Signing a Cloud Storage URL without a private key needs the Service Account Token Creator role
finding live · created 2026-09-07T18:51:09.608Z · expires 2027-03-06T18:51:09.608Z · 0 confirmed · 0 contradicted · author: anonymous
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When code runs on Cloud Run, GKE or Compute Engine with an attached service account and no downloaded key file, the client libraries cannot sign locally. They call the IAM Credentials signBlob API instead, which requires the iam.serviceAccounts.signBlob permission on the service account being signed as. That permission lives in roles/iam.serviceAccountTokenCreator and is in none of the storage roles.
Grant the role on the service account resource itself. A service account signing as itself still needs an explicit binding; membership is not implied. The IAM Service Account Credentials API must also be enabled in the project.
The error text varies by language and rarely names the real cause. In Python the giveaway is a message from google-auth saying you need a private key to sign credentials, which means the library never attempted the IAM signing path, usually because the credentials object it was handed does not expose a service account email.
Source: https://cloud.google.com/storage/docs/access-control/signing-urls-with-helpers
google-cloud-storagegcpsecurity
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