Tenable has disclosed that its Tenable Cloud Research Team has discovered a vulnerability in Google Cloud Platform (GCP), involving its Cloud Function serverless compute service and its Cloud Build CI/CD pipeline service. GCP has remediated Confused Function for future Cloud Build accounts, however existing Cloud Build instances remain at risk with immediate evasive action required.
Cloud Functions in GCP are event-triggered, serverless functions. They automatically scale and execute code in response to specific events like HTTP requests or data changes. When a GCP user creates or updates a Cloud Function, a multi-step backend process is triggered. This process, among other things, attaches a default Cloud Build service account to the Cloud Build instance that is created as part of the function’s deployment. This default Cloud Build service account gives the user excessive permissions. This process happens in the background and isn’t something that ordinary users would be aware of.
An attacker who gains access to create or update a Cloud Function can take advantage of the function’s deployment process to escalate privileges to the default Cloud Build service account and other GCP services including Cloud Storage, and Artifact Registry or Container Registry. By exploiting the deployment flow and the flawed trust between services an attacker could run code as the default Cloud Build service account.
“The Confused Function vulnerability highlights the problematic scenarios that may arise due to software complexity and inter-service communication in a cloud provider’s services,” explains Liv Matan, senior research engineer, Tenable. “To support backward compatibility, GCP has not changed the privileges from Cloud Build service accounts created before the fix was implemented. This means that the vulnerability is still affecting existing instances and we highly recommend customers take immediate action.”
GCP confirmed it had remediated ConfusedFunction, to some extent, for Cloud Build accounts created after February 14, 2024. While the fix has reduced the severity of the problem for future deployments, it hasn’t completely eliminated it. For every cloud function using the legacy Cloud Build service account, the advice is to replace it with a least-privilege service account.