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GCP Storage Security: Guide to Preventing Silent Takeovers

By William OGOU
GCP Storage Security GCS Security Cloud Security IAM Subdomain Takeover Dangling Bucket CISO
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Google Cloud Storage (GCS) is everywhere — simple, powerful, and trusted to hold everything from website files to sensitive customer data. But that simplicity can be dangerous.

One overlooked setting. One abandoned DNS record. That’s all it takes to turn your secure bucket into an open door.

Forget just “public buckets” — today’s threats are smarter. Dangling bucket takeovers are the new silent breach.

This guide is your no-fluff blueprint to lock down GCS and stay one step ahead of attackers.

The Core Truths: Why GCS Buckets Are a Unique Security Challenge

To defend GCS effectively, you must first understand two fundamental properties that make it different from a simple file server.

  • Every Bucket is a Global DNS Entry: This is the single most important concept to grasp. A GCS bucket name is not just a label within your project; it is a globally unique namespace. When you create a bucket named my-company-assets, you are claiming my-company-assets.storage.googleapis.com. No one else in the world can create a bucket with that name. This property is the root cause of the powerful subdomain takeover attacks we will explore.
  • The Dual Permission Model: IAM vs. Legacy ACLs: GCP provides two ways to control access to buckets:
    • IAM (Identity and Access Management): The modern, recommended, and organization-wide method for granting roles and permissions to principals (users, groups, service accounts).
    • Access Control Lists (ACLs): A legacy, object-specific permission system.

This duality can create immense confusion. A user might have IAM permissions at the project level but be blocked by a more restrictive ACL on a specific object. The key rule to remember is that GCP always enforces the most restrictive policy. If an IAM policy allows access but an ACL denies it, access is denied. To simplify this and prevent conflicts, Google strongly recommends using Uniform Bucket-Level Access (UBLA), which disables ACLs and makes IAM the single source of truth for permissions.

The Modern Attack Vectors: Beyond Just “Public Buckets”

While accidentally public buckets are still a major problem, sophisticated attackers have a wider array of tools in their arsenal.

1. The Open Door: Accidental Public Access

This is the classic misconfiguration. A developer, needing to share a file quickly, might grant access to allUsers (making it fully public) or allAuthenticatedUsers (accessible to anyone with any Google account). While seemingly temporary, these settings are often forgotten, leaving sensitive data exposed indefinitely.

2. The Forgotten Key: Leaked Service Account Credentials

Your bucket’s security is only as strong as the identities that can access it. If a service account key with permissions like storage.objects.list and storage.objects.get is accidentally leaked—for instance, hardcoded in a public GitHub repository—an attacker can use that key to exfiltrate all the data in your private buckets, completely bypassing network-level controls. This is part of a broader GCP IAM privilege escalation threat landscape.

3. The Hijacked Signpost: The “Dangling Bucket Takeover”

This is the subtle, high-impact threat that Google’s security team has explicitly warned about. It’s a type of subdomain takeover that leverages the global uniqueness of bucket names.

Here’s the kill chain:

  • The Legitimate Setup: Your team creates a GCS bucket, my-app-assets-prod, and configures a user-friendly DNS CNAME record, assets.mycompany.com, to point to it. This is a very common practice for serving static content.
  • The “Dangling” Mistake: Months later, the application is decommissioned. A well-meaning engineer deletes the GCS bucket, my-app-assets-prod, to save costs. However, they forget to delete the CNAME record in the company’s DNS zone. The signpost, assets.mycompany.com, is now “dangling”—pointing to a GCS bucket that no longer exists.
  • The Attacker’s Move: An attacker, continuously scanning for such dangling DNS records, discovers that assets.mycompany.com points to a non-existent bucket.
  • The Takeover: Because bucket names are globally unique and the original has been deleted, the attacker simply creates a new GCS bucket in their own Google Cloud project and gives it the exact same name: my-app-assets-prod.
  • Full Compromise: The attacker now controls the bucket that your corporate domain, assets.mycompany.com, points to. They can use this to:
    • Host a phishing site under your legitimate domain, destroying user trust.
    • Serve malicious JavaScript to visitors of your other web properties that reference this domain.
    • Distribute malware from a domain that your users and security tools implicitly trust.

This attack is devastating because it turns your own trusted domain into a weapon against your users and your brand.

The Blueprint for a Fortified GCS: A 4-Pillar Defense Strategy

Securing your GCS environment requires a proactive, multi-layered strategy that goes beyond just checking for public buckets.

Pillar 1: Establish Foundational, Preventative Guardrails

Prevent misconfigurations from happening in the first place with powerful, top-down controls.

  • Enforce Public Access Prevention: Use the Organization Policy Service to apply the storage.publicAccessPrevention constraint at the organization or folder level. This makes it impossible for anyone to create a public bucket, regardless of their IAM permissions. It’s a simple, powerful, and essential guardrail.
  • Use Domain Restricted Sharing: If you need to share data with external partners, don’t open it up to the world. Use the iam.allowedPolicyMemberDomains constraint to ensure that IAM policies can only grant access to identities within your organization and a list of trusted partner organizations.
  • Leverage IAM Deny Policies: For your most critical buckets, create an IAM Deny Policy that explicitly forbids permissions like storage.setIamPolicy for all but a highly-secured “break-glass” administrator group. A deny policy always overrides an allow, creating an unbreakable safeguard against accidental or malicious tampering. This is part of implementing defense in depth for GCP IAM.

Pillar 2: Enforce the Principle of Least Privilege

  • Mandate Uniform Bucket-Level Access (UBLA): Make UBLA the default for all new buckets. This disables legacy ACLs and makes IAM the single, unambiguous source of truth for permissions, drastically simplifying audits and reducing the risk of conflicting policies.
  • Use Granular IAM Roles: Avoid granting overly broad primitive roles like Owner, Editor, or Viewer. Use specific, predefined roles like Storage Object Viewer (roles/storage.objectViewer) or Storage Object Creator (roles/storage.objectCreator). Create custom roles if necessary to grant only the precise permissions required.

Pillar 3: Implement a Secure Decommissioning Process

This is the direct countermeasure to the dangling bucket takeover threat.

  • Order of Operations is Critical: Your decommissioning runbooks must enforce a strict order:
    1. First, delete the DNS CNAME record that points to the bucket.
    2. Wait for the DNS change to propagate (respecting the TTL).
    3. Only then, delete the GCS bucket itself.
  • Automate This Process: Do not rely on manual checklists. Build this logic into your Infrastructure as Code (IaC) or scripting workflows for decommissioning resources.

Pillar 4: Employ Continuous Monitoring and Detection

You cannot assume your environment will remain secure. Continuous visibility is key.

  • Leverage Security Command Center: Google Cloud’s Security Command Center (SCC) is your central hub for posture management. It will automatically detect and surface findings for publicly accessible buckets and other high-risk GCS misconfigurations.
  • Monitor Cloud Audit Logs: Create alerts for high-risk IAM actions on GCS, such as any principal granting allUsers or allAuthenticatedUsers permissions, or any attempt to modify the IAM policy on a critical bucket (storage.setIamPolicy).
  • Actively Scan for Dangling DNS: Regularly scan your DNS zones for CNAME records that point to external resources (not just GCS) and cross-reference them against your live cloud assets to proactively identify dangling records before an attacker does.

Conclusion: Fortify Your Buckets Before They Become Breach Headlines

Google Cloud Storage is powerful, reliable, and deceptively simple — and that’s exactly what makes it risky. A single misstep in configuration or lifecycle management can open the door to serious security incidents.

To truly secure your storage, you need to shift from reactive fixes to a proactive, layered defense. Treat your GCS buckets not as simple storage, but as critical assets worth protecting like vaults.

Set strong organizational policies. Enforce least-privilege access with IAM and UBLA. Decommission buckets securely to eliminate takeover risks. And above all, monitor continuously — because threats don’t wait.

This isn’t just best practice — it’s your blueprint for making GCS a secure foundation, not a silent liability.

To further enhance your cloud security and implement Zero Trust, contact me on LinkedIn Profile or [email protected].

GCP Storage Security FAQ

  • What is the biggest security risk with GCS buckets? While accidental public access is the most common risk, the most insidious threat is arguably the “dangling bucket takeover,” where an attacker exploits a forgotten DNS record to hijack your subdomain by creating a GCS bucket with the same name as one you deleted.
  • What is a “dangling CNAME”? A dangling CNAME is a DNS record (e.g., assets.mycompany.com) that points to a resource (like a GCS bucket) that has been deleted. This allows an attacker to “claim” the destination by creating a new resource with the same name, effectively taking over the subdomain.
  • What is Uniform Bucket-Level Access (UBLA) and why should I use it? UBLA is a GCS setting that disables legacy, per-object ACLs and makes IAM the sole authority for access control on a bucket. You should use it because it dramatically simplifies permission management, eliminates the risk of conflicting policies, and makes your security posture easier to audit and understand.
  • How can I prevent my team from creating public GCS buckets? The most effective way is to use Google Cloud’s Organization Policy Service to enforce the storage.publicAccessPrevention constraint across your entire organization or specific folders. This acts as a top-down guardrail that overrides any IAM permissions.