Mastering Google Cloud Pub/Sub Security: Controlling Cross-Project Access

Overview

Google Cloud Pub/Sub is a powerful messaging service that enables scalable, asynchronous communication between applications. It’s the backbone of many event-driven architectures on GCP, decoupling services and improving resilience. However, this flexibility comes with a critical governance challenge: managing Identity and Access Management (IAM) policies. One of the most significant risks is uncontrolled cross-project access, where a Pub/Sub topic in one project allows service accounts from other, potentially untrusted projects to publish messages.

While intentional cross-project communication is a valid architectural pattern for centralized logging or data aggregation, accidental or forgotten permissions create a major security vulnerability. A compromised service account in a non-production environment could gain a direct line into your production data pipeline.

This misconfiguration is not just a security flaw; it’s a FinOps concern. It undermines the logical separation of environments, complicates cost allocation, and can lead to unpredictable spending. Effectively governing Pub/Sub access is essential for maintaining a secure, cost-efficient, and well-managed GCP environment.

Why It Matters for FinOps

From a FinOps perspective, unmanaged cross-project access introduces significant business risk and financial waste. When the logical boundaries between projects are breached, the ability to practice effective cloud financial management is compromised. This impacts the organization through increased cost, operational drag, and governance failures.

The most direct financial impact comes from potential Denial of Service (DoS) attacks. An unauthorized publisher can flood a topic with messages, leading to massive, unexpected spikes in data volume and delivery charges. This waste is difficult to trace and can blow through project budgets.

Operationally, a security incident caused by malicious data injection can halt critical business processes, requiring significant engineering time to investigate, purge bad data, and restore service. For teams practicing chargeback or showback, this cleanup effort complicates cost attribution. Furthermore, in regulated industries, allowing data flow from an untrusted project can expand the scope of compliance audits for frameworks like PCI DSS, SOC 2, and HIPAA, dramatically increasing their complexity and cost.

What Counts as “Idle” in This Article

In this article, we define a risky configuration not as a truly "idle" resource but as an "improperly permissive" one. The focus is on identifying Google Cloud Pub/Sub topics that grant publishing permissions to service accounts belonging to external or untrusted GCP projects. This represents a governance gap and a violation of the principle of least privilege.

The primary signal of this misconfiguration is an IAM policy on a Pub/Sub topic that contains a binding for the roles/pubsub.publisher permission (or an equivalent custom role) assigned to a service account. The issue arises when the project ID associated with that service account does not match the topic’s own project ID and is not on a pre-approved list of trusted projects. This indicates a connection that bypasses standard organizational segmentation and requires immediate review.

Common Scenarios

Scenario 1

A developer, working on a feature in a temporary sandbox project, grants their test service account publishing rights to a production Pub/Sub topic for a quick integration test. After the test, the permission is forgotten. This "shadow IT" configuration leaves a direct, unmonitored pathway from an insecure environment into the production data stream.

Scenario 2

An organization has a centralized project dedicated to data analytics. Several application projects are configured to send telemetry and event data to Pub/Sub topics within this central project. This is a legitimate and intentional use of cross-project access, provided the application projects are formally documented and included in a "trusted project" list for governance purposes.

Scenario 3

A business integrates with a third-party SaaS provider that requires permission to publish events to a Pub/Sub topic in the company’s GCP project. This creates a supply chain risk. If the vendor’s environment is compromised, the attacker could gain access to the company’s internal message bus. This configuration is a high-risk violation unless the vendor has been thoroughly vetted and explicitly approved.

Risks and Trade-offs

The primary goal is to enforce the principle of least privilege by removing unauthorized access. However, the main trade-off is balancing immediate security hardening against the risk of operational disruption. Rushing to remove cross-project permissions without proper investigation can break critical, albeit poorly documented, business workflows that rely on those connections.

Before revoking access, it’s crucial to perform discovery and analysis. Teams must confirm whether a specific cross-project permission is actively used and serves a legitimate business purpose. This often involves collaborating with application owners and architects to understand the data flow. The risk of inaction is a potential security breach or cost overrun, while the risk of hasty action is an application outage. A phased approach of auditing, notifying owners, and then remediating is the safest path forward.

Recommended Guardrails

To prevent risky Pub/Sub configurations, organizations should implement a set of proactive governance policies and technical controls. These guardrails help shift the posture from reactive cleanup to automated prevention.

• Ownership and Tagging: Enforce a strict policy requiring all Pub/Sub topics to have a clear owner and cost center identified through resource tags. This ensures accountability and simplifies investigations.
• Trusted Project Perimeter: Formally define and maintain an organization-wide list of trusted GCP project IDs. This list should be the single source of truth for all automated security and compliance checks.
• Budget Alerts: Configure budget alerts on Pub/Sub services within projects to detect anomalous spikes in usage. A sudden, massive increase in message volume can be an early indicator of a DoS attack or a misbehaving publisher.
• Organization Policies: Use GCP Organization Policies to enforce broad security constraints. For example, the iam.allowedPolicyMemberDomains constraint can prevent IAM roles from being granted to identities outside your organization’s domain entirely.

Provider Notes

GCP

In Google Cloud, this entire scenario is governed by the interplay between Google Cloud Pub/Sub, the resource being secured, and Identity and Access Management (IAM), the service that controls access to it. The risk arises because IAM policies can grant permissions to principals (like service accounts) from any GCP project, not just the one hosting the resource. To manage this at scale, organizations should leverage Organization Policies to set coarse-grained restrictions on resource sharing, creating a secure-by-default environment that prevents many common misconfigurations before they happen.

Binadox Operational Playbook

Binadox Insight: Misconfigured IAM policies are a primary source of hidden security risk and financial waste in GCP. Cross-project permissions, while powerful, erode environmental boundaries and complicate unit economics if not governed with extreme prejudice.

Binadox Checklist:

• Audit all Pub/Sub topics to identify IAM policies with cross-project service account permissions.
• Establish and document an official "trusted project" list with business justification for each entry.
• Engage with resource owners to validate the business need for any identified cross-project access.
• Remediate unauthorized or unnecessary permissions in a controlled manner to avoid service disruption.
• Implement GCP Organization Policies to prevent the creation of new, unauthorized cross-project bindings.
• Set up continuous monitoring to alert on any new deviations from your defined governance policy.

Binadox KPIs to Track:

• Number of Pub/Sub topics with untrusted cross-project publisher roles.
• Mean Time to Remediate (MTTR) for discovered IAM misconfigurations.
• Percentage of Pub/Sub resources with clearly defined ownership tags.
• Number of policy violations blocked by preventative guardrails per quarter.

Binadox Common Pitfalls:

• Revoking permissions without analyzing usage, causing production outages for legitimate but undocumented integrations.
• Maintaining an incomplete or outdated "trusted project" list, leading to false positives in alerting.
• Failing to include third-party and vendor projects in the risk assessment process.
• Relying solely on manual audits instead of implementing automated discovery and prevention controls.

Conclusion

Securing Google Cloud Pub/Sub is more than a technical exercise; it is a fundamental practice in sound cloud governance and financial management. By treating unvetted cross-project access as a significant risk, teams can protect their data pipelines from abuse, prevent unexpected cost overruns, and maintain compliance with industry regulations.

The path forward involves moving beyond reactive fixes. A proactive approach, built on clear policies, automated guardrails, and continuous monitoring, ensures that your event-driven architecture remains both a powerful enabler of innovation and a secure, cost-effective component of your cloud estate.