Detect Additional Email Delegate Permissions in CrowdStrike LogScale
Adversaries may grant additional permission levels to maintain persistent access to an adversary-controlled email account. Using cmdlets like Add-MailboxPermission in Exchange/Office 365, or assigning folder-level permissions, attackers can ensure continued access to target mailboxes. This technique is commonly used in BEC incidents and persistent threat campaigns (APT28, APT29, Magic Hound) to maintain covert email access, enable internal spearphishing, and evade detection by reading communications without triggering login alerts.
MITRE ATT&CK
- Tactic
- Persistence Privilege Escalation
- Technique
- T1098 Account Manipulation
- Sub-technique
- T1098.002 Additional Email Delegate Permissions
- Canonical reference
- https://attack.mitre.org/techniques/T1098/002/
LogScale Detection Query
// T1098.002 — Additional Email Delegate Permissions
// Requires CrowdStrike Falcon with O365 Audit Log integration in LogScale
#repo = "o365" OR #source = "o365_audit"
| Operation in ("Add-MailboxPermission", "Add-MailboxFolderPermission", "Set-MailboxFolderPermission", "Add-RecipientPermission", "Set-Mailbox", "New-ManagementRoleAssignment")
| Parameters = /(?i)FullAccess|SendAs|SendOnBehalf|ApplicationImpersonation|ChangePermission|ChangeOwner/ OR Operation = "New-ManagementRoleAssignment"
| eval(is_impersonation := Parameters = /(?i)ApplicationImpersonation/)
| eval(is_high_risk := Parameters = /(?i)FullAccess|SendAs|SendOnBehalf/)
| eval(is_default_anon := Parameters = /(?i)\"Default\"|\"Anonymous\"/)
| eval(is_folder_perm := Operation = /(?i)MailboxFolderPermission/)
| eval(risk_score := case(
Parameters = /(?i)ApplicationImpersonation/, 3,
Parameters = /(?i)FullAccess|SendAs/, 2,
true(), 1
))
| eval(risk_level := case(
risk_score >= 3, "CRITICAL",
risk_score = 2, "HIGH",
true(), "MEDIUM"
))
| table([timestamp, UserId, Operation, ClientIP, Parameters, is_impersonation, is_high_risk, is_default_anon, is_folder_perm, risk_score, risk_level])
| sort(timestamp, order=desc)CrowdStrike LogScale (Humio CQL) query detecting Exchange mailbox permission grants via the Falcon O365 Audit Log integration. Evaluates a risk score per event: ApplicationImpersonation scores 3 (CRITICAL), FullAccess/SendAs scores 2 (HIGH), and all other suspicious rights score 1 (MEDIUM). Computes boolean indicators for impersonation, high-risk rights, default/anonymous scope, and folder-level permission changes to aid triage. Mirrors the risk scoring logic from the reference SPL query.
Data Sources
Required Tables
False Positives & Tuning
- Email compliance platforms (Mimecast Cloud Archive, Barracuda Cloud Archiving) with pre-approved ApplicationImpersonation service accounts registered in Exchange
- IT teams bulk-assigning FullAccess to shared department mailboxes during organisational restructuring or employee transitions
- Automated Microsoft 365 provisioning scripts that configure EA-to-executive mailbox delegate pairs as part of standard onboarding runbooks
Other platforms for T1098.002
Testing Methodology
Validate this detection against 4 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Grant FullAccess Mailbox Permission via PowerShell
Expected signal: Office 365 Unified Audit Log: Operation='Add-MailboxPermission', RecordType='ExchangeAdmin', UserId=<admin UPN>, [email protected], Parameters containing 'FullAccess' and '[email protected]'. PowerShell Event ID 4104 (ScriptBlock) if logging is enabled on the workstation running the cmdlet.
- Test 2Grant ApplicationImpersonation Role for Tenant-Wide Mailbox Access
Expected signal: Office 365 Unified Audit Log: Operation='New-ManagementRoleAssignment', RecordType='ExchangeAdmin', Parameters containing 'ApplicationImpersonation' and '[email protected]'. Azure AD Audit Logs may also show a role assignment event. PowerShell ScriptBlock Log (Event ID 4104) captures the New-ManagementRoleAssignment cmdlet with parameters.
- Test 3Set Default User Reviewer Permission on Mailbox Inbox Folder
Expected signal: Office 365 Unified Audit Log: Operation='Set-MailboxFolderPermission' and 'Add-MailboxFolderPermission', Parameters containing 'Default' and 'Reviewer'. Two separate audit events expected — one for Inbox and one for root folder. RecordType='ExchangeAdmin'.
- Test 4Grant SendAs Permission to Attacker Account for BEC
Expected signal: Office 365 Unified Audit Log: Operation='Add-RecipientPermission', RecordType='ExchangeAdmin', [email protected], Parameters containing 'SendAs' and '[email protected]'. ClientIP of the admin session performing the grant.
References (10)
- https://attack.mitre.org/techniques/T1098/002/
- https://docs.microsoft.com/en-us/powershell/module/exchange/mailboxes/add-mailboxpermission
- https://www.mandiant.com/resources/blog/remediation-and-hardening-strategies-for-microsoft-365-to-defend-against-unc2452
- https://www.crowdstrike.com/blog/hiding-in-plain-sight-using-the-office-365-activities-api-to-investigate-business-email-compromises/
- https://static.carahsoft.com/concrete/files/1015/2779/3571/M-Trends-2018-Report.pdf
- https://www.slideshare.net/slideshow/shmoocon-2019-becs-and-beyond-investigating-and-defending-office-365/128744511
- https://support.google.com/a/answer/7223765?hl=en
- https://learn.microsoft.com/en-us/microsoft-365/compliance/mailbox-audit-logging
- https://learn.microsoft.com/en-us/exchange/permissions/role-assignments
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1098.002/T1098.002.md
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