GCP - Public Buckets Privilege Escalation

Buckets Privilege Escalation

If the bucket policy allowed either “allUsers” or “allAuthenticatedUsers” to write to their bucket policy (the storage.buckets.setIamPolicy permission), then anyone can modify the bucket policy and grant himself full access.

Check Permissions

There are 2 ways to check the permissions over a bucket. The first one is to ask for them by making a request to https://www.googleapis.com/storage/v1/b/BUCKET_NAME/iam or running gsutil iam get gs://BUCKET_NAME.

However, if your user (potentially belonging to allUsers or allAuthenticatedUsers") doesn't have permissions to read the iam policy of the bucket (storage.buckets.getIamPolicy), that won't work.

The other option which will always work is to use the testPermissions endpoint of the bucket to figure out if you have the specified permission, for example accessing: https://www.googleapis.com/storage/v1/b/BUCKET_NAME/iam/testPermissions?permissions=storage.buckets.delete&permissions=storage.buckets.get&permissions=storage.buckets.getIamPolicy&permissions=storage.buckets.setIamPolicy&permissions=storage.buckets.update&permissions=storage.objects.create&permissions=storage.objects.delete&permissions=storage.objects.get&permissions=storage.objects.list&permissions=storage.objects.update

Escalating

In order to grant Storage Admin to allAuthenticatedUsers it's possible to run:

gsutil iam ch allAuthenticatedUsers:admin gs://BUCKET_NAME

Another attack would be to remove the bucket an d recreate it in your account to steal th ownership.

References

https://rhinosecuritylabs.com/gcp/google-cloud-platform-gcp-bucket-enumeration/