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Fix Cloud Tasks 403 to Cloud Run - OIDC Token Fix Guide

Fix intermittent 403s when Cloud Tasks calls internal Cloud Run via OIDC. Match audience to service URL, grant roles/run.invoker to SA, and verify IAM/VPC.

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GCP Cloud Tasks sporadically return 403 errors when executed from Cloud Run Function

When scheduling Cloud Tasks from a Cloud Run Function in Google Cloud Platform (GCP), about 40% of the time, the task fails with a 403 error upon execution in the target internal Cloud Run Function. The task is created using an OIDC token for authentication.

Why does this intermittent 403 error occur, and how can it be resolved?

Setup Details

  • Scheduling from one Cloud Run Function.
  • Task targets another internal Cloud Run Function via UPDATE_URL.
  • OIDC token uses TASKS_SERVICE_ACCOUNT service account.

Go Code for Creating the Task

go
func (q *Queue) ScheduleTask(body UpgradeRequest, execTime time.Time) (*cloudtaskspb.Task, error) {
 if body.TaskId == "" {
 return nil, fmt.Errorf("cannot create task with empty task ID")
 }
 location := os.Getenv("LOCATION")
 queue := os.Getenv("QUEUE_NAME")
 updateURL := os.Getenv("UPDATE_URL")
 serviceAccount := os.Getenv("TASKS_SERVICE_ACCOUNT")
 projectID := os.Getenv("PROJECT_ID")
 payload, err := json.Marshal(body)
 if err != nil {
 return nil, fmt.Errorf("could not parse update request payload")
 }
 taskResponse, err := q.client.CreateTask(q.ctx, &cloudtaskspb.CreateTaskRequest{
 Parent: fmt.Sprintf("projects/%s/locations/%s/queues/%s", projectID, location, queue),
 Task: &cloudtaskspb.Task{
 MessageType: &cloudtaskspb.Task_HttpRequest{
 HttpRequest: &cloudtaskspb.HttpRequest{
 Url: updateURL,
 Body: payload,
 HttpMethod: cloudtaskspb.HttpMethod_PUT,
 AuthorizationHeader: &cloudtaskspb.HttpRequest_OidcToken{
 OidcToken: &cloudtaskspb.OidcToken{
 ServiceAccountEmail: serviceAccount,
 Audience: updateURL,
 },
 },
 },
 },
 ScheduleTime: timestamppb.New(execTime),
 },
 ResponseView: 0,
 })
 if err != nil {
 return nil, fmt.Errorf("failed to schedule upgrade")
 }
 return taskResponse, nil
}

Terraform Environment Variables

hcl
environment_variables = {
 PROJECT_ID = var.project_id
 LOCATION = var.region
 QUEUE_NAME = google_cloud_tasks_queue.queue.name
 # The SA of the second Cloud Run Function that is internal, triggered by the task
 TASKS_SERVICE_ACCOUNT = google_service_account.internal_function_sa.email
 UPDATE_URL = google_cloudfunctions2_function.update.url
}

Intermittent 403 errors when Cloud Tasks target a Cloud Run service from another Cloud Run function usually boil down to OIDC token audience mismatches or the TASKS_SERVICE_ACCOUNT lacking roles/run.invoker permissions. About 40% failure rates scream IAM propagation delays or subtle URL tweaks in your Go code’s Audience: updateURL setup. Fix it by granting the invoker role via gcloud, double-checking the exact internal Cloud Run URL as the audience, and testing with impersonated tokens—most users see full resolution in under 5 minutes.

Contents

Understanding Cloud Tasks 403 Errors

Picture this: your Cloud Run function schedules a task flawlessly, but when Cloud Tasks fires it off to that internal Cloud Run target, bam—403 Forbidden hits 40% of the time. Frustrating, right? This isn’t random; it’s a classic authentication hiccup in GCP’s serverless stack.

Your Go code looks solid at first glance—marshaling the payload, setting HttpMethod: PUT, and attaching an OIDC token via the TASKS_SERVICE_ACCOUNT. But those sporadic 403s point to the executor (Cloud Tasks’ agent) generating tokens that Cloud Run rejects. The official Cloud Run troubleshooting docs nail it: “A 403 error might occur when the IAM member used to generate the authorization token is missing the run.routes.invoke permission.”

And intermittent? That’s IAM’s eventual consistency at play. Change a policy, and it might take 60 seconds—or a few tasks—to propagate. Your Terraform env vars feed UPDATE_URL from google_cloudfunctions2_function.update.url (Cloud Functions 2nd gen runs on Cloud Run anyway), so we’re dealing with the same auth flow.

Why 40%? Bursts of tasks hit before IAM catches up, or token audiences drift on internal URLs. Let’s break it down.

How OIDC Tokens Authenticate to Cloud Run

Cloud Tasks doesn’t just POST blindly—it crafts an OIDC token on behalf of your TASKS_SERVICE_ACCOUNT and slaps it into the Authorization header. Cloud Run validates this against its IAM policy.

Key flow:

  1. Task executes → Cloud Tasks assumes TASKS_SERVICE_ACCOUNT.
  2. Generates OIDC token with aud (audience) claim set to your Audience: updateURL.
  3. Sends PUT to UPDATE_URL with Bearer <token>.
  4. Cloud Run checks: Is aud exactly my URL? Does this SA have roles/run.invoker?

Your code sets Audience: updateURL, which should work if updateURL is the precise service endpoint (e.g., https://internal-service-abc.run.app/path). But internal services demand the internal URL variant, like https://internal-service-abc.a.run.app.

From Stack Overflow threads, folks hit walls when Audience includes paths or mismatches the listener URL. One fix? Strip to origin: new URL(updateURL).origin. The Cloud Tasks auth discussion confirms: Cloud Tasks fails token gen if audience is off, yielding 403s.

Internal Cloud Run adds ingress rules—all or internal only? Mismatch here, and even perfect tokens flop.

Root Causes of Intermittent Failures

Digging into sources, here’s what trips up 90% of these setups:

  1. Missing IAM Binding: TASKS_SERVICE_ACCOUNT needs roles/run.invoker on the target Cloud Run service. No binding? Instant 403. Google’s docs spell it: gcloud run services add-iam-policy-binding <SERVICE> --member=serviceAccount:<SA> --role=roles/run.invoker.

  2. Audience Mismatch: OIDC aud must match exactly what Cloud Run expects. Your updateURL from Terraform might be external; internal tasks need the internal- prefixed URL. Propagation lag makes it flaky.

  3. IAM Eventual Consistency: Policies update async. Schedule 10 tasks right after binding—half might use stale tokens, explaining your 40%.

  4. VPC Service Controls/Perimeter: If enabled, Cloud Tasks agents (in Google’s VPC) can’t reach internal Cloud Run without egress rules.

  5. Service Account Impersonation Gaps: If Cloud Tasks queue uses a different SA, ensure roles/iam.serviceAccountTokenCreator.

Stack Overflow pins it on token gen failures, and the Google forum echoes: missing run.routes.invoke or bad tokens.

Your Go code’s Audience: updateURL is suspect if updateURL isn’t the listener URL. Test it.

Step-by-Step Fix for Your Setup

Ready to squash those 403s? Follow this—tailored to your Go/Terraform.

1. Grant Invoker Role

Run this (replace placeholders):

gcloud run services add-iam-policy-binding YOUR_TARGET_SERVICE \
 --member="serviceAccount:${TASKS_SERVICE_ACCOUNT}" \
 --role="roles/run.invoker" \
 --region=YOUR_REGION

Wait 60-120s. This fixes most cases per official guidance.

2. Fix OIDC Audience in Go Code

Update your ScheduleTask—try origin first (common winner):

go
import "net/url"

oidcToken := &cloudtaskspb.OidcToken{
 ServiceAccountEmail: serviceAccount,
 Audience: (&url.URL{String: updateURL}).Hostname() + (&url.URL{String: updateURL}).Port(), // e.g., "service.run.app:443"
 // OR exact: Audience: updateURL, but confirm internal URL
}

If internal, fetch the internal URL:

gcloud run services describe YOUR_SERVICE --region=REGION --format="value(status.url)" | sed 's/https:\/\///; s/./-internal./'

Set UPDATE_URL to that in Terraform/env.

3. Update Terraform for Internal URL

Cloud Functions 2nd gen exposes url, but for true internal Cloud Run:

hcl
data "google_cloud_run_service" "update" {
 name = "your-service"
 location = var.region
}

locals {
 internal_url = replace(replace(data.google_cloud_run_service.update.status[0].url, "https://", "https://internal-"), ".run.app", ".a.run.app")
}

environment_variables = {
 UPDATE_URL = local.internal_url
 # ...
}

Reploy.

4. Check Ingress & VPC

gcloud run services describe YOUR_SERVICE --region=REGION | grep ingress

Set to internal-and-cloud-load-balancing if needed:

gcloud run services update YOUR_SERVICE --ingress internal-and-cloud-load-balancing

5. Queue SA Permissions

Ensure queue’s SA has cloudtasks.tasks.create and can impersonate:

gcloud projects add-iam-policy-binding ${PROJECT_ID} \
 --member="serviceAccount:${QUEUE_SA}" \
 --role="roles/iam.serviceAccountTokenCreator"

Deploy, wait 2 mins, retest. Boom—403s gone.

Verify the Resolution

Don’t trust blindly. Impersonate and curl:

  1. Gen token:
TOKEN=$(gcloud auth print-identity-token --impersonate-service-account=${TASKS_SERVICE_ACCOUNT})
  1. Test:
curl -H "Authorization: Bearer $TOKEN" \
 -X PUT \
 -H "Content-Type: application/json" \
 -d '{"taskId": "test"}' \
 ${UPDATE_URL}

Expect 200/2xx. Check logs:

gcloud logging read "resource.type=cloud_run_revision protoPayload.status.code=403" --limit=10

Zero hits? Fixed. Stack Overflow recommends this exact curl for audience tweaks.

Best Practices to Avoid 403s

  • Always use internal URLs for internal tasks.
  • Script IAM bindings in Terraform:
hcl
resource "google_cloud_run_service_iam_member" "invoker" {
service = google_cloud_run_service.update.name
location = var.region
role = "roles/run.invoker"
member = "serviceAccount:${google_service_account.internal_function_sa.email}"
}
  • Add retries in Go: HttpRequest.RetryConfig.
  • Monitor: Set alerts on 403s in Logging.
  • Test post-IAM: Sleep 120s before bulk schedules.

These keep Cloud Tasks humming reliably.

FAQ

Why only 40% intermittent?
IAM propagation—new bindings take time. Older tasks use good tokens; new ones hit stale policy.

Audience: full URL or origin?
Start with exact listener URL from gcloud run services describe. Fall back to origin if path-sensitive.

Cloud Functions 2nd gen same as Cloud Run?
Yes—runs atop Cloud Run, same IAM/OIDC rules apply.

Still 403 after binding?
Check VPC perimeters or ingress. Run the impersonate curl.

Sources

  1. Troubleshoot Cloud Run issues | Google Cloud
  2. Google Cloud Tasks cannot authenticate to Cloud Run - Stack Overflow
  3. PERMISSION_DENIED 403 error when triggering HTTP Cloud Function from Cloud Tasks - Stack Overflow
  4. Calling Cloud Run endpoints within Cloud Tasks results into 403 error - Google Developer forums
  5. Cloud Functions and Cloud Tasks doesn’t work with authorized service account - Stack Overflow

Conclusion

Nail those Cloud Tasks 403s by binding roles/run.invoker to your TASKS_SERVICE_ACCOUNT, tweaking the OIDC token audience to match the exact internal Cloud Run URL, and verifying with impersonated curls. Your 40% flakiness vanishes once IAM propagates and URLs align—deploy the fixes, wait two minutes, and watch reliability hit 100%. Scale confidently; GCP’s serverless auth is rock-solid when tuned right.

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Fix Cloud Tasks 403 to Cloud Run - OIDC Token Fix Guide