IAM deny policies in GCP - Deep Dive

Overview - IAM deny policies

What is it?

IAM deny policies are rules in Google Cloud that explicitly block certain actions or access to resources, even if other permissions allow them. They work alongside allow policies to add an extra layer of security by preventing specific users or groups from performing certain tasks. This helps organizations control access more precisely and protect sensitive data or operations.

Why it matters

Without deny policies, if a user has permission granted somewhere else, they might still access or change resources you want to protect. Deny policies stop this by explicitly blocking actions, reducing risks like accidental data leaks or unauthorized changes. This makes cloud environments safer and more trustworthy.

Where it fits

Before learning deny policies, you should understand basic IAM concepts like roles, permissions, and allow policies. After mastering deny policies, you can explore advanced security topics like conditional access, organization policies, and audit logging to monitor and enforce security.

Mental Model

Core Idea

Deny policies explicitly block actions, overriding any allowed permissions to ensure certain access is never granted.

Think of it like...

Imagine a building with a master key that opens many doors (allow policies). Deny policies are like special locks on some doors that stop even the master key from opening them.

┌───────────────┐
│   User tries  │
│   to access   │
└──────┬────────┘
       │
       ▼
┌───────────────┐
│ Deny Policy?  │───Yes──► Access Denied
│ (Explicit No) │
└──────┬────────┘
       │No
       ▼
┌───────────────┐
│ Allow Policy? │───Yes──► Access Granted
│ (Explicit Yes)│
└──────┬────────┘
       │No
       ▼
  Access Denied

Build-Up - 7 Steps

1

Foundation

Basics of IAM Permissions

Concept: Understand what permissions and roles are in IAM.

IAM controls who can do what in Google Cloud. Permissions are specific actions like 'read storage' or 'start VM'. Roles group permissions together. When you assign a role to a user, you allow them to perform those actions.

Result

You know how permissions grant access to resources.

Understanding permissions and roles is essential because deny policies work by overriding these permissions.

2

Foundation

Difference Between Allow and Deny

Concept: Learn that IAM traditionally uses allow policies, and deny policies add explicit blocks.

Allow policies say what a user can do. Deny policies say what a user cannot do, even if allowed elsewhere. Deny policies are a newer feature that add stronger control.

Result

You can distinguish between permissions that grant access and rules that block access.

Knowing this difference helps you see why deny policies are powerful for security.

3

Intermediate

How Deny Policies Override Allows

🤔Before reading on: do you think deny policies can be bypassed by allow policies? Commit to your answer.

Concept: Deny policies take priority over allow policies in access decisions.

When a user tries to do something, Google Cloud checks deny policies first. If any deny policy blocks the action, access is denied immediately, no matter what allow policies say. Only if no deny blocks exist, allow policies are checked.

Result

You understand that deny policies are a strong security tool that cannot be overridden by allow permissions.

Knowing deny policies override allows prevents security gaps where permissions might accidentally grant access.

4

Intermediate

Structure of a Deny Policy

🤔Before reading on: do you think deny policies list allowed actions or blocked actions? Commit to your answer.

Concept: Deny policies specify which permissions are blocked and to whom.

A deny policy includes: the resource it applies to, the identities (users/groups) it affects, and the permissions it denies. It can also include conditions like time or device type. This lets you block specific actions for specific people under certain situations.

Result

You can read and write deny policies that precisely block unwanted access.

Understanding the structure helps you create effective deny policies tailored to your security needs.

5

Intermediate

Using Conditions in Deny Policies

🤔Before reading on: do you think deny policies can block access only sometimes, based on conditions? Commit to your answer.

Concept: Deny policies can include conditions to apply blocks only in certain cases.

Conditions let you specify when a deny policy applies, such as only during weekends, from certain IP addresses, or on specific devices. This adds flexibility, so you don't block access unnecessarily.

Result

You can create deny policies that adapt to context, improving security without blocking needed access.

Knowing about conditions lets you balance security and usability effectively.

6

Advanced

Combining Deny and Allow Policies Safely

🤔Before reading on: do you think combining deny and allow policies can cause unexpected access denials? Commit to your answer.

Concept: Learn how to manage conflicts and order between deny and allow policies.

When both deny and allow policies exist, deny always wins. But complex setups can cause users to lose access unexpectedly if deny policies are too broad. Careful planning and testing are needed to avoid blocking legitimate access.

Result

You can design IAM policies that use deny rules without breaking workflows.

Understanding policy interactions prevents accidental outages and security holes.

7

Expert

Internal Evaluation Order and Performance

🤔Before reading on: do you think deny policies are checked after allow policies or before? Commit to your answer.

Concept: Explore how Google Cloud evaluates deny policies internally for efficiency and security.

Google Cloud evaluates deny policies first to quickly reject forbidden requests. This saves resources by avoiding unnecessary allow checks. Internally, deny policies are cached and optimized for fast evaluation even in large organizations.

Result

You appreciate the design choices that make deny policies both secure and performant.

Knowing the evaluation order helps you troubleshoot access issues and optimize policy design.

Under the Hood

When a user requests access, Google Cloud's IAM system first checks deny policies attached to the resource or its parents. If any deny policy matches the user and the requested permission, access is immediately denied. If no deny policy blocks the request, the system then checks allow policies to see if access is granted. This two-step evaluation ensures deny policies have the highest priority. Deny policies are stored and evaluated using efficient indexing and caching to handle large-scale environments with many policies and users.

Why designed this way?

Deny policies were introduced to fix security gaps where allow policies alone could not prevent unwanted access. Before deny policies, administrators had to rely on complex role assignments or resource hierarchy tricks. Explicit deny rules simplify security by clearly stating what is forbidden. The design prioritizes deny checks first to minimize risk and improve performance by rejecting unauthorized requests early.

┌───────────────┐
│ User Request  │
└──────┬────────┘
       │
       ▼
┌───────────────┐
│ Check Deny    │
│ Policies      │
└──────┬────────┘
       │
   Deny│Allow
       ▼    ▼
┌───────────────┐
│ Deny Found?   │
└──────┬────────┘
       │Yes       │No
       ▼          ▼
┌───────────────┐  ┌───────────────┐
│ Access Denied │  │ Check Allow   │
└───────────────┘  │ Policies      │
                   └──────┬────────┘
                          │
                   Allow  │Deny
                          ▼    ▼
                   ┌───────────────┐
                   │ Access       │
                   │ Granted or   │
                   │ Denied      │
                   └───────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Do deny policies only apply if no allow policy exists? Commit to yes or no.

Common Belief: Deny policies only matter if there is no allow policy granting access.

Reality: Deny policies always take precedence and block access even if allow policies grant it.

Why it matters: Believing this can cause administrators to miss critical blocks, leading to unintended access.

Quick: Can deny policies be applied to individual permissions or only entire roles? Commit to your answer.

Common Belief: Deny policies can only block entire roles, not specific permissions.

Reality: Deny policies can block specific permissions within roles, allowing fine-grained control.

Why it matters: Misunderstanding this limits the ability to create precise security rules.

Quick: Do deny policies automatically apply to all resources in a project? Commit to yes or no.

Common Belief: Deny policies set on a resource automatically apply to all resources in the project.

Reality: Deny policies apply only to the resource they are attached to and its children, not the entire project unless set at the project level.

Why it matters: Assuming broad application can cause unexpected access denials or gaps.

Quick: Can deny policies be bypassed by using a different identity? Commit to yes or no.

Common Belief: If a user is denied, they can just use another identity to bypass deny policies.

Reality: Deny policies apply to specified identities; using another identity not covered by the deny policy may allow access.

Why it matters: This shows the importance of comprehensive identity management alongside deny policies.

Expert Zone

1

Deny policies can be combined with conditions to create dynamic security rules that adapt to context, such as time or device type.

2

Overly broad deny policies can unintentionally block critical system accounts or services, causing outages.

3

Deny policies are evaluated in a hierarchical manner, so policies on parent resources affect child resources, requiring careful planning.

When NOT to use

Avoid using deny policies when simple allow policies suffice or when the environment is small and easy to manage. For very dynamic or temporary access control, consider using conditional allow policies or access approval workflows instead.

Production Patterns

In production, deny policies are often used to block risky actions like deleting logs, modifying billing, or accessing sensitive data. They are combined with allow policies and monitored with audit logs to enforce least privilege and detect policy violations.

Connections

Firewall Rules

Both deny policies and firewall rules explicitly block unwanted access at different layers.

Understanding deny policies helps grasp how security controls can explicitly block actions, similar to how firewalls block network traffic.

Legal Contracts

Deny policies act like clauses that forbid certain actions regardless of other permissions, similar to legal clauses that prohibit specific behaviors despite general agreements.

This connection shows how explicit prohibitions provide stronger guarantees than permissions alone.

Traffic Lights

Deny policies are like red lights that stop traffic regardless of other signals allowing movement.

Recognizing deny policies as absolute stops helps understand their priority in access control.

Common Pitfalls

#1 Creating deny policies that are too broad and block essential service accounts.

Wrong approach:

Deny policy denying 'all users' from 'storage.objects.delete' without exceptions.

Correct approach:

Deny policy denying 'specific user groups' from 'storage.objects.delete' while excluding service accounts.

Root cause:

Not understanding the scope and impact of deny policies on all identities leads to accidental service disruptions.

#2 Assuming deny policies apply globally without setting them on the correct resource level.

Wrong approach:

Setting deny policy on a single VM instance and expecting it to block access to the whole project.

Correct approach:

Setting deny policy at the project or folder level to cover all relevant resources.

Root cause:

Misunderstanding resource hierarchy and policy inheritance causes ineffective deny policies.

#3 Not testing deny policies before deployment, causing unexpected access denials.

Wrong approach:

Deploying deny policies directly in production without simulation or audit review.

Correct approach:

Using IAM policy simulator and audit logs to test deny policies before applying them.

Root cause:

Overlooking the importance of testing leads to operational issues and user frustration.

Key Takeaways

IAM deny policies explicitly block permissions and always override allow policies, providing a strong security layer.

They are structured with identities, permissions, and optional conditions to precisely control access.

Deny policies are evaluated first in the access decision process to quickly reject forbidden requests.

Careful design and testing of deny policies are essential to avoid accidental service disruptions.

Understanding deny policies helps build secure, manageable cloud environments by clearly stating what is forbidden.