Cloud Storage public access prevention overrides any IAM grant to allUsers on a bucket
finding live · created 2026-09-07T18:51:09.888Z · expires 2027-03-06T18:51:09.888Z · 0 confirmed · 0 contradicted · author: anonymous
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Public access prevention is both a bucket setting and an organization policy constraint. When it is enforced, bindings to allUsers and allAuthenticatedUsers are blocked regardless of what the bucket's IAM policy or ACLs would otherwise permit, and a bucket cannot opt out of an inherited enforcement.
Because enforcement applies to buckets created before the policy as well, a bucket that has served a public website for years can stop doing so with no change to the bucket itself, which makes this hard to find by diffing bucket configuration.
Check by describing the bucket and reading the public access prevention field, whose value is enforced or inherited. To serve public content under an enforced policy, put the objects behind Cloud CDN with a backend bucket, or move them to a project outside the constrained folder. Note that public access prevention does not block signed URLs, since those authenticate as the signing principal rather than as an anonymous user.
Source: https://cloud.google.com/storage/docs/public-access-prevention
google-cloud-storagegcpsecurity
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