[Rule Tuning] Tuning Suspicious Mailbox Permission Delegation in Exchange Online

[terrancedejesus]

Pull Request

Issue link(s):

• https://github.com/elastic/sdh-protections/issues/574

Summary - What I changed

Adjusts FNs and FPs for Suspicious Mailbox Permission Delegation in Exchange Online. This rule identifies the assignment of rights to access content from another mailbox.

How To Test

• Please see the SDH for a detailed explanation.
• Query can be tested in TRADE serverless stack.

Checklist

• Added a label for the type of pr: bug, enhancement, schema, maintenance, Rule: New, Rule: Deprecation, Rule: Tuning, Hunt: New, or Hunt: Tuning so guidelines can be generated
• Added the meta:rapid-merge label if planning to merge within 24 hours
• Secret and sensitive material has been managed correctly
• Automated testing was updated or added to match the most common scenarios
• Documentation and comments were added for features that require explanation

Contributor checklist

• Have you signed the contributor license agreement?
• Have you followed the contributor guidelines?

[github-actions]

Rule: Tuning - Guidelines

These guidelines serve as a reminder set of considerations when tuning an existing rule.

Documentation and Context

• Detailed description of the suggested changes.
• Provide example JSON data or screenshots.
• Provide evidence of reducing benign events mistakenly identified as threats (False Positives).
• Provide evidence of enhancing detection of true threats that were previously missed (False Negatives).
• Provide evidence of optimizing resource consumption and execution time of detection rules (Performance).
• Provide evidence of specific environment factors influencing customized rule tuning (Contextual Tuning).
• Provide evidence of improvements made by modifying sensitivity by changing alert triggering thresholds (Threshold Adjustments).
• Provide evidence of refining rules to better detect deviations from typical behavior (Behavioral Tuning).
• Provide evidence of improvements of adjusting rules based on time-based patterns (Temporal Tuning).
• Provide reasoning of adjusting priority or severity levels of alerts (Severity Tuning).
• Provide evidence of improving quality integrity of our data used by detection rules (Data Quality).
• Ensure the tuning includes necessary updates to the release documentation and versioning.

Rule Metadata Checks

• updated_date matches the date of tuning PR merged.
• min_stack_version should support the widest stack versions.
• name and description should be descriptive and not include typos.
• query should be inclusive, not overly exclusive. Review to ensure the original intent of the rule is maintained.

Testing and Validation

• Validate that the tuned rule's performance is satisfactory and does not negatively impact the stack.
• Ensure that the tuned rule has a low false positive rate.

[Mikaayenson]

lgtm, and ty for the SDH notes!

[imays11]

@terrancedejesus Thank you for the details in the SDH, are you sure about changing this to a new terms rule? I'm not fully opposed to it just wondering the justification for that. Is this behavior not something we'd want to investigate every instance of?

[terrancedejesus]

@terrancedejesus Thank you for the details in the SDH, are you sure about changing this to a new terms rule? I'm not fully opposed to it just wondering the justification for that. Is this behavior not something we'd want to investigate every instance of?

Hey @imays11! Thanks for taking the time to review. You bring up a great question about how frequent we expect the analyst to review these alerts. I had pondered this as well and here are a few reasons why I chose the New Terms.

• FP ratio: We have ~320 alerts for this rule globally from the telemetry we collect where each cluster contributes 5% so we know that globally, we are likely generating FPs consistently.
• Delegating permissions: From what I have read, it is not uncommon to delegate permissions of an M365 email to another user principal in Enterprise. For example, if an M365 admin needs to delegate permissions of Marketing@tenant[.]onmicrosoft.com to Alice who needs these delegated permissions then that is not a threat. However, if the admin does not delegate permissions often, maybe that is when we investigate, but we would at least reduce windows where delegated permissions may be recurring in a 2 week period where organization changes, operational changes, etc. may be taking place.
• Threat origin: Lastly, I attempted to identify some historical campaigns where this was specifically used based on OSINT and was unable to find any. Mainly I found some red team blogs or email delegation references in MITRE for APTs but their techniques/behavior were not specific to this. Thus we know this is plausible, but actual use in-the-wild is very limited from an OSINT perspective.

Let me know if you want to discuss more, happy to get more thoughts on this.