Detect Email Forwarding Rule in Google Chronicle
Adversaries may set up email forwarding rules to covertly collect and monitor victim email communications. By creating inbox rules, mailbox-level SMTP forwarding configurations, or Exchange transport rules, adversaries can silently redirect all or targeted messages to attacker-controlled accounts — internal or external — without the victim's awareness. This technique provides persistent intelligence access even after compromised credentials are reset, because forwarding rules survive password changes. Adversaries may also use the Microsoft Messaging API (MAPI) to create hidden inbox rules not visible through Outlook, OWA, or standard Exchange administration tools, enabling long-term covert collection. Threat groups including LAPSUS$, Scattered Spider, Kimsuky, Star Blizzard, and Silent Librarian have actively abused this technique. LAPSUS$ notably created tenant-level Exchange transport rules to forward all organizational email to newly created attacker-controlled accounts, achieving org-wide collection with a single rule.
MITRE ATT&CK
- Tactic
- Collection
- Technique
- T1114 Email Collection
- Sub-technique
- T1114.003 Email Forwarding Rule
- Canonical reference
- https://attack.mitre.org/techniques/T1114/003/
YARA-L Detection Query
rule email_forwarding_rule_t1114_003 {
meta:
author = "Argus Detection Engineering"
description = "Detects creation or modification of email forwarding rules in Microsoft 365 Exchange, including inbox rules, transport rules, and mailbox-level SMTP forwarding. Covers external forwarding, org-wide transport rules, and hidden rules associated with T1114.003."
mitre_attack_tactic = "Collection"
mitre_attack_technique = "T1114.003"
severity = "HIGH"
priority = "HIGH"
events:
$e.metadata.product_name = "Office 365"
$e.metadata.vendor_name = "Microsoft"
$e.metadata.product_event_type in~ (
"New-InboxRule", "Set-InboxRule", "Enable-InboxRule",
"New-TransportRule", "Set-TransportRule", "Enable-TransportRule",
"Set-Mailbox"
)
(
$e.security_result.rule_name = "ForwardTo" or
$e.security_result.rule_name = "ForwardAsAttachmentTo" or
$e.security_result.rule_name = "RedirectTo" or
$e.security_result.rule_name = "ForwardingSmtpAddress" or
$e.security_result.rule_name = "ForwardingAddress" or
$e.security_result.rule_name = "DeliverToMailboxAndForward" or
$e.security_result.rule_name = "RedirectMessageTo" or
$e.security_result.rule_name = "BlindCopyTo" or
re.regex($e.target.resource.attribute.labels["parameters"], `(?i)(ForwardTo|ForwardAsAttachmentTo|RedirectTo|ForwardingSmtpAddress|ForwardingAddress|DeliverToMailboxAndForward|RedirectMessageTo|BlindCopyTo)`)
)
$e.principal.user.userid != ""
condition:
$e
}Chronicle YARA-L 2.0 rule detecting email forwarding rule abuse in Microsoft 365 Exchange via the Chronicle UDM. Matches against Exchange audit log events for inbox rule, transport rule, and Set-Mailbox operations that include forwarding-related parameters, enabling detection of T1114.003 tactics including external forwarding and hidden rules.
Data Sources
Required Tables
False Positives & Tuning
- Exchange administrators using PowerShell automation scripts to configure forwarding for onboarding/offboarding workflows during business hours from known admin workstations
- Email security gateway products that create transport rules for spam filtering, DLP enforcement, or message hygiene policies
- Managed service providers (MSPs) administering tenant Exchange configurations on behalf of customers, generating forwarding rule events from external IP ranges
Other platforms for T1114.003
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 1Create Inbox Forwarding Rule via Exchange Online PowerShell
Expected signal: OfficeActivity (Sentinel) and o365:management:activity (Splunk): Operation=New-InboxRule, [email protected], [email protected], Parameters array contains {Name:ForwardTo, Value:[email protected]}. Event appears in Unified Audit Log within 15-60 minutes. Azure AD Sign-In Logs will record the Exchange Online session from the admin account.
- Test 2Set Mailbox-Level SMTP Forwarding via Set-Mailbox
Expected signal: OfficeActivity: Operation=Set-Mailbox, Parameters array contains {Name:ForwardingSmtpAddress, Value:[email protected]} and {Name:DeliverToMailboxAndForward, Value:True}. This forwarding is NOT visible via Get-InboxRule — only via Get-Mailbox -Identity victim | fl ForwardingSmtpAddress, ForwardingAddress, DeliverToMailboxAndForward.
- Test 3Create Org-Wide Transport Rule to Blind-Copy All Mail (LAPSUS$ Pattern)
Expected signal: OfficeActivity: Operation=New-TransportRule, [email protected], Parameters contains {Name:BlindCopyTo, Value:[email protected]} and {Name:FromScope, Value:InOrganization}. This is NOT logged to individual mailbox audit logs — only appears in Exchange Admin Audit Log and the Unified Audit Log at the tenant level. Transport rule changes take effect within minutes.
- Test 4Bulk Enumerate All Mailbox Forwarding Configurations (Reconnaissance Phase)
Expected signal: OfficeActivity: Multiple Operation=Get-InboxRule events from admin account accessing many mailboxes in rapid succession. High-volume Exchange admin read operations within a short time window generate multiple OfficeActivity records. Azure AD Sign-In Logs record the session. This pattern can be detected with anomaly-based analytics on Exchange admin read volume.
References (10)
- https://attack.mitre.org/techniques/T1114/003/
- https://blog.compass-security.com/2018/09/hidden-inbox-rules-in-microsoft-exchange/
- https://blogs.technet.microsoft.com/timmcmic/2015/06/08/exchange-and-office-365-mail-forwarding-2/
- https://learn.microsoft.com/en-us/exchange/security-and-compliance/mail-flow-rules/mail-flow-rules
- https://www.us-cert.gov/ncas/alerts/TA18-086A
- https://www.microsoft.com/en-us/security/blog/2022/03/22/dev-0537-criminal-actor-targeting-organizations-for-data-exfiltration-and-destruction/
- https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/external-email-forwarding
- https://github.com/sensepost/ruler
- https://learn.microsoft.com/en-us/powershell/module/exchange/new-inboxrule
- https://learn.microsoft.com/en-us/powershell/module/exchange/new-transportrule
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