ACL Debugging

ACL Debugging in ServiceNow – A Complete Guide for CAD Exam Preparation

Introduction

Access Control Lists (ACLs) are fundamental to ServiceNow's security model, governing who can read, write, create, and delete records across the platform. However, when ACLs don't behave as expected, diagnosing the issue can be challenging. This is where ACL Debugging becomes an essential tool. For the ServiceNow Certified Application Developer (CAD) exam, understanding ACL debugging is critical because it tests your ability to troubleshoot security configurations effectively.

Why Is ACL Debugging Important?

ACL debugging is important for several key reasons:

1. Troubleshooting Access Issues: When users report they cannot access certain records, fields, or tables, ACL debugging helps you identify exactly which ACL rule is denying or granting access.

2. Understanding Evaluation Order: ServiceNow evaluates ACLs in a specific order. Without debugging, it can be extremely difficult to determine which rule in the evaluation chain is causing unexpected behavior.

3. Validating Security Configurations: After implementing new ACL rules, debugging allows developers to verify that the rules are functioning as intended before deploying them to production.

4. Reducing Development Time: Rather than guessing which ACL is causing a problem, debugging provides a clear, systematic view of the evaluation process, saving significant time.

5. Preventing Security Gaps: Misconfigured ACLs can either block legitimate users or inadvertently grant unauthorized access. Debugging helps catch these issues early.

What Is ACL Debugging?

ACL debugging is a built-in ServiceNow diagnostic feature that allows administrators and developers to see, in real time, how the platform evaluates ACL rules for a specific user session. When enabled, it provides a detailed breakdown of:

- Which ACL rules are being evaluated
- The order in which they are evaluated
- Whether each rule passed (true) or failed (false)
- The specific condition, script, or role check that caused the pass or failure

ACL debugging is session-specific, meaning it only affects the session of the user who enables it. It does not impact other users or system performance globally.

How ACL Debugging Works

Here is a step-by-step breakdown of how ACL debugging works in ServiceNow:

1. Enabling ACL Debugging

To enable ACL debugging, you can use one of the following methods:

- Navigate to the System Diagnostics > Session Debug > Debug Security (ACL) module.
- Alternatively, type debugsecurity in the application navigator filter.
- You can also use the URL parameter: append &sysparm_debug=true or use session debug ACL.

Once enabled, the debugging output appears at the bottom of the page or in the System Log.

2. Understanding the Debug Output

When ACL debugging is active, each time you access a page, table, or record, the system outputs detailed information including:

- ACL Rule Name: The specific ACL being evaluated (e.g., incident.* [read]).
- Type of Check: Whether the evaluation involves a role, a condition, or a script.
- Result: Whether the check returned true (access granted) or false (access denied).
- Evaluation Order: ACLs are evaluated from the most specific to the most general (field-level → record-level → table-level).

3. ACL Evaluation Order

Understanding the evaluation order is crucial for debugging:

- Match Rules (Table.Field): The system first looks for a rule matching the specific table and field.
- Match Rules (Table.*): If no field-specific rule exists, it checks for a wildcard rule on the table.
- Match Rules (*.*): If no table-specific rule exists, it falls back to the global default rule.

Within each ACL rule, the system evaluates three components, all of which must pass for the ACL to grant access:

- Roles: Does the user have the required role(s)?
- Conditions: Does the record meet the specified conditions?
- Script: Does the advanced script return true?

If any one of these three checks fails, the entire ACL rule fails, and access is denied (unless another matching ACL grants access).

4. Multiple Matching ACLs

When multiple ACL rules match the same resource (e.g., two rules for incident.* [read]), the system evaluates all matching rules. The user must pass at least one of the matching ACL rules to gain access. This is an important concept – matching ACLs are evaluated with an OR logic between them, but within a single ACL, the role, condition, and script are evaluated with AND logic.

5. Disabling ACL Debugging

To disable debugging, navigate to System Diagnostics > Session Debug > Disable All or type debugoff in the navigator. It is important to disable debugging after troubleshooting to avoid unnecessary overhead on your session.

Practical Example

Suppose a user with the itil role cannot edit the short_description field on incident records. With ACL debugging enabled:

1. Navigate to an incident record.
2. Review the debug output for incident.short_description [write].
3. You might see output like:

ACL: incident.short_description [write] - Evaluating roles: PASS (itil)
ACL: incident.short_description [write] - Evaluating condition: FAIL (active=true required, record active=false)
ACL: incident.short_description [write] - DENIED

This tells you the user has the correct role, but the condition on the ACL requires the record to be active, and it is not. The issue is the ACL condition, not the role assignment.

Key Properties Related to ACL Debugging

- glide.security.debug: Enables or disables ACL debugging at the system level (not recommended for production).
- glide.security.acl.debug: Another property that can control ACL debug output.

Important Notes for the CAD Exam

- ACL debugging is a session-level activity; it does not affect other users.
- The debug output shows the evaluation of each component (role, condition, script) within each matching ACL.
- Remember the AND/OR logic: AND within a single ACL rule, OR between multiple matching ACL rules.
- ACL debugging can be enabled through the Application Navigator or through System Diagnostics.
- It is a diagnostic tool only – it does not change any security settings.

Exam Tips: Answering Questions on ACL Debugging

1. Know How to Enable It: Expect questions about where to find the debugging option. Remember: System Diagnostics > Session Debug > Debug Security (ACL). The keyword to remember is debugsecurity.

2. Understand Evaluation Logic: Many exam questions test whether you understand that within a single ACL, all three components (role, condition, script) must pass (AND logic), while between multiple matching ACLs, only one needs to pass (OR logic).

3. Session-Specific Behavior: If a question asks about the impact of enabling ACL debugging, the correct answer is that it only affects the current user's session. It does not impact other users or degrade overall system performance.

4. Read the Debug Output Carefully: Scenario-based questions may present debug output and ask you to identify the cause of an access issue. Look for which component (role, condition, or script) returned false.

5. Evaluation Order Matters: Questions may ask about which ACL takes precedence. Remember the order: field-level > record-level > table-level (most specific to least specific). Also remember that if no specific ACL exists, the system looks for wildcard (*) rules.

6. Don't Confuse Debugging with Impersonation: ACL debugging shows your ACL evaluations. If you need to see ACL evaluations for another user, you must first impersonate that user and then enable ACL debugging.

7. Eliminate Wrong Answers: If an answer choice suggests that ACL debugging modifies ACL rules or permanently logs all security evaluations, it is incorrect. ACL debugging is purely a read-only diagnostic tool.

8. Combine with Impersonation: A common exam scenario involves troubleshooting access for a specific user. The correct approach is to impersonate the user first, then enable security debugging to see the ACL evaluation from that user's perspective.

9. Remember to Disable: Best practice is to disable debugging after use. If a question asks about best practices, selecting the option that includes disabling debug after troubleshooting is typically correct.

10. Watch for the elevate roles concept: Some ACLs require elevated privileges (security_admin). If debugging shows that a role check fails for a role you know the user has, consider whether the role requires elevation.

Summary

ACL debugging is a powerful, session-specific diagnostic tool in ServiceNow that provides real-time visibility into how the platform evaluates access control rules. For the CAD exam, focus on understanding how to enable it, how to interpret the output, the AND/OR evaluation logic, and the ACL evaluation order from most specific to least specific. Mastering these concepts will help you confidently answer any ACL debugging question on the exam.