.discovery.build('vision', 'v1', credentials=credentials)

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EDIT:

I forgot to add that in order for Compute Engine instances to be able to work with Google APIs, it will have to be granted with the https://www.googleapis.com/auth/cloud-platform scope (in fact, this is the same as choosing the Allow full access to all Cloud APIs). This is documented in the GCE Service Accounts best practices, but you are right that this would allow full access to all resources and services in the project.

Alternatively, if you are concerned about the implications of allowing "access-all" scopes, in this other documentation page it is explained that you can allow full access and then perform the restriction access by IAM roles.

In any case, if you want to grant only the Vision scope to the instance, you can do so by running the following gcloud command:

gcloud compute instances set-service-account INSTANCE_NAME --zone=INSTANCE_ZONE --scopes=https://www.googleapis.com/auth/cloud-vision

The Cloud Vision API scope (https://www.googleapis.com/auth/cloud-vision) can be obtained, as for any other Cloud API, from this page.

Additionally, as explained in this section about SA permissions and access scopes, SA permissions should be compliant with instance scopes; that means that most restrictive permission would apply, so you need to have that in mind too.