How to Resolve a False Positive on the F Module Quickly and Safely

False positives in F5 ASM (Application Security Manager) can disrupt application functionality and reduce user confidence. Here’s a structured approach to resolving them efficiently while maintaining security.

Common Causes of False Positives

1. Application Changes Without Notice – The application behavior changed, but the ASM policy wasn’t updated.
2. Insufficient Policy Staging – The Enforcement Readiness Period (ERP) was too short, or the policy wasn’t properly tested.

Key Considerations

• Users often delay reporting issues, so internal logging is critical.
• Never revert to transport mode as a fix—this exposes the entire application.

The Fix (3 Steps)

Step 1: Identify the Cause

Use ASM logs and learning suggestions to pinpoint:

• Disallowed characters/values
• Signature violations
• Unexpected traffic patterns

Step 2: Refine Wildcard Entities

If blocking occurs on a wildcard entity (URL/Parameter/Filetype):

• Extract the violating entity from the wildcard.
• Create a specific matching entity to avoid broad blocking.

Step 3: Stage and Tune the New Entity

• Gradually adjust the new entity to allow legitimate traffic.
• Avoid skipping staging—application changes may require further tuning.

Best Practices

✔ Verify False Positives – Just because a user reports an issue doesn’t mean ASM is wrong.
✔ Boost Confidence – Quick, accurate fixes prove ASM’s effectiveness.

You Should Know: Essential F5 ASM Commands & Log Analysis

1. Check ASM Logs for Blocked Requests

tail -f /var/log/asm | grep "Blocked"

2. List Active Security Policies

tmsh list asm policy

3. Export ASM Logs for Analysis

tmsh save sys log asm /var/tmp/asm_log_export.gz

4. Modify an Entity in ASM Policy

tmsh modify asm policy <policy_name> entities add { <entity_name> { ... } }

5. Simulate Requests to Test Policy Changes

curl -X POST -H "Content-Type: application/json" -d '{"param":"value"}' http://app.example.com

6. Enable Learning Suggestions

tmsh modify asm policy <policy_name> learning-suggestions enabled

7. Force Policy Update Without Restart

tmsh apply asm policy <policy_name>

What Undercode Say

F5 ASM is a powerful WAF, but false positives require careful handling. Always:
– Analyze logs first (/var/log/asm).
– Avoid broad disablements—use granular entity tuning.
– Test in staging before enforcing changes.

For deeper troubleshooting:

grep "Violation" /var/log/asm –A 5

Expected Output:

[/bash]

[ASM][Violation] Parameter ‘user_input’ blocked (Disallowed metacharacter)

[ASM][Suggestion] Add parameter to allowed list or adjust signature threshold
[bash]

Related Resources:
– F5 ASM Documentation
– OWASP WAF Best Practices

References:

Reported By: Grahammattingley This – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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