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THREAT-Impact-BECFinancialTheftWireTransfer

Financial Theft — BEC Financial Theft — Fraudulent Wire/ACH Transfer Initiation via Mailbox Tampering and Invoice Fraud

Impact Last updated:

This detection covers the final, financially damaging stage of a Business Email Compromise (BEC) intrusion: fraudulent wire or ACH transfer initiation (T1657 — Financial Theft) that follows mailbox rule tampering and invoice/payment-detail modification. Having already gained access to a mailbox (via phishing, OAuth token theft, or credential stuffing), the attacker creates hidden inbox rules that forward, redirect, or delete messages matching finance-related keywords so the victim never sees replies from the real vendor or their own finance team. The attacker then sends or intercepts an invoice, purchase order, or payroll email and modifies the banking details — swapping the legitimate account/routing number or IBAN for an attacker-controlled mule account — while adding urgency language ("process today", "do not call, I am in a meeting") to pressure the victim into skipping out-of-band verification. Groups such as Cosmic Lynx (Russian-speaking, CEO-fraud wire transfers averaging $1.27M) and SilverTerrier (West African vendor-invoice-swap BEC) follow this exact three-stage pattern: mailbox tampering, payment-detail modification, then transfer initiation. Because banking-portal telemetry is rarely centralized in a SIEM, this detection correlates the two upstream stages that ARE consistently observable in mail and identity telemetry — malicious inbox rule creation and financial-fraud-pattern email content — as the highest-fidelity proxy for an active wire fraud attempt, with escalation criteria pointing responders at the actual funds-transfer confirmation step.

What is THREAT-Impact-BECFinancialTheftWireTransfer BEC Financial Theft — Fraudulent Wire/ACH Transfer Initiation via Mailbox Tampering and Invoice Fraud?

BEC Financial Theft — Fraudulent Wire/ACH Transfer Initiation via Mailbox Tampering and Invoice Fraud (THREAT-Impact-BECFinancialTheftWireTransfer) is a sub-technique of Financial Theft (T1657) in the MITRE ATT&CK framework. It maps to the Impact tactic — the adversary is trying to manipulate, interrupt, or destroy your systems and data.

This page provides production-ready detection logic for BEC Financial Theft — Fraudulent Wire/ACH Transfer Initiation via Mailbox Tampering and Invoice Fraud, covering the data sources and telemetry it touches: Office 365 Unified Audit Log (OfficeActivity), Microsoft Defender for Office 365 (EmailEvents), Microsoft 365 Message Trace. The queries below are rated critical severity at high confidence, and ship for 7 SIEM platforms — KQL, SPL, Elastic, QRadar, Sumo, YARA-L, LogScale.

MITRE ATT&CK

Tactic
Impact
Microsoft Sentinel / Defender
kusto
// THREAT-Impact-BECFinancialTheftWireTransfer (T1657 Financial Theft)
// Correlates mailbox rule tampering with financial-fraud-pattern email content to surface active BEC wire/ACH fraud
let Lookback = 7d;
let CorrelationWindowHours = 72;
let FinanceKeywords = dynamic(["wire transfer", "wire the funds", "ach transfer", "routing number", "account number", "swift code", "iban", "beneficiary", "new bank details", "updated payment details", "remittance", "invoice attached", "change of bank account", "payment instructions", "vendor payment"]);
let UrgencyKeywords = dynamic(["urgent", "immediately", "asap", "time sensitive", "confidential", "do not call", "cannot talk", "process today", "before end of day"]);
// Stage 1: mailbox rule tampering that hides finance-thread replies from the victim
let MailboxTampering = OfficeActivity
| where TimeGenerated > ago(Lookback)
| where Operation in ("New-InboxRule", "Set-InboxRule", "Set-Mailbox", "UpdateInboxRules")
| where Parameters has_any ("ForwardTo", "RedirectTo", "ForwardAsAttachmentTo", "DeleteMessage", "MoveToFolder", "StopProcessingRules")
| where Parameters !has "Junk Email"
| project TimeGenerated, UserId, ClientIP, Operation, Parameters
| extend Stage = "MailboxTampering";
// Stage 2: inbound/outbound email matching wire-fraud content and urgency-pressure language
let FraudulentFinanceEmail = EmailEvents
| where TimeGenerated > ago(Lookback)
| where Subject has_any (FinanceKeywords)
| where Subject has_any (UrgencyKeywords) or EmailDirection == "Outbound"
| project TimeGenerated, UserId = SenderFromAddress, RecipientEmailAddress, Subject, SenderIPv4, EmailDirection
| extend Stage = "FinanceEmailContent";
// Correlate: same mailbox shows tampering followed by finance-fraud email content within the window
MailboxTampering
| join kind=inner (FraudulentFinanceEmail) on UserId
| where datetime_diff('hour', TimeGenerated1, TimeGenerated) between (0 .. CorrelationWindowHours)
| project TamperTime = TimeGenerated, EmailTime = TimeGenerated1, UserId, ClientIP, Operation, Parameters, RecipientEmailAddress, Subject, SenderIPv4, EmailDirection
| extend ThreatType = "BEC_WireFraud_MailboxTamper_Correlation"
| sort by TamperTime desc

Two-stage correlation over Office 365 audit and Defender for Office 365 mail telemetry: (1) mailbox rule creation/modification with forwarding, redirection, or deletion parameters that would hide replies from the real vendor or finance team, and (2) email content matching wire-transfer/ACH/bank-detail-change keywords combined with urgency-pressure language. The inner join surfaces mailboxes where tampering occurred within 72 hours of a finance-fraud-pattern email — the strongest observable proxy for an active BEC wire-fraud attempt short of the actual funds-transfer confirmation.

critical severity high confidence

Data Sources

Office 365 Unified Audit Log (OfficeActivity) Microsoft Defender for Office 365 (EmailEvents) Microsoft 365 Message Trace

Required Tables

OfficeActivity EmailEvents

False Positives

  • Legitimate finance staff creating inbox rules to file paid invoices into an archive folder, coinciding by chance with routine vendor payment correspondence
  • Accounts payable teams sending genuine wire/ACH confirmation emails with urgency language during month-end close
  • Treasury or payroll automation mailboxes that both apply mail-flow rules and send high volume of payment-related email as part of normal operations
  • Legal/compliance holds that create forwarding rules to an e-discovery mailbox for finance-related correspondence

Sigma rule & cross-platform mapping

The detection logic for BEC Financial Theft — Fraudulent Wire/ACH Transfer Initiation via Mailbox Tampering and Invoice Fraud (THREAT-Impact-BECFinancialTheftWireTransfer) above is provided in a vendor-neutral form so you can deploy it on any SIEM. The same logic is shipped here as native KQL (Microsoft Sentinel / Defender), SPL (Splunk), Elastic (Elastic Security (EQL)), QRadar (IBM QRadar (AQL)), Sumo (Sumo Logic CSE), YARA-L (Google Chronicle / SecOps), LogScale (CrowdStrike LogScale (CQL)) queries. In Sigma terms, this detection targets the following logsource:

logsource:
  product: azure

Browse the community-maintained Sigma rules for this technique:


Testing Methodology

Validate this detection against 3 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.

  1. Test 1Create BEC-Style Mailbox Forwarding Rule Targeting Finance Keywords

    Expected signal: OfficeActivity records a New-InboxRule operation with Parameters containing ForwardTo, DeleteMessage, and the finance-related SubjectContainsWords condition.

  2. Test 2Send Simulated Vendor Invoice Email with Bank-Detail-Change and Urgency Language

    Expected signal: EmailEvents records the inbound message with Subject matching finance/urgency keyword patterns; message trace shows sender domain external-test.com.

  3. Test 3End-to-End BEC Wire Fraud Simulation (Rule Tampering + Fraud Email in Sequence)

    Expected signal: OfficeActivity and EmailEvents both record events for the same UserId within the 72-hour correlation window.

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Related Detections

Tactic Hub

Detection Variants (1)

Different telemetry and tradecraft for the same technique — pick the one that matches the data you collect.