Why do GCP deny policies create coverage gaps for least privilege?

Because deny policies only work for permissions that have an IAM v2 representation, while many grantable permissions still do not. A team can think it has central denial in place and still leave large parts of the permission set controlled only by allow policies and inherited access.

Why This Matters for Security Teams

GCP deny policies are attractive because they look like a clean way to centralise least privilege, but the coverage is only as complete as the permissions exposed through IAM v2. If a permission cannot be represented there, it cannot be denied centrally, which leaves security teams with a false sense of control. That matters because least privilege depends on both removing excess access and preventing new access paths from reappearing through inherited bindings, legacy roles, or service account sprawl.

This is the same governance gap highlighted in NHI research: NHIs already outnumber human identities by 25x to 50x in modern enterprises, and NHI privilege tends to accumulate faster than teams can review it. NHI Mgmt Group’s Top 10 NHI Issues and Ultimate Guide to NHIs – Key Challenges and Risks both frame this as a visibility and lifecycle problem, not just a policy syntax problem. Current guidance suggests matching deny controls with continuous entitlement review, because “deny first” does not help if the underlying permission model is incomplete. In practice, many security teams discover the missing coverage only after a service account has already exercised an inherited permission that nobody realised remained allow-only.

How It Works in Practice

GCP deny policies work by overriding allowed actions, but only where the permission has an IAM v2 representation. That means the policy engine can enforce denial for a subset of the effective permission graph, while other permissions still rely on allow bindings, custom roles, service account delegation, or inherited access from folders and organisations. The result is fragmented least privilege: one control plane says “no,” while adjacent permissions remain governed by older or less expressive mechanisms.

For security teams, the practical response is to treat deny policies as one layer, not the whole control strategy. Pair them with entitlement inventory, role pruning, and resource hierarchy review. NIST’s Cybersecurity Framework 2.0 emphasises governance and continuous risk management, which maps well to this problem. NIST SP 800-207 also reinforces that access decisions should be evaluated within a zero trust model, not assumed safe because a broad deny rule exists. For NHI-heavy environments, that means reviewing service accounts, workload identities, and inherited bindings together, rather than assuming central denial closes the loop.

• Inventory every grantable permission and compare it against deny-policy coverage.
• Identify permissions that remain allow-only because they lack IAM v2 deny support.
• Review folder, project, and organisation inheritance for hidden effective access.
• Use short-lived credentials and workload-scoped identities where possible.

Where this guidance breaks down is in large, fast-changing GCP estates with many custom roles and cross-project service account impersonation, because effective access changes faster than manual policy reviews can keep up.

Common Variations and Edge Cases

Tighter deny controls often increase operational overhead, requiring organisations to balance stronger central restriction against slower change management and more exception handling. That tradeoff becomes especially sharp when platform teams rely on custom roles, shared service accounts, or automation that needs broad but temporary access.

There is no universal standard for this yet, but best practice is evolving toward layered authorisation: combine deny policies with just-in-time access, explicit role design, and periodic verification of effective permissions. If a permission is not representable in deny policy, it should be treated as an exception requiring compensating control, not as “covered.” The Lifecycle Processes for Managing NHIs research is useful here because many gaps are really offboarding and rotation failures in disguise. For implementation guidance, the OWASP Non-Human Identity Top 10 is a practical reference for spotting excessive privilege and weak lifecycle controls.

Edge cases also appear when organisations use inherited roles from third-party managed services or when policy evaluation depends on context that deny policies cannot express. In those environments, least privilege needs continuous testing, not just a one-time policy rollout, because the gap is usually in the permission model itself rather than in the administrator’s intent.

Related resources from NHI Mgmt Group

• Why do NHIs complicate zero trust and least privilege efforts?
• When does least privilege break down for machine identities?
• Why do autonomous assistants complicate least-privilege design?
• Why do VPN-based remote access models still create privilege risk?