Overview
Google Cloud Pub/Sub is a powerful messaging service that forms the backbone of modern event-driven architectures on GCP. Its flexibility allows services across different projects to communicate seamlessly. However, this same flexibility can introduce significant security and cost management challenges, particularly through misconfigured cross-project Identity and Access Management (IAM) policies.
When a Pub/Sub topic or subscription grants permissions to a user or service account from an external or untrusted GCP project, it creates a direct pathway for unauthorized data access. This misconfiguration can easily go unnoticed, silently exposing sensitive information and creating hidden cost drivers. Effective governance requires a clear understanding of these cross-project relationships to ensure that data only flows where it is explicitly intended and approved.
Why It Matters for FinOps
From a FinOps perspective, unmanaged cross-project access in GCP Pub/Sub introduces multifaceted risks. The most direct financial impact comes from unexpected data egress costs. An unauthorized subscription in an external project can pull massive volumes of data, leading to significant and unforeseen charges on the source project’s bill. This uncontrolled consumption represents pure waste.
Beyond direct costs, the business risks are substantial. Unchecked access is a primary vector for data exfiltration, potentially leading to breaches of customer data, intellectual property theft, and severe regulatory fines under frameworks like GDPR, HIPAA, or PCI-DSS. This failure of governance not only damages customer trust but also creates operational drag, as teams must divert resources to investigate and remediate security incidents instead of focusing on value-generating activities.
What Counts as “Idle” in This Article
In the context of this article, we aren’t focused on idle compute or storage. Instead, we define "idle" or, more accurately, "unauthorized access," as any IAM permission on a Pub/Sub resource that grants access to an identity outside of a defined trust boundary. This is not about zero usage, but about risky, unvetted, or forgotten access.
Signals of this type of waste and risk include IAM policy bindings that reference service accounts from unknown or non-allowlisted GCP projects. Other indicators are permissions granted to development or sandbox project identities on production resources, or access granted to third-party vendors without a formal review and approval process. These configurations represent a latent threat and a potential violation of cost and security governance.
Common Scenarios
Scenario 1
A valid "hub-and-spoke" architecture is a common use case. A central logging or security project may need to subscribe to topics in dozens of application projects to aggregate data. In this scenario, cross-project access is intentional and necessary for operations. The key is that the central "hub" project is explicitly defined, documented, and included in a trusted project allowlist.
Scenario 2
A developer troubleshooting a production issue might grant a service account from their development project temporary access to a production Pub/Sub subscription. This action, often done with good intentions, directly exposes sensitive production data to a lower-security environment. If this "temporary" permission is never revoked, it becomes a persistent and dangerous security vulnerability.
Scenario 3
Integrating with a third-party SaaS analytics platform often requires granting their GCP service account permission to publish to or subscribe from your topics. While this may be a legitimate business need, it introduces risk. The external project ID must be formally vetted, approved, and monitored. Without proper governance, this creates a blind spot where your data is flowing to an external entity with minimal oversight.
Risks and Trade-offs
The primary risk of misconfigured cross-project access is data exfiltration, where sensitive information is leaked outside of its intended environment. Another significant risk is data poisoning; if an untrusted identity gains publisher permissions, it can inject malicious or malformed messages, potentially causing denial of service or corrupting downstream data stores.
However, remediation carries its own risks. A heavy-handed approach of revoking all external access without proper analysis could break critical business processes. A legitimate data pipeline to a trusted analytics project or a central security service might be inadvertently severed, causing operational disruption. The trade-off is between immediate security hardening and ensuring business continuity. A careful, audit-first approach is necessary to avoid breaking production workflows.
Recommended Guardrails
Effective governance over Pub/Sub access requires proactive, policy-driven guardrails. Start by establishing a clear data classification and tagging strategy to identify topics and subscriptions that handle sensitive information. Based on this, develop an explicit allowlist of "trusted projects" that are authorized to interact with your core services.
Implement an approval workflow for any new cross-project IAM binding requests, ensuring that each has a documented business justification, an owner, and a review date. Use GCP’s native tooling to set budgets and create alerts that trigger on anomalous cost spikes, which can be an early indicator of unauthorized data consumption. Enforcing these policies ensures that access is granted based on the principle of least privilege and is actively managed throughout its lifecycle.
Provider Notes
GCP
In Google Cloud, managing this risk centers on two key services. Access control for Google Cloud Pub/Sub is managed through Identity and Access Management (IAM) policies applied to individual topics and subscriptions. To prevent misconfigurations at scale, you can leverage Organization Policies to enforce constraints like iam.allowedPolicyMemberDomains, which restricts IAM permissions to principals within your own organization, effectively blocking access from external or personal accounts.
Binadox Operational Playbook
Binadox Insight: A single, forgotten IAM policy on a Pub/Sub resource can bypass your entire network security perimeter. This makes vigilant IAM hygiene just as critical as firewall management for preventing data breaches and controlling cloud costs.
Binadox Checklist:
- Inventory all Pub/Sub topics and subscriptions with cross-project IAM bindings.
- Develop and maintain a formal "Trusted Project" allowlist for your organization.
- Review all existing external permissions against the allowlist and business justifications.
- Revoke unauthorized or unnecessary permissions after verifying they are not in active use.
- Implement GCP Organization Policies to restrict sharing with untrusted domains.
- Configure alerts for IAM policy changes that add external principals to sensitive resources.
Binadox KPIs to Track:
- Number of Pub/Sub resources with policies granting access to untrusted projects.
- Mean Time to Remediate (MTTR) for discovered unauthorized access policies.
- Percentage of projects covered by restrictive Organization Policies.
- Number of approved exceptions for third-party service account access.
Binadox Common Pitfalls:
- Revoking access without first analyzing usage, causing production outages.
- Focusing only on subscriber permissions while ignoring the risk of unauthorized publishers.
- Failing to create and enforce a clear definition of a "trusted" project.
- Lacking an automated process to continuously scan for new misconfigurations.
Conclusion
Securing Google Cloud Pub/Sub is a critical component of a mature cloud governance strategy. By treating unauthorized cross-project access as a source of both security risk and financial waste, organizations can take proactive steps to mitigate the threat.
The path forward involves moving from a reactive to a proactive stance. Begin by auditing your current environment to understand your exposure. Then, implement automated guardrails and clear governance policies to ensure that your messaging infrastructure remains a secure and efficient asset, not a hidden liability.
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